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Environmental factors on secondary metabolism in medicinal plants: exploring accelerating factors

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  • Medicinal plants are vital in synthesizing crucial substrates, fortifying stress resilience, and serving clinical and industrial domains. The optimization of pharmacological potential necessitates a nuanced understanding of the factors governing the synthesis of secondary metabolites sourced from plants. Cultivation success hinges upon many factors dictating the production of these vital compounds. Biotic factors, encompassing pathogens and herbivores, alongside abiotic factors such as light exposure, altitude, temperature variations, irrigation patterns, soil fertility, drought susceptibility, and salinity levels, collectively orchestrate medicinal plants' growth, development, and metabolic pathways. This comprehensive review delves into the intricate interplay of factors influencing the formation of secondary metabolites, exploring the roles of endophytes, pathogens, light availability, temperature fluctuations, drought stress, pollution impacts, and plant growth regulators. Grasping the dynamics of these factors is imperative for devising strategic interventions to enhance secondary metabolite production, thereby ensuring the sustainable and efficient cultivation of medicinal plants.
  • Agricultural management practices impact soil physicochemical properties to a remarkable extent. Degradation of soil health has led to a contraction in agricultural production and soil biodiversity particularly due to conventional farming practices, indiscriminate use of inorganic fertilizers (INF) and inadequate input of residues[1]. Organic or inorganic fertilizers have been regarded as a critical component of agriculture to accomplish global food security goals[2]. The exogenous supply of fertilizers could easily alter soil properties by restoring the nutrients that have been absorbed by the plants[2]. Thus, implementing adequate nutrient management strategies could boost plant yield and sustain plant health. Tillage affects the soil, especially for crop production and consequently affects the agro-ecosystem functions. This involves the mechanical manipulation of the soil to modify soil attributes like soil water retention, evapotranspiration, and infiltration processes for better crop production. Thus, tillage practices coupled with fertilizer inputs may prove a viable strategy to improve soil health components such as nutrient status, biodiversity, and organic carbon.

    Soil serves as a major reservoir of nutrients for sustainable crop production. Intensive cultivation due to growing population burden has led to the decline of soil nutrient status that has adversely affected agricultural production. Various researchers have assessed the soil nutrient budget and the reasons behind decline of nutrient content in soil[3]. Soil management strategies have assisted in overcoming this problem to a greater extent. Tillage practices redistribute soil fertility and improve plant available nutrient content due to soil perturbations[4]. Different tillage and fertilization practices alter soil nutrient cycling over time[5]. Fertilization is an important agricultural practice which is known to increase nutrient availability in soil as well as plants[6]. A report has been compiled by Srivastava et al.[7], which assessed the effectiveness of different fertilizers on soil nutrient status in Indian soils.

    Soil biota has a vital role in the self-regulating functions of the soil to maintain soil quality which might reduce the reliance on anthropogenic activities. Soil microbial activities are sensitive to slight modifications in soil properties and could be used as an index of soil health[8]. Maintenance of microbial activity is essential for soil resilience as they influence a diverse range of parameters and activities including soil structure formation, soil SOM degradation, bio-geochemical cycling of nutrients etc.[9]. Various researchers have identified microbial parameters like microbial biomass carbon (MBC), potentially mineralizable nitrogen (PMN), soil respiration, microbial biomass nitrogen (MBN), and earthworm population as potential predictors of soil quality. Geisseler & Scow[10] have compiled a review on the affirmative influence of long-term mineral fertilization on the soil microbial community.

    Being the largest terrestrial carbon (C) reservoir, soil organic carbon (SOC) plays a significant role in agricultural productivity, soil quality, and climate change mitigation[11]. Manure addition, either solely or along with INF augments SOC content which helps in the maintenance and restoration of SOM more effectively as compared to the addition of INF alone[12]. Enhancement of recalcitrant and labile pools of SOC could be obtained through long-term manure application accentuating the necessity of continuous organic amendments for building up C and maintaining its stability[13]. Generally, compared with manure addition, INF application is relatively less capable of raising SOC and its labile fractions[14]. Alteration in SOC content because of management strategies and/or degradation or restoring processes is more prominent in the labile fraction of soil C[15]. Several fractions of soil C play vital roles in food web and nutrient cycles in soils besides influencing many biological properties of soil[16]. Thus, monitoring the response of SOC and its fractions to various management practices is of utmost importance.

    A positive impact on SOC under manure application coupled with INF in rice-wheat systems has been reported, as compared to sole applications of INFs[17]. Although ploughing and other mechanical disturbances in intensive farming cause rapid OM breakdown and SOC loss[18], additional carbon input into the soil through manure addition and rational fertilization increases carbon content[13]. Wei et al.[19] in light sandy loam soil of China found that the inclusion of crop straw together with inorganic N, P and K fertilizers showed better results for improving soil fertility over sole use of inorganic fertilizers. Zhu et al.[20] studied the influence of soil C through wheat straw, farmyard manure (FYM), green manure, and rice straw on plant growth, yield, and various soil properties and found that the recycling of SOM under intensive cultivation is completely reliant on net OM input and biomass inclusion. However, most of the studies on residue management and organically managed systems could not provide clear views regarding the relations between the quality of OM inputs and biological responses towards it. The disintegration of soil aggregates due to ploughing, use of heavy machinery, and residue removal has been reported widely under conventional tillage (CT) practices[21]. On the contrary, improvement in SOC stabilization has also been observed by some scientists[22]. Under CT, the disintegration of macro-aggregates into micro-aggregates is a prominent phenomenon, while conservation tillage has been identified as a useful practice for increasing macro-aggregates as well as carbon sequestration in agricultural soils[23]. By and large, the ploughing depth (0–20 cm) is taken into consideration for evaluating the impact of tillage and straw retention on soil aggregation[24], while degradation in deeper layers of soil is becoming a major constraint towards soil quality together with crop yield[25].

    Hence, the present review would be useful in determining how tillage practices and inorganic and organic fertilization impact nutrient availability in the soil, microbial composition and SOC fractions besides stocks under different land uses.

    Agricultural production is greatly influenced by nutrient availability and thus nutrient management is required for sustaining higher yields of crop. The term 'nutrient availability' refers to the quantity of nutrients in chemical forms accessible to plant roots or compounds likely to be converted to such forms throughout the growing season in lieu of the total amount of nutrients in the soil. For optimum growth, different crops require specifically designed nutrient ratios. Plants need macronutrients [nitrogen (N), phosphorus (P), potassium (K) in higher concentrations], secondary nutrients [calcium (Ca), magnesium (Mg), sulphur (S) in moderate amounts as compared to macronutrients] and Micronutrients [Zn (zinc), Fe (iron), Cu (copper), B (boron), Mn (manganese), Mo (molybdenum) in smaller amounts] for sustainable growth and production[26]. Fertilizers assist the monitoring of soil nutrient levels by direct addition of required nutrients into the soil through different sources and tillage practices may alter the concentration of available plant nutrients through soil perturbations. Various studies on the influence of fertilization and tillage practices on available plant nutrients have been discussed below.

    Yue et al.[27] reported that long-term fertilization through manure/INF improved the macronutrient content of Ultisol soil in China. Two doses of NPK (2NPK) considerably improved soil properties over a single dose (NPK). Combined application (NPK + OM) resulted in higher hydrolysable N and available P over the sole OM application. The total K content was higher under the treatments NPK, 2NPK and NPK + OM than sole OM treatment, whereas available K was higher in treatments NPK + OM and 2NPK over the sole OM and NPK. Likewise, OM, INF, and OM + INF were evaluated for their potential to regulate the soil macronutrient dynamics. Organic manure significantly improved the soil N content, whereas INF showed comparable results to that of the control treatment. Besides, all the treatments improved available P and exchangeable K concentration[28].

    Hasnain et al.[29] performed comparative studies of different ratios of INF + compost and different application times for the chemical N fertilizer on silty loamy soils of China. The available nitrogen and phosphorous content were greater in conjoint OM + INF application over the bare INF and control application irrespective of N application time. Soil quality substantially improved with increasing ratio of compost and 70:30 (INF to compost ratio) was found to be most suitable to maintain soil fertility and nutrient status. Another study by Liu et al.[30] reported the superior effects of NPK + pig manure and NPK + straw to improve soil available P and K over the control and sole NPK treatments. However, total N concentration did not exhibit any significant variation under any treatment.

    Shang et al.[31] accounted the positive impact of vermicompost and mushroom residue application on grassland soil fertility in China. The addition of organic manures improved available P and K content to a considerable extent. Under moisture-deficit winter wheat-fallow rotation, another study quantified the influence of residue management approaches and fertilizer rate on nutrient accrual. Residue burning resulted in no decline in soil macronutrient content, whereas the perpetual addition of FYM for 84 years significantly improved total N and extractable K and P concentration. Thus, residue incorporation along with FYM application may prove beneficial in reducing the temporal macronutrient decline[32].

    Ge et al.[33] examined the effects of NPK and NPK along with manure (NPKM) addition on the macronutrient status of Hapli-Udic Cambisol soil. The NPKM application resulted in the highest increase in total N, available-P and K concentration as compared to NPK and control. Likewise, mineral fertilization reduction and partial substitution with organic amendments have posed a significant influence on soil macronutrient status. Soil available P and K decreased after INF reduction[34]. Chen et al.[35] evinced that integrated application of manure and mineral fertilizers to red clay soil (typical Ultisols) improved hydrolyzed nitrogen and available P due to an increase in the decomposition of organic matter (OM) and N bio-fixation than sole mineral fertilizers and control.

    A long-term experiment was carried out out by Shiwakoti et al.[36] to ascertain the influence of N fertilization and tillage on macronutrient dynamics in soil. Nitrogen fertilization produced higher crop biomass which might have improved total N and P concentration in soil. Moreover, the reduced interaction between soil colloids and residue or greater cation exchange sites due to tillage practices could have augmented K concentration in 0−10 cm soil depth. Likewise, among tillage systems combined organic (poultry manure) and inorganic (lime and fertilizers) fertilization, no-tillage, and reduced tillage with organic fertilization resulted in higher availability of P owing to minimal disturbance of soil which decreases contact surface between phosphate ions and adsorption sites. Greater losses of K in runoff water under NT resulted in lower K availability under NT than CT[37].

    The influence of tillage systems on soil nutrient dynamics showed that minimal soil disturbances under zero tillage prohibited redistribution of soil nutrients and resulted in the highest available N, P, and K in the surface soil[38]. The influence of tillage timing on soil macronutrient status has also been assessed under tillage treatments that are fall tillage (FT), spring tillage (ST), no tillage (NT), and disk/chisel tillage (DT/CT) on mixed mesic Typic Haploxerolls soil. All the tillage systems differed in the quantity of residues generated. Thus, variation in the decomposition of crop residue and mineralization of SOM resulted in variable rates of nutrient release. The FT and ST had the highest N content over DT/CT and NT systems at corresponding depth. The N content also decreased with soil depth irrespective of tillage treatment. The available P and extractable K were highest under NT at the top 10 cm soil depth and increased over time[39]. Residue management in combination with tillage treatments (ST and CT) has been reported to affect the soil macronutrient status in Bangladesh. Tillage treatments enhanced the total N content to a considerable extent. Moreover, 3 years of residue retention led to a higher concentration of total N, available P and K in the soil.

    The combinations of N, P, and K in different ratios together with two rates of organic fertilizer (OF) applied on the aquic Inceptisol having sandy loam texture influenced the micronutrient status of the soil[40]. Soil Zn content decreased with time when no fertilizer was applied as compared to organic fertilizer (OF) application. The mineral fertilizer treatments led to a substantial increase in DTPA-extractable micronutrients in the soil. The higher micronutrient concentration due to higher OM highlights the importance of maintaining OM for soil fertility and higher crop production. Further studies revealed that long-term application of sole N fertilizers led to a significant decline in total Zn and Cu, whereas Mn and Fe status improved through atmospheric deposition. Phosphorus and OF addition along with straw incorporation markedly increased total Zn, Cu, Fe, and Mn. The DTPA-extractable Mn, Zn, Fe, and Cu were also higher in OF treatment, thus demonstrating the beneficial effects of constant OM application for maintaining the nutrient status of soil[41].

