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Comparing resource use efficiencies in hydroponic and aeroponic production systems

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  • With depleting sources of fresh water, approaches must be taken to reduce the use of water in agricultural systems. Along with reduced usage, research should focus on using resources more efficiently. Hydroponic production systems offer higher resource use efficiency, such as water and fertilizer, compared to traditional agriculture systems. Notably, water usage can be reduced by more than 90% and fertilizer by 60% depending upon the system and crop. This review focuses on water and nutrient use efficiency of different crops in greenhouse production systems to further elucidate the current accomplishments and future needs in this research area. This is important because water and nutrient use efficiency is highly dependent upon multiple factors like type of crop, cultivars, environment, type of system used, nutrient concentration and form, flow rate of water, water depth, location, etc. Herein, nutrient film technique (NFT), deep-water culture, aeroponics, drip, and other systems were compared for different water and nutrient use efficiencies. Because different crops were used in these studies, direct comparison was limited, but we found that crop type and cultivars, NFT channel depth, and fertilization rate were among the most influential factors affecting nutrient use efficiency in hydroponic systems. Surprisingly, water use efficiency in aeroponic systems was greater when more nozzles were used. Aeroponic systems also showed greater water use efficiency when compared to NFT systems. Overall, this review highlights the resource use efficiency of different vegetable crops in hydroponic production systems and highlights opportunities for future research.
  • Huangqin tea (HQT, Fig. 1), also called Huangjin tea in Chinese, has a long history of consumption in China[1]. It is a healthy herbal tea crafted from the aerial parts of Scutellaria baicalensis Georgi, S. scordifolia Fisch, S. amoena C. H. Wrigh, and S. viscidula Bung[24]. Unlike traditional tea made from Camellia sinensis (L.) O. Kuntze, HQT lacks stimulating components and is renowned for clearing heat and dry dampness, eliminating toxins, promoting digestion, and soothing fire[1,5]. It can be enjoyed both as a hot beverage and as a cold drink, and modern research suggests that HQT exhibits several pharmacological properties, such as anti-inflammatory, chemopreventive effects of colorectal cancer[6], anti-aging[7], cardiovascular protection, hypoglycemic effect, hypolipidemic effect, anti-tumor, anti-bacterial, anti-influenza virus, and enhance human resistance[812]. The predominant compounds found in HQT are flavonoids and essential oils and its potential health benefits are likely attributed to the presence of distinctive flavonoids, including isocarthamidin-7-O-β-D-glucuronide, carthamidin-7-O-β-D-glucuronide, apigenin-7-O-β-D-glucopyranside, chrysin-7-O-β-D-glucuronide, scutellarin, baicalin, wogonoside, and chrysin[1318].

    Figure 1.  The aerial parts of S. baicalensis Georgi, dried and brewed Huangqin tea.

    HQT is made using simple ingredients and has a straightforward preparation method. Traditionally, the above-ground portions of Scutellaria species have been utilized for crafting HQT, with the differentiation between stems and leaves remaining unessential. During the hot summer season (July to August), the aerial parts (stem, leaves, flowers) of these species of Scutellaria from a single source are collected and cut into small sections, which are then directly dried for later use. Alternatively, freshly harvested branch and leaf sections may undergo a transformative process through multiple rounds of steaming and subsequent drying within a steamer. Once suitably conditioned, they find their place within sealed containers, poised for extended storage periods. The brewing ritual commences by incorporating 7−8 g of this tea into 2 L of water, with subsequent replenishments of water at intervals 2−3 times. As the brewing ritual concludes, HQT graces the senses with a resplendent golden infusion—earning it another name, 'Huangjin tea'.

    HQT boasts a range of health advantages and is frequently gathered from the upper sections of Scutellaria species plants cultivated in the vicinity, intended for tea consumption. In recent years, large planting bases have been established in several regions, such as Beijing, Inner Mongolia Wuchuan, Yakeshi, Hebei Chengde, Shanxi Province, and Shandong Province, to comprehensively develop S. baicalensis resources. HQT has gained more attention and is now produced by specialized operating companies in various grades, such as bulk tea, bagged tea, and unique tea culture[12].

    To provide up-to-date information on the chemical composition, bioactivity, and safety aspects of the Scutellaria genus from HQT, this review paper has collected related literature from various databases, such as PubMed, Web of Science, Sci Finder, Scopus, Baidu Scholar, China National Knowledge Internet (CNKI), Wanfang and Weipu Data. This paper aims to draw attention to the need for further research and application of HQT in preventing and managing certain chronic diseases.