    López-Fando & Pardo[42] quantified the impact of various tillage practices including NT, CT, minimum tillage (MT), and zone-tillage (ZT) on soil micronutrient stocks. Tillage systems did exhibit a significant influence on plant available Fe stocks in the topsoil; however, diminished with depth under ZT, NT and MT. Manganese was higher in NT and ZT at all depths and increased with soil depth. Zinc was highest under NT and other results did not vary significantly as in the case of Cu. The SOC levels were also found to be responsible to affect micronutrients due to tillage practices. Likewise, in Calciortidic Haploxeralf soil the distribution of soil micronutrients (Zn, Mn, Fe, Cu) was ascertained under different tillage practices (CT, MT, and NT). The micronutrient status was highest under NT in the upper layers due to the higher SOC level[43].

    Sharma & Dhaliwal[44] determined that the combined application of nitrogen and rice residues facilitated the transformation of micronutrients (Zn, Mn, Fe, Cu). Among different fractions, the predominant fractions were crystalline Fe bound in Zn, Mn, and Cu and amorphous Fe oxide in Fe with 120 kg N ha˗1 and 7.5-ton rice residue incorporation. The higher content of occluded fractions adduced the increment in cationic micronutrient availability in soil with residue incorporation together with N fertilization due to increased biomass. Rice straw compost along with sewage sludge (SS) and INF also affected the micronutrient availability under the RW cropping system. Nitrogen fertilization through inorganic fertilizers and rice straw compost and sewage sludge (50% + 50%) improved soil micronutrient status due to an increase in SOM over sole NPK fertilizers[45]. Earlier, Dhaliwal et al.[46] in a long-term experiment determined that different combinations of NPK along with biogas slurry as an organic source modified the extractable micronutrient status of the soil.

    A comparative study was carried out by Dhaliwal et al.[47] to ascertain the long-term impact of agro-forestry and rice–wheat systems on the distribution of soil micronutrients. The DTPA-extractable and total Cu, Zn, Fe, and Mn were greater in the RW system due to the reduced conditions because of rice cultivation. Under the RW system Zn removal was higher which was balanced by continuous Zn application. The higher availability of Fe under the RW system was due to reduced conditions. Contrarily, Mn was greater under the agro-forestry system owing to nutrient recycling from leaf litter.

    The long-term impact of integrated application of FYM, GM, WCS (wheat-cut straw) and INF on the soil micronutrients (Zn, Mn, Cu, and Fe) have been studied by Dhaliwal et al.[48]. The FYM application substantially improved DTPA-extractable Zn status followed by GM and WCS, whereas Cu content was maximum in the plots with OM application. The highest Fe concentration was recorded in treatment in which 50% recommended N supplied through FYM. This could be ascribed to the release of micronutrients from OM at low soil pH.

    Shiwakoti et al.[49] studied the dual effects of tillage methods (MP, DP, SW) and variable rates of N (0, 45, 90, 135 and 180 kg ha−1) on the distribution of micronutrients under a moisture-deficit winter wheat-fallow system. The soil Mn content was highest under the DP regime. Inorganic N application reduced Cu content in the soil. Comparative studies with adjacent undisturbed grass pasture indicated the loss of Zn and Cu to a significant extent. Thus, DP along with nitrogen added through inorganic fertilizers could improve micronutrient concentration in the soil. Moreover, the results implied that long-term cultivation with nitrogen fertilization and tillage results in the decline of essential plant nutrients in the soil. Thus, organic amendments along with INF may prove an effective approach to increase soil micronutrient content. In another study conducted by Lozano-García & Parras-Alcántara[50] tillage practices such as NT under apple orchard, CT with the wheat-soybean system and puddling (PD) in the rice-rice cropping system were found to affect nutrient status. Under CT, Cu content was lowest and Zn content was highest. On the contrary, puddling caused an increase in Fe and Mn concentration owing to the dispersion of soil aggregates which reduced the percolation of water and created an anaerobic environment thereby enhancing the availability of Fe and Mn.

    Tillage practices along with gypsum fertilization have been known to affect secondary nutrient concentrations in soil. In a long-term experiment, FYM application showed maximum response to increased S concentration due to the maximum addition of OM through FYM over other treatments as S is an essential component of OM and FYM[32]. Higher Mg content was recorded in FYM and pea vine treatments because the application of organic matter through organic manure or pea vines outright led to Mg accrual. The lower Mg concentration in topsoil than the lower layers was due to the competition between Mg and K for adsorbing sites and thus displacement of Mg by K. Han et al.[28] while ascertaining the impact of organic manures and mineral fertilizers (NPK) on soil chemical attributes determined that INF application reduced exchangeable calcium, whereas no significant changes were exhibited in the magnesium concentrations. The OM application significantly increased both the calcium and magnesium concentrations in the soil.

    While ascertaining the effect of different tillage treatments such as CT, NT, and MT on exchangeable and water-soluble cations, Lozano-García & Parras-Alcántara[50] recorded that NT had greater content of exchangeable Ca2+ and Mg2+ than MT and CT. The exchangeable Ca2+ decreased with depth, however, opposite results were observed for Mg2+ which might be due to the higher uptake of Mg2+ by the crop. On another note, there might be the existence of Mg2+-deficient minerals on the surface horizon. Alam et al.[51] studied the temporal effect of tillage systems on S distribution in the soil and observed that available S was 19%, 31%, and 34% higher in zero tillage than in minimum tillage, conventional tillage, and deep tillage, respectively.

    Kumar et al.[38] appraised the impact of tillage systems on surface soil nutrient dynamics under the following conditions: conventional tillage, zero till seeding with bullock drawn, conventional tillage with bullock drawn seeding, utera cropping and conservation tillage seeding with country plough and observed that tillage had a significant impact on the available S content. Compared with conventional tillage, zero and minimum tillage had higher S content as there was none or limited tillage operations which led to the accumulation of root stubble in the soil that decomposed over time and increased S concentration.

    Soil is considered a hotspot for microbial biodiversity which plays an important role in building a complex link between plants and soil. The microbial components exhibit dynamic nature and, therefore, are characterized as good indicators of soil quality[52]. These components include MBC, MBN, PMN and microbial respiration which not only assist in biological transformations like OM conversion, and biological nitrogen fixation but also increase nutrient availability for crop uptake. Management strategies such as fertilizer inputs and tillage practices may exert beneficial effects on soil biota as discussed below.

    Soil is an abode to a considerable portion of global biodiversity. This biodiversity not only plays a pivotal role in regulating soil functions but also provides a fertile ground for advancing global sustainability, especially agricultural ventures. Thus, the maintenance of soil biodiversity is of paramount importance for sustaining ecosystem services. Soil biodiversity is the diverse community of living creatures in the soil that interact not only with one another but also with plants and small animals to regulate various biological activities[53]. Additionally, it increases the fertility of soil by converting organic litter to SOM thereby enhancing SOC content. Thus, the SOM measures the number and activity of soil biota. Furthermore, the quality and amount of SOC, as well as plant diversity have a considerable impact on the soil microbial community structure[54].

    Dangi et al.[55] ascertained the impact of integrated nutrient management and biochar on soil microbial characteristics and observed that soil amended with biochar or the addition of organic manures influenced microbial community composition and biomass and crop yield. After two years, the higher rates of biochar significantly enhanced the levels of gram-positive and gram-negative bacterial phospholipid fatty acid (PLFA), total arbuscular mycorrhizal fungal (AMF) than lower rates, unfertilized and non-amended soil. Luan et al.[56] conducted a comparison study in a greenhouse to assess the effects of various rates of N fertilizer and kinds (inorganic and organic) on enzyme activities and soil microbial characteristics. Microbial growth (greater total PLFAs and microbial biomass carbon) and activity were promoted by manure substitution of mineral fertilizer, particularly at a higher replacement rate. On account of lower response in bacterial over fungal growth, manure addition led to a greater fungi/bacteria ratio. Furthermore, manure application significantly enhanced microbial communities, bacterial stress indicators and functional diversity. Lazcano et al.[57] determined the influence of different fertilization strategies on microbial community structure and function, soil biochemical properties and crop yield three months after addition of fertilizer. The integrated fertilizer regimes augmented microbial growth with improved enzyme activity as compared to sole inorganic amendments. Bacterial growth showed variable response with variation in fertilizer regime used whereas fungal growth varied with the amount of fertilizer added. Compared to mineral fertilizers, manure application led to a rapid increase in PLFA biomarkers for gram-negative bacteria. The organic amendments exhibited significant effects even at small concentration of the total quantity of nutrients applied through them; thus, confirming the viability of integrated fertilizer strategies in the short term.

    Kamaa et al.[58] assessed the long-term effect of crop manure and INF on the composition of microbial communities. The organic treatments comprised of maize (Zea mays) stover (MS) at 10 t ha−1 and FYM @ 10 t ha−1, INF treatments (120 kg N, 52.8 kg P-N2P2), integrated treatments (N2P2 + MS, N2P2 + FYM), fallow plot and control. The treatment N2P2 exhibited unfavourable effects on bacterial community structure and diversity that were more closely connected to the bacterial structure in control soils than integrated treatments or sole INF. In N2P2, fungal diversity varied differently than bacterial diversity but fungal diversity was similar in the N2P2 + FYM and N2P2 + MS-treated plots. Thus, the total diversity of fungal and bacterial communities was linked to agroecosystem management approaches which could explain some of the yield variations observed between the treatments. Furthermore, a long-term experiment was performed by Liu et al.[59] to study the efficiency of pig manure and compost as a source for N fertilization and found unique prokaryotic communities with variable abundance of Proteobacteria under compost and pig manure treatments.

    Recently, Li et al.[60] assessed the influence of different tillage practices (no-tillage, shallow tillage, deep tillage, no-tillage with straw retention, shallow tillage with straw retention and deep tillage with straw retention) on microbial communities and observed that tillage practices improved the bacterial Shannon index to a greater extent over the no-tillage plots in which the least value was recorded. Another research study by He et al.[61] reported the effect of tillage practices on enzyme activities at various growth stages. Across all the growth stages, enzyme activities of cellobiohydrolase (CBH), β-xylosidase (BXYL), alkaline phosphatase (AP), β-glucosidase (BG), β-N-acetylglucosamines (NAG) were 17%−169%, 7%−97%, 0.12%−29%, 3%−66%, 23%−137% greater after NT/ST, NT, ST, ST/PT, and PT/NT treatments as compared to plow tillage. The NT/ST treatment resulted in highest soil enzyme activities and yield, and thus was an effective and sustainable method to enhance soil quality and crop production.

    Microbes play a crucial role in controlling different soil functions and soil ecology and microbial community show significant variation across as well as within the landscape. On average, the total biomass of microbes exceeds 500 mg C kg soil−1[62]. Microbial biomass carbon is an active constituent of SOM which constitutes a fundamental soil quality parameter because SOM serves as a source of energy for microbial processes and is a measure of potential microbial activity[48,63]. Soil systems that have higher amounts of OM indicate higher levels of MBC. Microbial biomass carbon is influenced by many parameters like OM content in the soil, land use, and management strategies[64]. The MBC and soil aggregate stability are strongly related because MBC integrates soil physical and chemical properties responds to anthropogenic activities.

    Microbial biomass is regarded as a determinative criterion to assess the functional state of soil. Soils having high functional diversity of microbes which, by and large, occurs under organic agricultural practices, acquire disease and insect-suppressive characteristics that could assist in inducing resistance in plants[65]. Dou et al.[66] determined that soil microbial biomass C (SMBC) was 5% to 8% under wheat-based cropping systems and zero tillage significantly enhanced SMBC in the 0−30 cm depth, particularly in the upper 0 to 5 cm. According to Liang et al.[67], SMBC and soil microbial biomass N (SMBN) in the 0−10 cm surface layer were greater in the fertilized plots in comparison to the unfertilized plots on all sampling dates whereas microbial biomass C and N were highest at the grain filling stage. Mandal et al.[68] demonstrated that MBC also varied significantly with soil depth. Surface soil possessed a maximum MBC value than lower soil layers due to addition of crop residues and root biomass on the surface soil. The MBC content was highest with combined application of INF along with farmyard manure and GM, whereas untreated plots showed minimum MBC values. The incorporation of CR slows down the rate of mineralization processes; therefore, microbes require more time to decompose the residues and utilize the nutrients released[69]. On the other hand, incorporation of GR having a narrow C:N ratio enhances microbial activity and consequently accelerates mineralization in the soil. Malviya[70] also recorded that the SMBC contents were significantly greater under RT than CT, regardless of soil depth which was also assigned to residue incorporation which increases microbial biomass on account of higher carbon substrate in RT.

    Naresh et al.[71] studied the vertical distribution of MBC under no-tillage (NT), shallow (reduced) tillage and normal cultivated fields. A shallow tillage system significantly altered the tillage induced distribution of MBC. In a field experiment, Nakhro & Dkhar[72] examined the microbial populations and MBC in paddy fields under organic and inorganic farming approaches. The organic source used was a combination of rock phosphate, FYM and neem cake, whereas a mixture of urea, muriate of potash and single super phosphate was used as an inorganic source. The organically treated plots exhibited the highest MBC compared to inorganically treated plots and control. Organic carbon exhibited a direct and significant correlation with bacterial and fungal populations. The addition of organic fertilizers enhanced the content of SOC and consequently resulted in higher microbial count and MBC. Ramdas et al.[73] investigated the influence of inorganic and organic sources of nutrients (as minerals or INF) applied over a five-year period on SOC, MBC and other variables. It was observed that the addition of FYM and conjoint application of paddy straw (dry) and water hyacinth (PsWh) (fresh) significantly increased the SOC content than vermicompost, Chromolaena adenophorum (fresh) and Glyricidia aculeate (fresh), and Sesbania rostrata (fresh).

    Xu et al.[74] evaluated the influence of long-term fertilization strategies on the SOC content, soil MBN, soil MBC, and soil microbial quotient (SMQ) in a continuous rice system and observed that MBC at the main growth stages of early and late rice under 30% organic matter and 70% mineral fertilizer and 60% organic matter and 40% mineral fertilizer treatments was greater as compared to mineral fertilizer alone (MF), rice straw residues and mineral fertilizer (RF), and no fertilizer (CK) treatments. However, SMBC levels at late growth stages were greater in comparison to early growth stages. A recent study by Xiao et al.[75] demonstrated that increasing tillage frequency (no-tillage, semi-annual tillage, and tillage after every four months, two months, and one month) decreased soil MBC. Microbial biomass carbon content was significantly greater in no-till treatment (597 g kg−1) than in tillage every four months (421 g kg−1), two months (342 g kg−1) and one month (222 g kg−1). The decrease in the content of MBC in association with tillage practices is due to soil perturbations which enhanced soil temperature, diminished soil moisture content, and resulted in the destruction of microbial habitat and fungal hyphae. Therefore, the MBC content eventually affected the N cycle.

    Li et al.[76] reported that in comparison to CT, NT and RT resulted in increased MBC content and NT significantly increased MBC by 33.1% over CT. Furthermore, MBC concentration was 34.1% greater in NT than RT. The increase in MBC concentration was correlated with the results of increase in SOC concentration. Site-specific factors including soil depth and mean annual temperature significantly affected the response ratio of MBC under NT as compared to the duration of NT.

    Microbial biomass nitrogen (MBN) is a prominent indicator of soil fertility as it quantifies the biological status of soil. Soil MBN is strongly associated with organic matter of the soil. The nitrogen in MBN has a rapid turnover rate thereby reflecting the changes in management strategies way before the transformations in total N are discernable[77].

    In an experiment on continuous silage maize cultivation with crop rotation, Cerny et al.[78] observed that organic fertilizers exerted an affirmative influence on the soil MBN. During the application of organic manure MBN decreased, but there was higher MBN content as compared to control. However, addition of mineral nitrogenous fertilizers exerted an adverse effect on MBN content in experiments with maize. El-Sharkawi[79] recorded that organic matter-treated pots resulted in maximum MBN content than urea-treated pots. The sludge application enhanced total MBN and, therefore, could implicitly benefit crop production particularly in poor soils[18]. Sugihara et al.[80] observed that during the grain-filling stage in maize, residue and/or fertilizer addition exerted a pronounced influence on soil microbial dynamics; however, a clear effect of residue and ⁄or fertilizer addition was not observed. Microbial biomass nitrogen reduced dramatically from 63–71 to 18–33 kg N ha˗1 and C:N ratio at the same time increased more than ten-fold in all plots.

    Malik et al.[81] apprised that the organic amendments significantly enhanced MBN concentrations up to 50% more than the unamended soil. Wang et al.[82] evaluated the influence of organic materials on MBN content in an incubation and pot experiment with acidic and calcareous soils. The results revealed that MBN content which was affected by the different forms of organic amendments, increased by 23.37%−150.08% and 35.02%−160.02% in acidic and calcareous soils, respectively. The MBN content of both soils decreased with the increase in the C/N ratio of the organic materials, though a higher C/N ratio was effective for sustaining a greater MBN content for a very long time.

    Dhaliwal & Bijay-Singh[52] observed higher MBN levels in NT soils (116 kg ha−1) than in cultivated soils (80 kg ha−1). Kumar et al.[83] ascertained that in surface layer, MBN content was 11.8 mg kg−1 in CT which increased to 14.1 and 14.4 mg kg−1 in ZT and RT without residue retention and 20.2, 19.1 and 18.2 mg kg−1 in ZT, RT and CT with residue incorporation, respectively (Table 1). In the subsurface layer, the increased tendency on account of tillage and crop residue retention was identical to those of 0−15 cm layer but the magnitude was comparatively meagre (Table 1). In comparison to control, the persistent retention of crop residues led to significant accrual of MBN in the surface layer.

    Table 1.  Effect of different treatments on contents of various fractions of soil organic carbon[38].
    TreatmentsPMN (mg kg−1)MBC (mg kg−1)MBN (mg kg−1)DOC (mg kg−1)
    Depths (cm)
    0−1515−300−1515−300−1515−300−1515−30
    Tillage practices
    ZTR12.411.2562.5471.120.218.9198.6183.6
    ZTWR8.57.6350.4302.114.112.6167.1159.2
    RTR10.69.9490.2399.319.117.2186.4171.6
    RTWR7.66.6318.1299.814.413.7159.5148.7
    CTR9.38.5402.9354.418.216.6175.9168.9
    CT6.75.6307.9289.511.89.7142.5134.6
    Nitrogen management
    Control3.62.8218.3202.910.810.4103.792.3
    80 kg N ha−15.34.4241.1199.414.912.2128.3116.9
    120 kg N ha−18.97.6282.7220.916.516.1136.8123.6
    160 kg N ha−19.88.4343.9262.919.418.1164.8148.9
    200 kg N ha−110.49.7346.3269.622.721.7155.7136.4
    ZTR = Zero tillage with residue retention, ZTWR = Zero tillage without residue retention; RTR = Reduced tillage with residue retention, RTWR = Reduced tillage without residue retention, CTR = Conventional tillage with residue incorporation; CT = Conventional tillage without residue incorporation.
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    Xiao et al.[75] determined that the MBN content decreased with tillage treatment having highest value in no tillage treatment, however, the difference among the treatments was negligible. Soil perturbations decreased the aggregate size and thus lower the soil aeration and exposure of fresh organic matter which restricted the growth of microorganisms. The results also concluded that MBN content is highly sensitive to tillage. Ginakes et al.[84] assessed the impact of zone tillage intensity on MBN in a corn-kura clover cropping sequence. Microbial biomass nitrogen was influenced by time and type of tillage treatment. Temporal studies revealed that MBN was higher after tillage treatment than the values possessed before tillage. Under different tillage treatments, higher values were recorded in ST (shank-till) and DT (double-till) over NT and RZT (zone-till) treatments.

    Another biological parameter, PMN, is a crucial parameter of soil fertility due to its association with soil N supply for crop growth. Also, PMN indicates the status of soil microbial community associated with PMN, whether it is improving or degrading. Forest soils are characterized by greater levels of PMN than CT receiving conventional chemical fertilizers which could be assignable to improved microbial activity in the former soils than the latter[48,77]. Aulakh et al.[85] assessed the effect of various combinations of fertilizer N, P, FYM and wheat residue (WR) applied to soybean and soybean residues added to wheat under CT and CA. The added fertilizers of N and P, FYM, and crop residue enhanced the mean weight diameter and water-stable aggregates thus favoured the development of macro-aggregates. The treatment INF + FYM + crop residue performed better among all the treatments. The net flux of mineral nitrogen from the mineralizable fraction is used to measure potentially mineralizable N which indicates the balance between mineralization and immobilization by soil microbes[77]. Nitrogen mineralization is widely used to assess the ability of SOM to supply inorganic nitrogen in the form of nitrate which is the most common form of plant-available nitrogen. Kumar et al.[83] observed an increase in PMN which was higher in surface soil than sub-surface soil thereby implying that high OC accumulation on account of crop residue retention was the most probable cause.

    Verma & Goyal[86] assessed the effect of INM and organic manuring on PMN and observed that PMN was substantially affected by different organic amendments. Potentially mineralizable nitrogen varied between 19.6−41.5 mg kg−1 soil with greater quantity (2.5%) in vermicompost applied plots than FYM treated plots. The INF treatments resulted in lower PMN content which could be due to nutrient immobilization by microbes. Mahal et al.[87] reported that no-till resulted in higher PMN content than conventional tillage treatments. This trend was due to the maintenance of SOM due to the residue cover and reduction of soil erosion under no-tillage system[88]. On the contrary, tillage practices led to the loss of SOC owing to loosened surface soil and higher mineralization of SOM.

    Soil respiration is referred as the sum of CO2 evolution from intact soils because of the respiration by soil organisms, mycorrhizae and roots[89]. Various researchers have proposed soil respiration as a potential indicator of soil microbial activity[52,77]. Gilani & Bahmanyar[90] observed that addition of organic amendments enhanced soil respiration more than the control and synthetic fertilizer treatments. Moreover, among organic amendment treatments, highest soil respiration was observed in sewage-sludge treated soils. Under controlled conditions in saline-sodic soil, Celis et al.[91] reported that sewage sludge resulted in a higher soil respiration rate than mined gypsum and synthetic gypsum. The application of gypsum because of minimal organic matter intake had little effect on soil respiration. The addition of organic matter especially during early spring led to higher microbial biomass and soil respiration albeit diminished levels of nitrate-N. Moreover, SOM hinders the leaching of nitrate ions thereby resulting in a better soil chemical environment[71].

    Faust et al.[92] observed that microbial respiration was associated with volumetric water content. The respiration declined with less availability of water, thus the lesser the tillage intensity, the more the volumetric water content which consequently resulted in higher microbial respiration. Another study by Bongiorno et al.[93] reflected the influence of soil management intensity on soil respiration. Reduced tillage practices resulted in 51% higher basal respiration than CT. Furthermore, this investigation suggested that microbial catabolic profile could be used as a useful biological soil quality indicator. Recently, Kalkhajeh et al.[94] ascertained the impact of simultaneous addition of N fertilizer and straw-decomposing microbial inoculant (SDMI) on soil respiration. The SDMI application boosted the soil microbial respiration which accelerated the decomposition of straw due to N fertilization. The C/N ratio did not affect the microbial respiration at elongation and heading stages, whereas N fertilization enhanced the microbial respiration to a greater extent than the unfertilized control. Additionally, the interaction between sampling time and basal N application significantly affected microbial respiration.

    Gong et al.[95] apprised the effect of conventional rotary tillage and deep ploughing on soil respiration in winter wheat and observed that deep ploughing resulted in a higher soil respiration rate than conventional rotary tillage. Soil moisture content and temperature are the dominating agents influencing soil respiration which is restricted by the soil porosity.

    Soil organic carbon plays a vital role in regulating various soil functions and ecosystem services. It is influenced by numerous factors like tillage practices and fertilization. Moreover, modified management practices may prove beneficial to avoid SOC loss by increasing its content. An exogenous supply of fertilizers may alter the chemical conditions of soil and thus result in transformation of SOC. Tillage practices lead to frequent soil disturbances which reduce the size of soil aggregates and accelerate the oxidation of SOC thereby reducing its content. The literature on the influence of fertilization and tillage practices on the transformation of SOC is discussed below.

    Soil organic carbon is a major part of the global carbon cycle which is associated not only with the soil but also takes part in the C cycling through vegetation, oceans and the atmosphere (Figs 1 & 2). Soil acts as a sink of approximately 1,500 Pg of C up to 1 m depth, which is greater than its storage in the atmosphere (approximately 800 Pg C) and terrestrial vegetation (500 Pg C) combined[96]. This dynamic carbon reservoir is continuously cycling in diverse molecular forms between the different carbon pools[97]. Fertilization (both organic and mineral) is one of the crucial factors that impart a notable influence on OC accretion in the soil. Many researchers have studied the soil C dynamics under different fertilizer treatments. Though inorganic fertilizers possess the advantage of easy handling, application and storage, they do not contribute to soil organic carbon. On the contrary, regardless of management method, plant residues are known to increase organic carbon content.