    HQT can be derived from several Scutellaria species. S. baicalensis is the most extensively cultivated, with a significant biomass in its aerial parts. Consequently, the primary source of HQT is the aerial part of S. baicalensis, and research focused on these aerial parts is also the most extensive.

    The aerial part and the root of S. baicalensis share similarities in their primary constituents, notably the significant presence of flavonoids, which are regarded as the main active components. However, there are specific chemical differences between the above-ground and underground parts of S. baicalensis. Our preliminary experiments have revealed the disparities in chemical composition between these two parts[18]. Notably, the root of S. baicalensis finds its primary use in medicinal applications, characterized by notably higher expression levels of specific 4′-deoxyflavones compared to the aerial organs. In contrast, the above-ground parts of the plant are employed to prepare tea. This differentiation in utilizing these plant components reflects the accumulated wisdom of generations cultivated through extensive periods of tasting and experience.

    This section provided an in-depth review of the chemical composition of the plant from which HQT is derived. Additionally, a specific emphasis on reviewing the aerial parts of S. baicalensis, which serve as the primary source of HQT has been placed.

    In the 1970s, researchers began to study the chemical composition of the aerial parts of S. baicalensis. The flavonoids are the most abundant chemical components in the aerial parts (flowers, stems, and leaves), and about 54 flavonoid compounds were identified (Table 1, Fig. 2). Most of them are flavonoids, dihydro flavonoids, and glycosides. The main glycoside-forming sugars are arabinose, glucose, and glucuronide, with the most abundant glucuronide glycosides. The optimization of extraction processes can yield a remarkable total flavonoid content of up to 5% in the stems and leaves of S. baicalensis[19].