    Figure 1.  Impact of different fertilization regimes on abundance of the microbial biomarker groups . Error bars represent the standard error of the means and different letters indicate significant differences at p < 0.05 among treatments. Source: Li et al.[60].
    Figure 2.  Soil organic carbon (SOC) dynamics in the global carbon cycle.

    Katkar et al.[98] reported a higher soil quality index under conjunctive nutrient management strategies comprising addition of compost and green leaves along with mineral nutrients. Mazumdar et al.[99] investigated the impact of crop residue (CR), FYM, and leguminous green manure (GM) on SOC in continuous rice-wheat cropping sequence over a 25-year period. At the surface layer, the maximum SOC content was recorded under NPK + FYM than NPK + CR and NPK + GM treatments. SOC was significantly lower under sole application of INFs (NPK) than the mixed application of organic and inorganic treatments. A higher range of SOC content was recorded at a depth of 0.6 m in the rice-wheat system (1.8–6.2 g kg−1) in farmyard manure (FYM)-treated plots than 1.7–5.3 g kg−1 under NPK, and 0.9–3.0 g kg−1 in case of unfertilized plots[100]. In a research study Dutta et al.[101] reported that rice residue had a higher decomposition rate (k¼ 0.121 and 0.076 day−1) followed by wheat (0.073 and 0.042 day−1) and maize residues (0.041 day−1) when their respective residues placed on soil surface than incorporated in the soils. Naresh et al.[102] found FYM and dhaincha as GM/ sulphitation press mud (SPM) treatments are potent enough to enhance the SOC. Maximum SOC content was noted in 0–5 cm depth that reduced gradually along the profile. In surface soil, the total organic content (TOC) under different treatments varied with source used to supply a recommended dose of nitrogen (RDN) along with conventional fertilizer (CF).

    Cai et al.[103] ascertained that long-term manure application significantly improved SOC content in different size fractions which followed the sequence: 2,000–250 μm > 250–53 μm > 53 μm fraction. Naresh et al.[22] determined that mean SOC content increased from 0.54% in control to 0.65% in RDF and 0.82% in RDF + FYM treatment and improved enzyme activity; thus, ultimately influenced nutrient dynamics under field conditions. The treatments RDF + FYM and NPK resulted in 0.28 Mg C ha−1 yr−1 and 0.13 Mg C ha−1 yr−1, respectively and thus higher sequestration than control. Zhao et al.[104] determined that in the surface layer, significant increase in SOC content in each soil aggregate was noticed under straw incorporation treatments over no straw incorporated treatments (Fig. 3). Moreover, the aggregate associated OC was significantly higher in the surface layer than the sub-surface layer. The highest increment in aggregate-associated OC was noted in both maize and wheat straw (MR-WR) added plots followed by MR and least in WR. Besides, all of the three straw-incorporated treatments exhibited notable increase in SOC stock in each aggregate fraction in the surface layer of the soil. In the subsurface (20−40 cm) layer under MR-WR, significant rise in SOC stock of small macro-aggregates was observed, whereas there was a reduction in SOC stock in the silt + clay fraction than other treatments. The straw-incorporated treatments increased the quantity of mineral-associated organic matter (mSOM) and intra-aggregate particulate organic matter, (iPOM) within small macro-aggregates and micro-aggregates especially in the topmost layer of the soil.

    Figure 3.  Distribution of OC in coarse iPOM (intra-aggregate particulate organic matter) fine iPOM, mSOM (mineral-associated matter), and free LF (free light fraction) of small macro-aggregates and micro-aggregates in the 0–20 cm and 20–40 cm soil layers under MR-WR (return of both maize and wheat straw), MR (maize straw return), WR (wheat straw return). Different lowercase and uppercase letters indicate significant differences at p < 0.05 among treatments and depths respectively[104].

    Srinivasarao et al.[105] reported that SOC content was reduced with the addition of INFs (100% RDN) alone as compared to the conjunctive application of inorganic and organic or sole FYM treatments. Earlier, Srinivasarao et al.[106] reported that FYM treated plots exhibited greater per cent increase in SOC stock than mineral fertilized plots and control. Tong et al.[107] ascertained that the application of NP and NPK significantly improved SOC stocks. On the contrary, fertilized soils could also exhibit decrease in carbon content than control. Naresh et al.[108] determined that higher biomass C input significantly resulted in greater particulate organic carbon (POC) content. Zhang et al.[109] ascertained that long-term addition of NPK and animal manures significantly improved SOC stocks by a magnitude of 32%−87% whereas NPK and wheat/ and or maize straw incorporation enhanced the C stocks by 26%−38% than control. Kamp et al.[110] determined that continuous cultivation without fertilization decreased SOC content by 14% than uncultivated soil. However, super optimum dose of NPK, balanced NPK fertilization and integration of NPK with FYM not only improved SOC content but also SOC stocks over the first year. In conventionally tilled cotton-growing soils of southern USA, Franzluebbers et al.[111] estimated that carbon sequestration averaged 0.31 ± 0.19 Mg C ha−1 yr−1. Mandal et al.[112] reported maximum SOC stock in the surface layer of the soil (0–15 cm) which progressively diminished with depth in each land use system. A significant decrease in SOC stock along the profile depth was also observed by Dhaliwal et al.[47] in both croplands and agroforestry. In the topmost soil layer, highest SOC stock was recorded in rice–fallow system while the lowest was in the guava orchard[112].

    Nath et al.[113] determined that there was accrual of higher TOC in surface layers as compared to lower layers of soil under paddy cultivation. This accrual could be adduced to left-over crop residues and remnant root biomass which exhibited a decreasing trend with soil depth. Das et al.[114] determined that integrated use of fertilizers and organic sources resulted in greater TOC as compared to control or sole fertilizer application. Fang et al.[115] observed that the cumulative carbon mineralization differed with aggregate size in top soils of broad-leaved forests (BF) and coniferous forests (CF). However, in deep soil it was greater in macro-aggregates as compared to micro-aggregates in BF but not in CF (Fig. 4). By and large, the percent SOC mineralized was greater in macro-aggregates as compared to micro-aggregates. Dhaliwal et al.[100] ascertained that SOC accrual was considerably influenced by residue levels and tillage in surface soil (0−20 cm); albeit no variation was observed at lower depth (20−40 cm). The SOC content was greater in zero-tilled and permanently raised beds incorporated with residues as compared to puddled transplanted rice and conventionally planted wheat. Pandey et al.[116] reported that no-tillage prior to sowing of rice and wheat increased soil organic carbon by 0.6 Mg C ha–1 yr–1. The carbon sequestration rate on account of no-tillage or reduced tillage ranged between 0−2,114 kg ha–1 yr–1 in the predominant cropping system of South Asia, Xue et al.[117] observed that the long-term conventional tillage, by and large, exhibited a significant decline in SOC owing to degradation of soil structure, exposing protected soil organic matter (intra-soil aggregates) to microbes. Therefore, the adoption of no-tillage could hamper the loss of SOC thereby resulting in a greater or equivalent quantity of carbon in comparison to CT (Fig. 5).

    Figure 4.  (a) Soil aggregate fractions of two depths in two restored plantations of subtropical China, (b) organic carbon and (c) its mineralization from various soil aggregates within 71 d at various soil depths in two restored plantations of subtropical China. Error bars show the standard error of the mean. The different letters represent significant differences among the different soil aggregate fractions within a depth at p < 0.05[115].
    Figure 5.  The concentrations of (a) SOC, (b) total nitrogen (TN), and (c) soil C:N ratio for 0–50 cm profile under different tillage treatments in 2012 and 2013. NT = no-till with residue retention; RT = rotary tillage with residue incorporation; PT = plow tillage with residue incorporation; and PT0 = plow tillage with residue removed. The lowercase letters indicate statistical difference among treatments at p < 0.05[117].

    Singh et al.[118] determined that carbon stock in the 0-40 cm layer increased by 39, 35 and 19% in zero-tilled clay loam, loam, and sandy loam soils, respectively as compared to conventional tilled soils over a period of 15 years. Kuhn et al.[119] also apprised about the advantages of NT over CT vis-a-vis SOC stocks across soil depths. In the surface layer (0−20 cm) NT, by and large, resulted in higher SOC stocks as compared to CT; however, SOC stocks exhibited a declining trend with soil depth, in fact, became negative at depths lower than 20 cm. Sapkota et al.[120] observed that over a period of seven years, direct dry-seeded rice proceeded by wheat cultivation with residue retention enhanced SOC at 0-60 cm depth by a magnitude of 4.7 and 3.0 t C ha−1 in zero-tillage (ZTDSR-ZTW + R) and without tillage (PBDSR-PBW + R), respectively. On the contrary, the conventional tillage rice-wheat cropping system (CTR-CTW) decreased the SOC up to 0.9 t C ha−1 (Table 2).

    Table 2.  Influence of tillage and crop establishment methods on SOC stock and its temporal variation under rice–wheat system[120].
    Tillage and crop establishment methodsDepths (m)
    0–0.050.05–0.150.15–0.30.3–0.60–0.6
    Total SOC (t/ha)
    CTR-CTW3.5e7.1c8.77.026.2c
    CTR-ZTW3.9d7.6bc8.86.526.7c
    ZTDSR-CTW4.2d7.5bc9.26.327.3c
    ZTDSR-ZTW4.9c8.9ab8.26.228.2bc
    ZTDSR-ZTW+R6.1a9.0ab9.86.831.8a
    PBDSR-PBW+R5.5b9.3a9.36.030.1ab
    MSD0.41.72.01.42.49
    Treatment effect
    (p value)
    < 0.0010.040.1580.267< 0.001
    Initial SOC content3.6 ±
    0.15
    8.1 ±
    1.39
    8.78 ±
    1.07
    6.7 ±
    0.73
    27.1 ±
    1.21
    Change in SOC over seven years (t/ha)
    CTR-CTW−0.16−0.99−0.040.28−0.90
    CTR-ZTW0.28−0.500.01−0.20−0.41
    ZTDSR-CTW0.62−0.570.45−0.340.16
    ZTDSR-ZTW1.340.84−0.62−0.461.09
    ZTDSR-ZTW+R2.490.961.040.164.66
    PBDSR-PBW+R1.891.220.51−0.642.98
    CTR-CTW = Conventionally tilled puddled transplanted rice followed by conventionally tilled wheat, CTR-ZTW = Conventionally tilled puddled transplanted rice followed by zero-tilled wheat, ZTDSR-CTW = Zero-tilled direct dry-seeded rice followed by conventionally tilled wheat, ZTDSR-ZTW = Zero-tilled direct dry-seeded rice followed by zero-tilled wheat, ZTDSR-ZTW+R = Zero-tilled direct dry-seeded rice followed by zero-tilled wheat with residue retention, PBDSR-PBW+R = Direct dry-seeded rice followed by direct drilling of wheat both on permanent beds with residue retention, MSD, minimum significant difference. Significant different letters indicate significant differences at p < 0.05.
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    Labile organic carbon (LC) is that fraction of SOC that is rapidly degraded by soil microbes, therefore, having the highest turnover rate. This fraction can turn over quickly on account of the change in land use and management strategies. From the crop production perspective, this fraction is crucial as it sustains the soil food cycle and, hence, considerably impacts nutrient cycling thereby altering soil quality and productivity. Short-term management could influence the labile fraction of carbon[121]. However, some site-specific problems and regional factors may influence their distribution in soil layers[102].

    Banger et al.[122] observed significant alteration in labile pools of C, for instance, particulate organic matter (POM), water-soluble C (WSC) and light fraction of C (LFC) because of the addition of fertilizers and/or FYM over a 16-year period. Particulate organic matter, LFC and WSC contributed 24%–35%, 12%–14% and 0.6%–0.8%, respectively, towards SOC. The increase in concentration of SOC including its pools like POC and the sequestration rate due to integrated nutrient management was also reported by Nayak et al.[123]. Gu et al.[124] observed that mulch-treated soils (straw and grass mulch) had significantly greater levels of LOC, POC, DOC and EOC as compared to no mulch-treated soils which could be adduced to the addition of straw, root and its sections into the soil. The content of labile C fractions across all treatments exhibited a decreasing trend with soil depth[23, 102, 125].

    In a long-term experiment, Anantha et al.[126] observed that the total organic carbon apportioned into labile carbon, non-labile, less labile, and very labile carbon constituted around 18.7%, 19.3%, 20.6% and 41.4% of the TOC, respectively (Table 3). Zhu et al.[20] determined that straw incorporation had a substantial impact on TOC and labile C fractions of the soil which were greater in straw incorporated treatments as compared to non-straw treatments across all the depths. Wang et al.[127] reported that the light fraction organic carbon (LFOC) and DOC were significantly greater in the straw-applied treatments than the control by a magnitude of 7%–129% for both the early and late season rice. The treatments NPK + FYM or NPK + GR + FYM resulted in greater content of very labile and labile C fractions whereas non-labile and less labile fractions were greater in control and NPK + CR treatment. There was 40.5% and 16.2% higher C build-up in sole FYM treated plots and 100% NPK + FYM, respectively over control. On the other hand, a net depletion of 1.2 and 1.8 Mg ha−1 in carbon stock was recorded under 50% NPK and control treatments, respectively. Out of the total C added through FYM, only 28.9% was stabilized as SOC, though an external supply of OM is a significant source of soil organic carbon[69]. Hence, to sustain the optimum SOC level at least an input of 2.3 Mg C ha−1 y−1 is required. A comparatively greater quantity of soil C in passive pools was observed in 100% NPK + FYM treatment. The increase in allocation of C into the passive pool was about 33%, 35%, 41% and 39% of TOC in control, suboptimal dose, optimal dose and super optimal dose of NPK which indicates that the concentration of passive pools increased with an increase in fertilization doses. Water-soluble carbon (WSC) was 5.48% greater in the upper soil layer as compared to lower layer of soil. In surface soil (0−15 cm), the values of light fraction carbon (LFC) were 81.3, 107.8, 155.2, 95.7, 128.8, 177.8 and 52.7 mg kg−1 in ZT without residue retention, ZT with 4 t ha−1 residue retention, ZT with 6 t ha−1 residue retention, FIRB without residue addition and FIRB with 4 and 6 t ha−1 residue addition and CT, respectively (Table 4). Tiwari et al.[128] determined that the decrease in POC was due to reduction in fine particulate organic matter in topsoil whereas decrement in dissolved organic carbon was observed largely in subsoil. Therefore, in surface soils fine POC and LFOC might be regarded as preliminary evidence of organic C alteration more precisely, while DOC could be considered as a useful indicator for subsoil. Reduction in allocations of fine POC, LFOC and DOC to SOC caused by tillage and straw management strategies indicated the decline in quality of SOC. A higher SOC concentration was recorded in the conjoint application of INF + FYM (0.82%) and sole application of INF (0.65%) than control (0.54%). Kumar et al.[83] reported that the CT without residue retention had significantly lower labile carbon fractions (27%–48%) than zero-tillage with 6-ton residue retention. Moreover, residue-retained fertilized treatments had significantly greater labile fractions of C than sole fertilized treatments[125]. Kumar et al.[83] reported highest change in DOC in zero-till with residue retention (28.2%) in comparison to conventional tillage practices. In ZT, absence of soil perturbations resulted in sustained supply of organic substrata for soil microbes which increases their activity. On the contrary, CT practices resulted in higher losses of C as CO2 due to frequent disturbances.

    Table 3.  Oxidisable organic carbon fractions in soils (g kg−1) at different layers[126].
    TreatmentDepths (cm)
    0−1515−3030−45Total
    Very Labile C
    Control3.6 ± 0.5c1.4 ± 0.3b1.3 ± 0.2a6.3 ± 0.4b
    50% NPK4.6 ± 0.3bc2.1 ± 0.7ab1.5 ± 0.1a8.1 ± 0.9a
    100% NPK4.4 ± 0.3bc2.3 ± 0.2a1.4 ± 0.5a8.0 ± 0.7a
    150% NPK5.0 ± 0.2ab2.6 ± 0.2a1.5 ± 0.1a9.0 ± 0.3a
    100% NPK + FYM4.8 ± 0.2ab2.0 ± 0.2ab1.3 ± 0.3a8.1 ± 0.2a
    FYM5.9 ± 1.3a2.2 ± 0.2a1.4 ± 0.3a9.5 ± 1.6a
    Fallow4.2 ± 0.7bc1.5 ± 0.5b0.7 ± 0.3b6.3 ± 0.8b
    Lbile C
    Control2.4 ± 0.3a1.0 ± 0.2a0.8 ± 0.4a4.2 ± 0.6a
    50% NPK1.7 ± 0.4ab0.9 ± 0.5a0.7 ± 0.2a3.3 ± 0.7a
    100% NPK1.8 ± 0.4ab0.8 ± 0.5a0.6 ± 0.3a3.2 ± 0.8a
    150% NPK1.2 ± 0.3b0.7 ± 0.2a0.9 ± 0.2a2.8 ± 0.4a
    100% NPK + FYM1.9 ± 0.3ab0.7 ± 0.2a0.7 ± 0.3a3.4 ± 0.2a
    FYM2.5 ± 0.9a0.7 ± 0.3a0.7 ± 0.2a3.9 ± 0.9a
    Fallow2.2 ± 1.0ab1.0 ± 0.3a1.0 ± 0.4a4.1 ± 1.1a
    Less labile C
    Control1.5 ± 0.3c0.6 ± 0.4c0.4 ± 0.0c2.6 ± 0.7d
    50% NPK1.8 ± 0.1c0.4 ± 0.1c0.5 ± 0.2c2.7 ± 0.1cd
    100% NPK2.5 ± 0.3ab0.8 ± 0.1bc1.1 ± 0.2ab4.4 ± 0.1b
    150% NPK2.6 ± 0.2a0.9 ± 0.1bc0.4 ± 0.2c3.9 ± 0.1b
    100% NPK + FYM2.7 ± 0.6a1.5 ± 0.2a1.4 ± 0.1a5.6 ± 0.7a
    FYM1.9 ± 0.7bc1.7 ± 0.2a1.0 ± 0.2b4.5 ± 0.7ab
    Fallow1.5 ± 0.3c1.3 ± 0.7ab0.9 ± 0.4b3.8 ± 1.2bc
    Non labile C
    Control1.2 ± 0.5b1.2 ± 0.3a0.2 ± 0.2b2.6 ± 0.5b
    50% NPK1.2 ± 0.9b1.7 ± 0.8a0.7 ± 0.4ab3.5 ± 1.8ab
    100% NPK1.3 ± 0.6b1.5 ± 0.6a0.5 ± 0.2ab3.3 ± 1.0ab
    150% NPK1.4 ± 0.3b1.5 ± 0.2a0.8 ± 0.1a3.7 ± 0.3ab
    100% NPK + FYM2.0 ± 0.8b1.3 ± 0.1a0.3 ± 0.3ab3.5 ± 0.7ab
    FYM3.7 ± 1.3a1.0 ± 0.2a0.5 ± 0.5ab5.1 ± 1.9a
    Fallow2.1 ± 0.2b1.4 ± 0.7a0.4 ± 0.2ab3.9 ± 0.9ab
    Values in the same column followed by different letters are significantly different at p < 0.001, ± indicates the standard deviation values of the means.
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    Table 4.  Influence of tillage and nitrogen management on distribution of carbon fractions in soil[83].
    TreatmentsWSOC
    (g kg−1)
    SOC
    (g kg−1)
    OC
    (g kg−1)
    BC
    (g kg−1)
    POC
    (mg kg)
    PON
    (mg kg−1)
    LFOC
    (mg kg−1)
    LFON
    (mg kg−1)
    Depths (cm)
    0−1515−300−1515−300−1515−300−1515−300−1515−300−1515−300−1515−300−1515−30
    Tillage practices
    ZTR28.826.223.119.39.619.134.694.281342.8967.9119.5108.1194.7154.814.812.3
    ZTWR25.324.618.414.87.877.213.763.19981.1667.494.686.5120.5104.711.810.3
    RTR27.025.922.418.28.688.174.133.871230.2836.9109.797.8170.9144.913.711.6
    RTWR23.721.818.114.27.667.073.122.96869.4604.482.676.6107.197.39.78.6
    CTR26.124.421.817.48.497.963.823.481099.1779.498.489.3143.8115.912.810.9
    CT21.820.916.113.16.215.642.892.63617.5481.869.257.690.873.69.67.9
    Nitrogen management
    Control21.114.916.113.16.135.481.581.07709.7658.631.726.3123.9104.36.45.8
    80 kg N ha−128.321.217.814.76.466.162.461.75860.7785.668.456.2132.8116.17.66.9
    120 kg N ha−129.522.119.116.17.256.713.262.18952.2808.989.578.5150.6127.69.78.6
    160 kg N ha−130.223.120.818.27.757.283.822.661099.5823.896.883.4168.5145.710.29.8
    200 kg N ha−131.125.421.318.77.937.484.153.421153.1898.4103.997.3176.2152.911.710.6
    WSOC = Water soluble organic carbon, SOC = Total soil organic carbon, OC = Oxidizable organic carbon, BC =Black carbon, POC = particulate organic carbon, PON = particulate organic nitrogen, LFOC = labile fraction organic carbon, and LFON = labile fraction organic nitrogen. ZTR = Zero tillage with residue retention, ZTWR = Zero tillage without residue retention; RTR = Reduced tillage with residue retention, RTWR = Reduced tillage without residue retention, CTR = Conventional tillage with residue incorporation; CT = Conventional tillage without residue incorporation.
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    The soil characteristics such as plant available nutrients, microbial diversity and soil organic carbon transformation are dwindling on account of intensive cultivation under conventional tillage practices, therefore, demand relevant management approaches for soil and crop sustainability. Long-term application of organic amendments significantly increases soil properties by increasing plant available macro, micro, secondary nutrients and soil organic C, whereas the increase in organic C by INF application is, by and large, due to increment in organic C content within macro-aggregates and in the silt + clay compartments. The soil organic carbon and other plant available nutrients were significantly greater in conservation tillage systems as compared to conventional tillage (CT) that conservation approaches could be an exemplary promoter of soil productivity by modifying soil structure thereby protecting SOM and maintaining higher nutrient content. The mean concentration of different fractions of carbon MBN, PMN and soil respiration under integrated nutrient management treatments was higher as compared with to control. Therefore, the conjoint use of organic manures or retention of crop residues with inorganic fertilizers is imperative to reduce the depletion of SOC while sustaining crop production as a realistic alternative. Future research should focus mainly on the usage of organic and mineral fertilizers in conjunction with conservation tillage approaches to sustain the soil environment.

    The authors confirm contribution to the paper as follows: study conception and design: Dhaliwal SS, Shukla AK, Randhawa MK, Behera SK; data collection: Sanddep S, Dhaliwal SS, Behera SK; analysis and interpretation of results: Dhaliwal SS, Gagandeep Kaur, Behera SK; draft manuscript preparation: Dhaliwal SS, walia, Shukla AK, Toor AS, Behera SK, Randhawa MK. All authors reviewed the results and approved the final version of the manuscript.

    Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

    The support rendered by the Departemnt of Soil Science, PAU, Ludhiana, RVSKVV, Gwailor, CSSRI, Karnal, IISS, Bhopal, School of Organic Farming, PAU Ludhiana and Washington State University, USA is fully acknowledged .

  • The authors declare that they have no conflict of interest.

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  • Cite this article

    Alami MM, Guo S, Mei Z, Yang G, Wang X. 2024. Environmental factors on secondary metabolism in medicinal plants: exploring accelerating factors. Medicinal Plant Biology 3: e016 doi: 10.48130/mpb-0024-0016
    Alami MM, Guo S, Mei Z, Yang G, Wang X. 2024. Environmental factors on secondary metabolism in medicinal plants: exploring accelerating factors. Medicinal Plant Biology 3: e016 doi: 10.48130/mpb-0024-0016

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Environmental factors on secondary metabolism in medicinal plants: exploring accelerating factors

Medicinal Plant Biology  3 Article number: e016  (2024)  |  Cite this article

Abstract: Medicinal plants are vital in synthesizing crucial substrates, fortifying stress resilience, and serving clinical and industrial domains. The optimization of pharmacological potential necessitates a nuanced understanding of the factors governing the synthesis of secondary metabolites sourced from plants. Cultivation success hinges upon many factors dictating the production of these vital compounds. Biotic factors, encompassing pathogens and herbivores, alongside abiotic factors such as light exposure, altitude, temperature variations, irrigation patterns, soil fertility, drought susceptibility, and salinity levels, collectively orchestrate medicinal plants' growth, development, and metabolic pathways. This comprehensive review delves into the intricate interplay of factors influencing the formation of secondary metabolites, exploring the roles of endophytes, pathogens, light availability, temperature fluctuations, drought stress, pollution impacts, and plant growth regulators. Grasping the dynamics of these factors is imperative for devising strategic interventions to enhance secondary metabolite production, thereby ensuring the sustainable and efficient cultivation of medicinal plants.