    Table 1.  Flavonoids of HQT.
    NumberNameFormulaSpeciesRef.
    12',5-Dihydroxy-3',6,7,8-tetramethoxyflavoneC19H18O8S. baicalensis[17]
    22',6-DihydroxyflavoneC15H10O4S. baicalensis[16]
    33',4',5,5',7-PentamethoxyflavoneC20H20O7S. baicalensis[16]
    44',5-Dihydroxy-3',5',6,7-tetramethoxyflavoneC19H18O8S. baicalensis[22,27]
    54',5-Dihydroxy-7-methoxyflavanoneC16H14O5S. baicalensis[16]
    65,4′-Dihydroxy-6,7,8,3′-tetramethoxyflavoneC19H18O8S. baicalensis[17]
    75,2′-Dihydroxy-6,7,8-trimethoxyflavoneC18H16O7S. baicalensis[17]
    85,2′-Dihydroxy-6,7,8,3′-tetramethoxyflavoneC19H18O8S. baicalensis[17]
    95,2′-Dihydroxy-7,8-dimethoxyflavoneC17H14O6S. baicalensis[17]
    105,2′-Dihydroxy-7,8,6′-trimethoxyflavoneC18H16O7S. baicalensis[17]
    115,6,7,3',4'-Pentahydroxyflavone-7-O-glucuronideC21H20O13S. baicalensis[13]
    125,6,7,4'-TetrahydroxydihydroflavoneC15H12O6S. baicalensis[17]
    135,6,7-Trihydroxy-4'-methoxyflavoneC16H12O6S. baicalensis[15]
    145,7-Dihydroxy-6-methoxyflavanonC16H14O5S. baicalensis[17]
    155,7,4′-Trihydroxy-6-methoxyflavanoneC16H14O6S. baicalensis[22]
    165,7,4'-TrihydroxyflavanoneC15H12O6S. baicalensis[13]
    17(2S)-5,7,8,4'-Tetrahydroxyflavanone 7-O-β-D-glucuronopyranosideC21H20O12S. baicalensis[21]
    18(2S)-5,6,7,4'-Tetrahydroxyflavanone 7-O-β-D-glucuronopyranosideC21H20O12S. baicalensis[21]
    196-Hydroxyluteolin-7-O-glucuronideC21H18O13S. baicalensis[13]
    207-MethoxychrysinC16H14O4S. baicalensis[16]
    21ApigeninC15H10O5S. baicalensis, S. amoena,
    S. scordifolia, S. viscidula
    [17,2125]
    22Apigenin-4'-glucopyransideC21H20O10S. baicalensis[17]
    23Apigenin-6-C-glucoside-8-C-arabinosideC26H28O14S. baicalensis[13]
    24Apigenin-7-O-β-D-glucopyransideC21H20O10S. baicalensis[17]
    25Apigenin-7-O-β-D-glucuronideC21H18O11S. baicalensis, S. amoena,
    S. scordifolia
    [13,23,24]
    26Apigenin-7-O-methylglucuronideC22H20O11S. baicalensis[28]
    27BaicaleinC15H10O5S. baicalensis, S. amoena,
    S. scordifolia, S. viscidula
    [2225]
    28Baicalein-7-O-D-glucopyransideC21H20O10S. baicalensis[22]
    29BaicalinC21H18O11S. baicalensis, S. amoena,
    S. scordifolia, S. viscidula
    [16,2325]
    30CarthamidinC15H12O6S. baicalensis[13,20]
    31Carthamidin-7-O-β-D-glucuronideC21H20O12S. baicalensis[22]
    32ChrysinC15H10O4S. baicalensis, S. amoena,
    S. scordifolia
    [13,2124]
    33Chrysin-7-O-β-D-glucuronideC21H20O9S. baicalensis, S. amoena[23,29]
    34DihydrobaicalinC21H20O11S. baicalensis[22,28]
    35Dihydrooroxylin AC21H20O11S. baicalensis[13]
    36GenkwaninC16H12O5S. baicalensis[16]
    37IsocarthamidinC15H10O6S. baicalensis[20,28]
    38Isocarthamidin-7-O-β-D-glucuronideC21H20O12S. baicalensis[28]
    39IsoschaftsideC26H28O14S. baicalensis[13,16]
    40IsoscutellareinC15H10O6S. baicalensis[21,30]
    41Isoscutellarein 8-O-β-D-glucuronideC21H18O12S. baicalensis[21]
    42Kaempferol 3-O-β-D-glucopyranosideC21H20O11S. baicalensis[13,28]
    43LuteolinC15H10O6S. baicalensis[22,30]
    44Norwogonin-7-O-glucuronideC21H18O12S. baicalensis, S. amoena[13,17,23]
    45Oroxylin AC16H12O5S. baicalensis, S. amoena[17,23]
    46Oroxylin A-7-O-D-glucopyransideC22H22O10S. baicalensis[22,28]
    47Oroxylin A-7-O-β-D-glucuronideC22H20O11S. baicalensis, S. amoena[13,23]
    48PinocembrinC16H12O5S. baicalensis[13]
    49Pinocembrin-7-O-glucuronideC21H20O11S. baicalensis[13,16,22]
    50SalvigeninC18H16O6S. baicalensis[21]
    51ScutellareinC15H10O6S. baicalensis[28]
    52ScutellarinC21H18O12S. baicalensis, S. amoena,
    S. scordifolia, S. viscidula
    [2325,29]
    53WogoninC16H12O5S. baicalensis, S. amoena,
    S. scordifolia, S. viscidula
    [2125]
    54WogonosideC22H20O11S. baicalensis, S. viscidula[13,25]
     | Show Table
    DownLoad: CSV
    Figure 2.  Chemical structure of flavonoids in HQT.

    In 1976, Takido et al.[20] isolated two flavanone derivatives, carthamidin and isocarthamidin, for the first time as natural products from the leaves of S. baicalensis. Later, Yukinori et al.[21] identified two new flavanones, (2S)-5,7,8,4'-tetrahydroxyflavanone 7-O-β-D-glucuronopyranoside and (2S)-5,6,7,4'-tetrahydroxyflavanone 7-O-β-D-glucuronopyranoside), in the leaves of S. baicalensis. Eight compounds of chrysin, wogonin, apigenin, salvigenin, scutellarein, isoscutellarein, apigenin 7-O-glucuronide, and isoscutellarein 8-O-glucuronide were also isolated. Wang et al.[15] used column chromatography to isolate seven flavonoids (wogonin, chrysin, 5,6,7-trihydroxy-4'-methoxyflavone, carthamindin, isocarthamidin, scutellarein, and chrysin 7-O-β-D-glucuronide) from a water extract of the leaves of S. baicalensis. Liu et al.[13] identified 21 flavonoids in the stems and leaves of S. baicalensis by HPLC-UV/MS and NMR, and found one flavonone (5,6,7,3',4'-Pentahydroxyflavone-7-O-glucuronide) was a new compound. Zhao[17] firstly isolated 5,6,7,4′-tetrahydroxyflavanone 7,5,7-dihydroxy-6-methoxyflavanone, oroxylin A, 5,4′-dihydroxy-6,7,8,3′-tetramethoxyflavone, 5,2′-dihydroxy-6,7,8,3′-tetramethoxyflavone, 5,2′-dihydroxy-7,8,6′-trimethoxyflavone, 5,2′-dihydroxy-7,8-dimethoxyflavone, 5,2′-dihydroxy-6,7,8-trimethoxyflavone, apigenin 4'-β-D-glucopyranoside, and apigenin-7-β-D-glucopyranoside from the aerial parts of S. baicalensis. Ma[22] firstly isolated 5,7,4'-trihydroxy-6-methoxyflavone, 5,4'-dihydroxy-6,7,3',5'-tetramethoxyflavone, from stems and leaves of S. baicalensis. Wang et al.[16] isolated 5,4'-dihydroxy-7-methoxyflavanone, genkwanin, 7-methoxychrysin, 3',4',5,5',7-pentamethoxyflavone from 60% ethanol extracts for stems and leaves of S. baicalensis for the first time. Also, the compounds of carthamidin-7-O-β-D-glucuronide, oroxylin A-7-O-β-D--glucuronide, and chrysin were isolated from this plant for the first time.