    • Throughout human history, medicinal plants have been the main source of secondary metabolites utilized in traditional medicine[1]. Medicinal plants offer a sustainable and eco-friendly approach to healthcare[2]. They can be cultivated in diverse climates, including urban areas, and often require fewer resources compared to synthetic drug production. As the global demand for sustainable solutions increases, medicinal plants can provide a viable alternative to conventional pharmaceuticals. Genetic engineering and plant tissue culture have been promising technologies for improving the formation of secondary metabolites in plants for the last decade[35]. The secondary metabolites are not vital for the basic functions of growth and development but often contribute to ecological interactions and defense mechanisms. Photoperiod, Light intensity, water availability, temperature, soil composition, and biotic interactions are key environmental variables shaping secondary metabolite synthesis[6,7]. For instance, studies have shown that increased light intensity can regulate the secondary metabolism of plants, such as terpenoids, alkaloids, and flavonoids[8].

      Similarly, temperature fluctuations can modulate secondary metabolite synthesis, with certain compounds being more abundantly under specific temperature regimes[9]. Water availability also plays a critical role; drought stress, for example, can induce the synthesize of secondary metabolites involved in osmotic regulation and antioxidant defense[10]. Furthermore, soil composition affects nutrient availability, influencing secondary metabolite biosynthesis[11]. Biotic factors such as herbivores or pathogen attacks can trigger the production of defense-related secondary metabolites as part of a plant's response to stress[12].

      Understanding the intricate interplay between environmental cues and secondary metabolite production is essential for optimizing the cultivation conditions of medicinal plants and the sustainable production of bioactive compounds for pharmaceutical and agricultural applications. Additionally, elucidating the molecular mechanisms underlying these responses can facilitate the development of strategies to manipulate secondary metabolite biosynthesis for desired traits in crops and other organisms.

    • An endophyte is a kind of endosymbiont that dwells inside a plant for at least part of its life cycle without producing visible illness. Endophytes are abundant within the tissues of live plants and play a vital role in these micro-ecosystems. The quality and quantity of crude pharmaceuticals derived from medicinal plants can be significantly impacted by the unique interaction that some co-existing endophytes and their host plants have established over time. This association can also substantially impact the synthesis of metabolic products in plants (Fig. 1)[13]. Nowadays, endophytes are often understood to be microorganisms (mostly bacteria and fungi) that can invade the internal tissues of healthy plants without producing indications of disease. However, when the host plant experiences senescence, it could become pathogenic. Various organisms called endophytes include fungi[14], bacteria[15], and archaea[16]. In addition, they live a large part of their life cycle within the living tissues of plants. Medicinal plants, as shown in Table 1, showed that stimulating factors like endophytes induced the production of metabolites, and it could be a better and more stainable way to use endophytes as green fertilizer for the significant increase of secondary metabolites in plants. Here, some common endophytes in medicinal plants are reviewed and the advantages, disadvantages, and applications are covered.

      Figure 1. 

      Medicinal response to the endophytes.

      Table 1.  Endophytes increase the accumulation of secondary metabolites in medicinal plants.

      Metabolites Endophytes Accumulation change Plants Ref.
      Ginsenoside Bacillus altitudinis KX230132.1 Increased Panax ginseng [17]
      Ginsenoside Rg3 Burkholderia sp. GE 17-7 Increased Panax ginseng [18]
      Camptothecin Kytococcus schroeteri Increased Ephedra foliata [19]
      Berberine Microbacterium and Burkholderia Increased Coptis teeta [20]
      Guignarderemophilanes A-E Guignardia mangiferae Increased Gelsemium elegans [21]
      Grignard dene A Guignardia mangiferae Increased Gelsemium elegans [21]
      Grignard lactone A Naphthomycins A, D, E, L, K, O-Q Streptomyces sp. Increased Maytenus hookeri [22]
      Cedarmycin B Daunorubicin Paenibacillus polymyxa Increased Ephedra foliata [19]
      Hookerolide Streptomyces sp. Increased Maytenus hookeri [22]
      Benzoic acid phthalic acid Bacillus atrophaeus and Bacillus mojavensis Increased Glycyrrhiza uralensis [23]
      5,7-Dimethoxy-4-phenylcoumarin Streptomyces aureofaciens Increased Zingiber officinale [24]
      Bis (2-ethylhexyl) phthalate Bacillus subtilis Increased Thymus vulgaris [25]
      1,3-dimethyl-, p-xylene dibutyl phthalate Tetracosane 1- –Heptacosano Nocardiopsis sp. Increased Zingiber officinale [26]
      Sesquiterpenoids Pseudomonas fluorescens Increased Atractylodes macrocephala [27]
      Essential oil Pseudomonas fluorescens Increased Atractylodes lancea [28]
      Ligustrazine Bacillus subtilis Increased Ligusticum chuanxiong [29]
      Morphine Marmoricola sp. and Acinetobacter sp. Increased Papaver somniferum L. [30]
      Forskolin Fusarium redolens, Phialemoniopsis cornearis, and Macrophomina pseudophaseolina Increased Coleus forskohlii [31]
    • Endophytic fungi are diverse, polyphyletic groups of microorganisms that can live in various healthy tissues of living plants, both above and below ground, without causing any symptoms. These tissues could consist of roots, leaves, and stems. Many different types of plants contain endophytic fungi[32]. It is believed that about one million different species of endophytic fungi may be found in nature. Endophytic fungus, not their host plants, are required to produce bioactive compounds that are specific to them. These substances are unique to endophytic fungus.

      Furthermore, these substances can promote the synthesis of numerous novels and well-known physiologically active secondary metabolites. Humans may be able to use and employ these metabolites as important sources of medicinal medicines. Alkaloids, diterpenes, flavonoids, and iso-flavonoids are only a few of the bioactive substances that endophytic fungi produce to increase the resistance of their host plant against biotic and abiotic stresses. Endophytic fungi would be the producers of these substances. Some endophytic fungi can potentially encourage the accumulation of secondary metabolites that plants initially created. These secondary metabolites may include essential therapeutic components or medicines. According to the analyzed references, these metabolites may have been created by the host plants or endophytic fungi. Huperzia serrata, found in the tropical zone, may yield compounds containing Huperzine-A that are thought to be activated by the endophytic fungus Acremonium and Shiraia species[33].

      It is well known that endophytic fungal colonization is not a chance phenomenon. Because of chemotaxis, a process that the host plants use to produce specific chemicals. Simultaneously, some secondary metabolites, including saponin and essential oils from medicinal plants, are produced through protracted co-evolution as a defense mechanism against the pathogens, most likely including endophytic fungi. The secondary metabolites consequently became barriers that stopped endophytic fungus from effectively colonizing the plant. To get beyond the defense mechanisms of the remaining host plants, endophytic fungi must develop the proper detoxifying enzymes, such as cellulases, lactases, xylanases, and proteases, to break down these secondary chemicals. It makes this obstacle surmountable for the endophytic fungus[34]. Endophytic fungi go into a dormant, or quiescent, stage when they have established themselves within the tissues of a host plant. This state can persist for the whole of the host plant's life (neutralism), or it can persist for a considerable amount of time (mutualism or antagonism) until either the host's ontogenetic state changes in a way that benefits the fungi or the environmental conditions become more favorable for endophytic fungi. It was shown that when cultured in vitro under axenic conditions, an endophytic fungus known as Taxomyces andreanae could produce taxol. A Taxus brevifolia tree's bark was used to isolate the fungus. Some endophytic fungi can create many phytochemicals, secondary metabolites originating in plants. One such toxin is podophyllotoxin, camptothecin and structural analogs, hypericin and emodin, deoxypodophyllotoxin, and azadirachtin. Many endophytic fungi that had colonized various host plants, such as Metarhizium anisopliae, and Tubercularia sp. strain TF5 were also found to produce taxol. Our research shows that certain endophytic fungi are the production of secondary metabolites in host plants. This enhancement affects both the quantity and quality of medicinal compounds. This insight explains why traditional Chinese medicine often emphasizes using specific medicinal plants from particular regions or habitats where desired chemical compounds are likely to be abundant. The key advantage lies in leveraging the abilities of endophytic fungi to promote the accumulation of secondary metabolites originally produced by plants. By harnessing this capability, we can enhance the synthesis and accumulation of bioactive compounds in medicinal plants, leading to higher-quality pharmaceuticals. Achieving this involves introducing specific endophytic fungi to the plants. This strategy, once the interaction between endophytic fungi and their host plants is well-understood, holds great promise for revolutionizing natural medicine production, offering a highly effective approach to enhancing medicinal qualities[35].

    • Endophyte species of archaea and bacteria make up prokaryotic endophytes, and it is currently thought that prokaryotic endophytes may be classified into anywhere from two to 21 different phyla[36,37]. Surprisingly, most prokaryotic endophytes are located inside one of four distinct phyla of bacteria: proteobacteria, bacteroidetes, firmicutes, and actinobacteria. By lowering ethylene concentration, bacterial endophytes help plants become more resilient to abiotic stress and alleviate it. When there is stress and a spike in ethylene concentration, it is crucial. Under stressful conditions, ethylene reduction in plant organs has been linked to the bacterial enzyme ACC (1-aminocyclopropane-1-carboxylate) deaminase. ACC deaminase converts ACC, which the plant generates and is an ethylene precursor, into 2-oxobutanoate and ammonia, inhibiting the ethylene signaling pathway[38].

      Additionally, cleaved chemicals provide nutrients to endophytic bacteria, and ACC deaminase promotes bacterial colonization of plants to maximize bacterial development. Further investigation is necessary to ascertain the limitations of ACC deaminase in plant-endophyte relationships. This is due to the lack of inconsistent studies about its ability to reduce the levels of ethylene and aid in accumulating secondary metabolites in medicinal plants under abiotic stress. Recent studies have found that bacterial endophytes benefit secondary metabolites. Numerous articles provided evidence for this[39]. For example, Mishra et al. reported that the two most potent bacterial endophytes, Bacillus amyloliquefaciens (MPE20) and Pseudomonas fluorescens (MPE115), were able to individually and in combination modulate the withanolide biosynthetic pathway and tolerance against Alternaria alternata in Withania somnifera. It's interesting to note that plants treated with the microbial consortium under A. alternata stress had higher expression levels of genes involved in the withanolide biosynthesis pathway (3-hydroxy-3-methylglutaryl co-enzyme A reductase, 1-deoxy-D-xylulose-5-phosphate reductase, farnesyl di-phosphate synthase, squalene synthase, cytochrome p450, sterol desaturase, sterol-7 reductase, and sterol glycosyl transferases)[40].

      According to the findings of some studies, the creation of secondary metabolites that have pharmacological action may be boosted by pathogenic bacteria or other naturally occurring elicitors[41]. Plant-growth-promoting rhizobacteria (PGPR) are bacteria that colonize the rhizospheres of plants. These bacteria promote plant development via various processes under normal settings and in situations detrimental to plant growth. Through a process that botanists call induced systemic resistance, PGPR can encourage the manufacture of secondary metabolites in plants. The PGPR proteins are efficient elicitors of the main enzymes engaged in secondary metabolites' biosynthesis pathways. These pathways are associated with the defensive responses of plants against pathogenic pathogens[42].

    • Plants do not possess innate immune systems like animals, yet plants can resist many diseases via secondary metabolites (Fig. 2). Some plant secondary metabolites (PSMs), known as phytoalexins, show antimicrobial actions. These PSMs serve as a line of defense for plants against various diseases[43]. During the process of plants defending themselves against pathogens, the need for large concentrations of PSMs causes their production to be triggered fast. In lupin (Lupinus angustifolius), secondary metabolites such as phenolics exhibit variations in their amounts as a defense mechanism against fungal infection caused by Colletotrichum lupini. Effector-triggered immunity and basal immunity are the two processes responsible for activating a plant's innate immune system in response to a pathogen assault. Plants develop innate immune systems as a defense mechanism against the invasion of pathogens. In the system of basal immunity, microbe-associated molecular patterns, also known as MAMPs, are recognized by pattern recognition receptors (PRR) located on the cell surface.

      Figure 2. 

      Medicinal plants response to the pathogens and herbivores.

      Conversely, the impact is brought about by effectors that stimulate immunity, which include low molecular weight natural products, phytotoxins, and microbial proteins or peptides[44]. Because they interpret these effectors as an infection signal, plants respond by turning on several metabolic pathways. The activation of these metabolic pathways results in the production of a variety of secondary metabolites. When a pathogen attacks, PSM concentrations rise to protect the plant; however, once the threat has passed, they diminish[45]. Consequently, raising the secondary metabolite content may strengthen the plant's resistance to diseases and infections. Furthermore, employing specific endophytes as green fertilizers may encourage the production of secondary metabolites and lessen the likelihood of plant illnesses.

    • Plants create PSMs, or secondary metabolites, to defend themselves against herbivores. These PSMs regulate the signaling pathways involved in plant defense and support defensive functions. Herbivore injuries set off a complex series of processes that ultimately lead to the synthesis and accumulation of PSMs. PSMs consist of amines, glucosinolates, quinones, phenolics, peptides, polyacetylenes, terpenes, and cyanogenic glucosides (Fig. 2)[12,41]. PSMs do not participate in the fundamental processes of a plant's existence, but they are essential to its ability to adapt to its environment and defend itself against herbivores. PSMs are generated through a series of metabolites and intermediates involved in plants' defensive mechanisms. Herbivores may interfere with the production of defense chemicals at multiple levels, including the last stages of the biosynthesis process. Therefore, integrating them directly into regulatory feedback loops could help plants monitor and control their defense mechanisms more effectively[46].

      While some insects serve as pollinators, others consume just plant matter. We may classify herbivores into polyphagous, oligophagous, and monophagous. Polyphagous herbivores eat a wide variety of plant species, whereas oligophagous herbivores have a preference for just a few plant types. Oligophagous herbivores have a narrower range of plant preferences than monophagous herbivores. The PSMs in their food plants presented a challenge for the herbivorous insects, and they still do. Many systems have been developed to allow them to tolerate or detoxify PSMs. Generally, generalists have highly active enzymes that either deactivate harmful PSMs (via the CYP pathway) or swiftly remove them (through the ABC transporter pathway). Another tactic is to consume not just one plant but also samples from different species (with low PSM concentrations), diluting the impact of any poisonous substance consumed. Herbivores often have a rapid digestive process, which allows them to absorb nutrients more rapidly than any poisons, which are then rapidly expelled via the feces. Some herbivores get the assistance they need for detoxification from symbiotic gut bacteria. These microorganisms may often break down or deactivate harmful substances. If there are enough of them, they may cause significant damage to the plants that serve as their hosts. This is something that may be seen in regions where there are a lot of Senecio jacobaea plants (which produce PAs). Senecio populations have a high risk of experiencing severe declines if the PA-specialized moth Tyria jacobaeae is found in the same region. However, even under these dire circumstances, Tyria cannot eradicate its host plants[47].

      Herbivores can adjust to secondary metabolites by sequestering or detoxifying toxic substances and by upregulating or downregulating genes related to sensory information processing. PSMs regulate the interactions of herbivores, pollinators, natural enemies, and hosts in a multitrophic environment. Monophagous insects who enjoy their deadly host plants exhibit certain specializations. These insects are known to exist. These specialists frequently actively sequester the toxic PSMs generated by the host plant in addition to being able to withstand them. As a result, these experts can stockpile significant quantities of poisonous PSMs and employ them as part of their defensive mechanisms against enemies. These researchers have examined a variety of hazardous alkaloids, including quinolizidines, pyrrolizidines, aconitine, and cyanogenic glucosides, as well as poisonous cardiac glycosides, aristolochic acids, and cyanogenic glucosides[48]. These professionals often display colors that serve as warnings. That is to say. They broadcast their potential toxicity to any possible predator under their aposematic nature.

      Since we cannot observe how these specialists circumvent the inherent toxicity of PSMs, we typically do not know how they manage it. In the case of certain insects that can sequester cardiac glycosides, it is feasible to show that point mutations have changed the binding site of their molecular target, the Na, K-ATPase, to prevent cardiac glycosides from attaching to it. Cardiac glycosides would be unable to sequester them as a result. As a result, heart glycoside concentrations that would be fatal to every other species, polyphagous or oligophagous, cannot be tolerated by monarch butterflies. In most other instances, we do not have clear information about how an act of insensitivity was carried out. It has been observed that herbivorous insects that eat by piercing or sucking plants cause ethylene release and JA buildup in the plants they consume[49].

    • Eliciting the production of phytochemicals in plants by applying chemical or physical stimuli is called abiotic elicitation. Experiments have been conducted using these elicitors alone and in combinations, in hydroponics and sprays, during various phases of plant development and even after harvesting[50,51] (Fig. 3).

      Figure 3. 

      Plant synthetic responses to the abiotic factors 1-5; majore group of secondary metabolites biosynthesis pathways.

    • The physical aspect of light influences many different plant species' metabolite synthesis. Light exposure in Zingiber officinale callus culture can stimulate the production of secondary metabolites, including zingiberene and gingerol. VIB The production of secondary metabolites is also enhanced by ultraviolet (UV) radiation. An increase in UV-B exposure causes plants in the field to produce more essential oils and phenolic content, whereas a decrease in toxic beta-asarone is observed. In addition to being essential for photosynthesis and plant growth, light controls the quantity and caliber of secondary metabolites (PSMs) plants produce. Sunlight exposure promotes coumarin accumulation in M. glomerata. Plants with longer light periods have much higher levels of coumarin in their leaves and stems, while shorter light periods result in much lower levels of coumarin[52]. The length of the photoperiod had a considerable impact on the amount of coumarin found in the stems and leaves. Therefore, the amount of accumulated PSMs is significantly influenced by both the light intensity and the photoperiod.

      Based on the varied expression patterns of terpene synthase genes seen throughout plant development and in response to both biotic and abiotic environmental stimuli, terpenoid metabolites have been linked to some ecological and physiological activities[53]. These links have been made possible by the observation that these genes are expressed differently. For instance, the ent-copalyl diphosphate (CPS1) and kaurene synthase (KS1) genes are activated in rice leaves when the plant is treated with an elicitor or ultraviolet light. Gibberellin is produced in plants via the action of these genes that control the process. Quantitative and qualitative investigations demonstrate that light exposure may impact the concentration of bioactive triterpenes in grasses like Centella asiatica, which has high levels of secondary metabolites. They had the greatest acetic acid concentration but the lowest asiaticoside concentration when grown under 70% shade[54]. The solid seasonal variations in the transcript levels of mRNA from 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXRs), the first committed enzyme in the 2-methyl-D-erythritol 4-phosphate (MEP) terpenoid biosynthetic pathway, isoprene synthase (ISPS), have been reported by Mayrhofer et al. These changes depended on the developmental stage and were strongly correlated with temperature and light levels. The process of terpenoid manufacture and the expression of genes encoding terpene synthase are generally significantly influenced by light intensity[55].

    • An essential environmental component that affects enzyme activity and metabolic pathways, influencing the synthesis of secondary metabolites, is temperature[56,57]. Research has demonstrated that temperature variations can impact the diversity, concentration, and makeup of secondary metabolites produced by marine species, microorganisms, and plants[58,59]. A rise in temperature speeds up the aging process of Panax quinquefolius's leaves and the buildup of secondary metabolites in the roots. It should be highlighted that thermal stress dramatically slows down plant development and causes senescence, even though it has also been shown to promote or decrease the synthesis of secondary metabolites in plants. When the temperature of the plants of the Panax quinquefolius genus is raised, the amount of root ginsenosides they contain increases. Temperate plants produce certain cryoprotectant molecules during the overwintering season. These molecules include soluble sugars (trehalose, stachyose, saccharose, and raffinose), sugar alcohols (sorbitol, ribitol, and inositol), low molecular weight nitrogenous substances (proline and glycine betaine, protective antifreeze proteins, and others). Plants exhibit enhanced resistance to low temperatures due to lignification and suberin deposition in their cell walls[17].

      The term 'cold stress' refers to temperatures below 20 °C, which have a deleterious effect on the growth and development of plants and drastically reduce their production (Table 2). It hinders plants from expressing their full genetic potential, which directly inhibits metabolic responses and indirectly inhibits water intake and cellular dehydration. This prevents plants from reaching their full genetic potential. The levels of chlorophyll a and total chlorophyll are reduced due to cold stress, whereas the quantity of apoplastic and total soluble protein in the leaf is increased[60]. According to the findings of the research, cold stress has a substantial influence on the variance of the number of PSMs present.

      Table 2.  List of metabolites that are affected by temperature variation in different medicinal plants.

      Metabolites Stress Affects Plants Ref.
      Flavonol, quercetin, kaempferol, and isorhamnetin Cold stress Increased Brassica oleracea L., var. sabellica [61]
      Terpenoids Cold stress Increased Polygonum minus [62]
      Carotenoid Cold stress Increased Sugarcane [62]
      Alkaloids Cold stress Increased Arabidopsis [17]
      Total phenolic Cold stress Decreased Eleutherococcus senticosus
      (Rupr. & Maxim.) Maxim.
      [63]
      Flavanol Cold stress Decreased Polygonum minus Huds. [62]
      Artemisinin Cold stress Increased Artemisia sp. [64]
      Autrescine Heat stress Increased Oryza sativa L. [65]
      Tryptophan, tyrosine, and phenylalanine Heat stress Decreased Ricinus communis [66]
      Flavonoids Heat stress Decreased Vigna radiata (L.) R. Wilczek [67]
      Hypericin, pseudohypericin, and hyperforin Heat stress Increased St. John's wort [17]
      Alkaloid ricine Heat stress Increased Caster bean [66]
    • Drought is one of the most important environmental stresses that can alter plants' physiological and biochemical features and increase the concentration of secondary metabolites in plant tissues. Drought is an abiotic physical elicitor[6870]. Drought is classified as a physical elicitor and is an example of an abiotic physical elicitor. There is a wide range of variability in the ability of plant species to withstand drought. The circumstances that lead to drought are characterized by low levels of available water and are coupled with elevated temperatures and intense levels of solar radiation[7173]. In Glechoma longituba, a lack of water results in a reduction in the total flavonoid content, and a water treatment of field capacity between 80 and 85% effective is ideal for achieving the highest possible total flavonoid content[74]. According to research published in 2015 by Gupta and colleagues, drought stresses significantly raise the amount of rebaudioside A in the suspension culture of Stevia rebaudiana more so than stevioside[75].

      Plant growth and photosynthesis are altered due to drought, which also changes the biochemical features of the plant[73]. Lack of water increases the amount of the secondary metabolite artemisinin in Artemisia, whereas it increases the amount of betulinic acid, quercetin, and rutin in Hypericum brasiliense. The concentration of secondary metabolites like hypericin and pseudohypericin, as well as the photosynthetic rate of the leaves, are both lowered by water stress. Conversely, in situations with insufficient water, the concentration of the major secondary metabolite hyperforin increases[73]. When exposed to water stress, herbs like sweet basil (Ocimum basilicum) and American basil (Ocimum americanum) lose vegetative growth, total carbohydrates, essential oil, proline, nitrogen, phosphorus, potassium, and protein content. Stress from water leads to an increase in the percentage of essential oils and a decrease in nitrogen, phosphate, potassium, and protein. For the production of herbs and essential oils, the field's water capacity must be at least 75% to maximize the potential for both species[76]. These results demonstrate that drought stress could fluctuate the biosynthesis of PSMs.

    • Salt in the environment encourages the development of many secondary metabolites in plants, such as phenols, terpenes, and alkaloids. Some plant species have a higher anthocyanin concentration in response to salt stress, whereas salt-sensitive species have a lower anthocyanin content. A correlation exists between an increase in salinity and an increase in the polyphenol content of certain plant tissues. In Chamomilla (Matricaria chamomilla), the essential oil content, number of branches per plant, number of flowers per plant, peduncle length, and other characteristics are all negatively affected by salinity and dryness[77]. Salt stress causes a rise in the content of the alkaloids reserpine and vincristine, which are both classified as plant stress metabolites (PSMs) in Rauvolfia tetraphylla and C. roseus, respectively. The alkaloid ricinine present in Ricinus communis was higher in the plant's shoots but lower in its roots. The enhancement of phenols in Mentha pulegium[78] and Nigella sativa[79] has been noticed under salt stress. Isabgol (Plantago ovata) was subjected to salt stress, which resulted in a rise in the plant's proline, flavonoids, and saponins content[80]. Plants' accumulation of various ions may also change the concentration of primary and secondary metabolites. This can happen in both cases. According to the findings of these investigations, salinity is a factor that positively influences the accumulation of PSMs.

    • Because of their ability to stimulate the gene expression of various photosynthetic pathways, several plant growth regulators have also been used for elicitation[81,82]. Salicylic acid and jasmonic acid are the most often used plant growth regulators as elicitors. These two plant growth regulators are important signals that influence gene production. Abscisic acid is a stress hormone in plants due to its rapid buildup in response to stress. In addition to producing systemic acquired resistance in plants in response to different pathogens, SA is also known to produce secondary metabolites in plants[83]. The JA signaling pathway is necessary for synthesizing several plant secondary metabolites, including terpenoids, flavonoids, alkaloids, and phenylpropanoids. It is acknowledged that this route is a crucial signal for this operation. The cyclopentanone class of compounds known as jasmonates, which includes JA and methyl jasmonates (MeJA), affects a range of plant responses and functions as an excellent elicitor to encourage the synthesis of secondary metabolites in vitro cultures. Jasmonate a and jasmonate b are the two categories into which jasmonates can be divided. They make up a significant subclass of elicitors for a wide variety of metabolic pathways, which is most often seen as the induction of secondary metabolite production in response to environmental challenges experienced by plants. The phytohormones jasmonic acid (JA) and salicylic acid (SA) are responsible for inducing plant defenses, and the linked pathways interact convolutedly at the transcript and protein levels. Following the application of JA and SA, it was shown that both chewing insects (Heliothis virescens) and sucking insects (Myzus persicae) had detrimental impacts on their ability to survive[84].

      In plant cell cultures, proteins, carboxylates, and enzymes may all function as biotic elicitors and set off defensive processes. Protein elicitors have been employed to understand how ion channels in plant cell membranes relate to the signal transmission brought on by outside stimuli. In plant cell cultures, glycoproteins trigger the creation of phytoalexins. Plant defense mechanisms include proteins that attach to carbohydrates and function as lectins or agglutinins, protecting plants from various animals that prey on them. It is associated with secondary metabolite production, which plays a part in plant defense mechanisms. Oligogalacturonides (OGAs), molecules that function as elicitors, are produced from the pectic polysaccharides in plant cell walls. Biosynthesis of phytoalexins is prompted in the cotyledons of Glycine max plants by OGAs, whereas Nicotiana tabacum plants' defensive mechanisms are activated. Chitin, a component of fungal cell walls, performs the function of a powerful elicitor signal in some plant-based systems[85].

      Different chemicals, such as minerals, heavy metals, fertilizers, pollutants, gaseous toxins (such as elevated CO2 and ozone), insecticides, and so on, may all contribute to developing chemical stress (Table 3). In Cassia angustifolia, the administration of micronutrients causes an increase in primary metabolites, which may generate a corresponding increase in secondary metabolites. The amounts of chlorophyll, protein, and phenol are not the only things affected by FeSO4, ZnSO4, Miczink, and CuSO4. The amount of flavonoid in the St. John's Wort plant (Hypericum perforatum) may be altered by adding nitrogen and phosphorus fertilizers. Nitrogen and phosphorous are essential nutrients that play a significant role in the development and maturation of the plant[86].

      Table 3.  Elevated CO2 affected the accumulation of secondary metabolites.

      Metabolites Chemicals Affects Plants Ref.
      Total phenolics Elevated CO2 Increased Populus tremula L. [87]
      Tannins Elevated CO2 Increased Zingiber officinale Roscoe [8890]
      Total and individual phenolics and antioxidant Elevated CO2 Decreased Oryza sativa L. [91]
      Terpene Elevated CO2 Increased Phaseolus lunatus L. and Gossypium hirsutum L. [92,93]
      Morphine, codeine, papaverine, and noscapine Elevated CO2 Increased Papaver setigerum L. [17]

      Furthermore, plants use nitrogen as fertilizer as signals to control the expression of specific genes, including those in Arabidopsis and other plant species. In response to the varying amounts of nitrogen available in their surroundings, plants have evolved a variety of responses. Scientists use various methods, including genetics and bioinformatics, to identify the regulatory pathways that plants use in response to different nitrogen concentrations. Plants respond to nitrogen supply by changing gene expression and metabolic, physiological, and developmental changes. Phosphorus influences not just the development of plants but also the secondary metabolites they produce. Phosphorus increases the leaf biomass, total phenolic concentrations, and rosmarinic acid (RA) concentrations in Salvia officinalis (garden sage), but does not influence the quality or quantity of the essential oils produced by the plant. Therefore, phosphorus is crucial in the growth and manufacture of secondary metabolites in plants[17].

      Apart from these food factors, plants also require specific atmospheric gases, such as nitrogen, oxygen, and carbon dioxide, to complete their biological activities and create secondary plant metabolites. A rise in CO2 concentration raises the concentration of secondary plant products in Taxus bacatta, H. perforatum, and Echinacea purpure, either directly or indirectly[94]. Plant metabolism is altered by heavy metals, which also impact the synthesis of carbohydrates, proteins, non-protein thiols, pigments used in photosynthetic processes, and sugars. Metals can alter secondary metabolism in certain ways, which could impact the synthesis of physiologically active molecules. The formation of secondary metabolites may be affected by various metal ions, including Ag+, Eu3+, Cd2+, La3+, and oxalate[95]. According to these studies on the chemical impacts on secondary metabolites, the chemicals that plants require for growth and development also influence PSM synthesis, which implies that plant concentrations of PSMs may vary in response to these chemicals. PSMs are necessary for plants' growth and development, so naturally, these substances would have this impact.

    • Mineral elements, especially mineral nitrogen, affect primary metabolism and the production of secondary metabolites. This has a major impact on the quality of the raw materials that plants generate and their growth and development. The amount of flavonol in the seedling tissues of Arabidopsis and tomato plants was found to have a highly significant inverse connection with the availability of nitrogen and phosphate. Furthermore, it was discovered that in both species, the concentrations of quercetin, kaempferol, and isorhamnetin rose in response to either phosphate or nitrogen stress. Labisia pumila Benth's capacity to produce total phenolics and flavonoids was noticeably influenced by the nitrogen levels present in the environment. In addition, the increase in fertilizing was associated with a modest rise in antioxidant activities[96].

      Furthermore, different genes encoding flavonoid biosynthesis enzymes may be affected differently by N-deficiency stress. For example, mRNA levels for chalcone synthase (CHS) and dihydroflavonol-4-reductase (DFR) increased in response to N stress, whereas mRNA levels for a chalcone isomerase homologous band (CHI) decreased. Insufficient amounts of specific nutrients may impact the concentration of alkaloids in the soil. However, the content of alkaloids was greatly enhanced by 9%–17% by nitrogenous fertilizers; the largest increase was observed upon application of NH4NO3. Under conditions of acute potassium deficit, the alkaloid contents in the seeds of sweet cultivars of lupins increased drastically by 205%. When magnesium and nitrogen were applied to seeds, the number of alkaloids present was reduced. When the rate of fertilization was raised in Datura innoxia, the number of total alkaloids that were produced also rose. The metabolic route that produces amino acids, nucleic acids, lipids, and enzymes, all of which immediately impact cell division, has nitrogen as one of its essential components. When Pseudotsuga menziesii was cultivated with a high concentration of ammonium nitrate fertilizer, the monoterpenoid concentrations showed an ascending pattern. When 200 milligrams of N fertilizer were applied to Thuja plicata during the active growth season, the amount of monoterpenoid increased. This was in contrast to the 400 mg high fertilizing amount. However, the higher amount of fertilizer applied resulted in a higher monoterpenoid content[97].

    • Plant secondary metabolites represent natural compounds plants synthesize, pivotal for their adaptation and survival mechanisms. These compounds, categorized by their chemical structures and biosynthetic pathways, intrigue humans due to their potential in pharmaceuticals, nutraceuticals, and various industrial applications. Both biotic and abiotic factors influence the synthesis of these metabolites in medicinal plants. Biotic factors encompass interactions with organisms like herbivores, pathogens, and symbionts. Herbivores can trigger secondary metabolite production as a defense mechanism, elevating their concentration. Pathogens also stimulate their production, often leading to compounds with antimicrobial properties. On the other hand, abiotic factors, including temperature, light, soil nutrients, and water availability, can significantly impact secondary metabolite biosynthesis. Understanding these factors is crucial for harnessing the full potential of plant secondary metabolites.

    • Plant tissue culture techniques, such as cell suspension cultures, organ cultures, and hairy root cultures can produce secondary metabolites in a controlled environment. By optimizing culture conditions, such as nutrient composition, hormone levels, and elicitation strategies, researchers can stimulate the production of secondary metabolites, offering the advantage of year-round output independent of seasonal variations. Applying stress factors, such as UV radiation, temperature changes, or exposure to specific chemicals can trigger the plant's defense mechanisms and stimulate secondary metabolite production through a process known as elicitation. Understanding the signaling pathways involved in stress responses enable the development of innovative approaches to induce secondary metabolite production in plants.

      Metabolic engineering involves manipulating the metabolic pathways of plants to redirect the flow of precursors toward the synthesis of desired secondary metabolites. This can be achieved through gene editing, pathway engineering, or modulation of enzyme activities, enabling the production of specific secondary metabolites or the creation of novel compounds with improved therapeutic properties. Biotechnological methods, such as using plant cell cultures, bioreactors, and genetic transformation can scale up secondary metabolite production. These approaches provide controlled and efficient systems for the large-scale production of bioactive compounds and can be combined with other techniques, like metabolic engineering, to enhance productivity further.

      Employing precision agriculture techniques coupled with remote sensing technologies can enable real-time monitoring of plant health and stress levels, facilitating timely interventions to mitigate adverse effects on biosynthesis. Additionally, harnessing microbial biofertilizers and elicitors can modulate plant metabolism, enhancing the synthesis of bioactive compounds. Furthermore, integrating traditional knowledge with modern biotechnological approaches holds promise for the sustainable cultivation of medicinal plants. Utilizing indigenous cultivation practices that promote plant resilience to environmental stressors and genetic enhancement through marker-assisted breeding and genetic engineering can further augment the yield and quality of medicinal crops. Continued research, advancements in biotechnology, and the integration of multidisciplinary approaches will contribute to unlocking the full potential of these valuable bioactive compounds.

    • The authors confirm contribution to the paper as follows: conceptualization: Alami MM, Yang G, Wang X; methodology: Alami MM, Guo S; resources: Mei Z; management: Mei Z, Yang G, Wang X; funding acquisition: Yang G, Wang X; writing – original draft preparation: Alami MM; writing – review & editing: Guo S, Mei Z, Yang G, Wang X. All authors reviewed the results and approved the final version of the manuscript.

    • Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

      • We thank the Chinese Scholarship Council (CSC) for providing scholarships for our Ph.D. studies. This study is supported by the enterprise entrusted project: Multiomics analysis and evaluation of core germplasm resources of Tinospora sagittata. and National Natural Science Foundation of China (81872948).

      • The authors declare that they have no conflict of interest.

      • Copyright: © 2024 by the author(s). Published by Maximum Academic Press, Fayetteville, GA. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (3)  Table (3) References (97)
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    Alami MM, Guo S, Mei Z, Yang G, Wang X. 2024. Environmental factors on secondary metabolism in medicinal plants: exploring accelerating factors. Medicinal Plant Biology 3: e016 doi: 10.48130/mpb-0024-0016
    Alami MM, Guo S, Mei Z, Yang G, Wang X. 2024. Environmental factors on secondary metabolism in medicinal plants: exploring accelerating factors. Medicinal Plant Biology 3: e016 doi: 10.48130/mpb-0024-0016

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