    The concentration of these chemical components in HQT varies depending on the plant part utilized. Employing the HPLC-DAD method, Shen et al.[18] established that the aerial parts (stems, leaves, and flowers) of S. baicalensis are rich in flavonoids, resembling the roots in composition but exhibiting significant disparities in content. The contents of isocarthamidin-7-O-β-D-glucuronide (106.66 ± 22.68 mg/g), carthamidin-7-O-β-D-glucuronide (19.82 ± 11.17 mg/g), and isoscutellarein-8-O-β-D-glucuronide (3.10 ±1.73 mg/g) were the highest in leaves. The content of apigenin-7-O-β-D-glucopyranoside (18.1 ± 4.85 mg/g) and chrysin-7-O-β-D-glucuronide (9.82 ± 5.51 mg/g) were the highest in flowers. HQT has a high content proportion of flavone glycosides, which is closely related to the activity of HQT. The concentrations of the nine main flavonoids in HQT infusions were measured using HPLC. The content of isocarthamidin-7-O-β-D-glucuronide (52.19 ± 29.81 mg/g) was the highest; carthamidin-7-O-β-D-glucuronide (31.48 ± 6.82 mg/g), chrysin-7-O-β-D-glucuronide (10.65 ± 0.40 mg/g) and apigenin-7-O-β-D-glucopyranside (5.39 ± 0.92 mg/g) were found at moderate levels in HQT samples. As for flavone aglycones, scutellarin (12.77 ± 1.14 mg/g), baicalin (1.88 ± 0.48 mg/g), isoscutellarein-8-O-β-D-glucuronide (2.84 ± 0.60 mg/g), wogonoside (0.23 ± 0.02 mg/g) and chrysin (0.03 ± 0.01 mg/g) has lower content in HQT[6].

    Although there are few studies on the chemical constituents of the aerial parts of S. amoena, S. scordifolia, and S. viscidula it has been shown that the compounds of the aerial parts are similar to S. baicalensis. The aerial parts of S. amoena contain the compounds of baicalein, baicalin, oroxylin A, oroxylin A-7-O-β-D-glucuronide, wogonin, chrysin, chrysin-7-O-β-D-glucuronide, norwogonin, 5,7-dihydroxy-6,8-dimethoxyflavone, scutellarin[23]. Zhang et al.[24] identified compounds of chrysin, wogonin, baicalein, apigenin, apigenin-7-O-β-D-glucoside, baicalin, and scutellarin in whole plants of S. scordifolia. The stems and leaves of S. viscidula all contain compounds of wogonoside, apigenin, baicalein, wogonin, baicalin, and scutellarin. The contents of baicalein, wogonoside, wogonin, and apigenin in the stem of S. viscidula were higher than those in the stem of S. baicalensis. In the leaves of the two species, the content of scutellarin was higher, while the content of other compounds was lower[25]. The content of scutellarin in S. viscidula was stem (2.30%) > leaf (1.78%) > flowers (0.38%)[26].

    The aerial parts of S. baicalensis are rich in essential oils, and the taste of HQT is closely related to this. The flowers of S. baicalensis are thought to have a Concord grape aroma, while HQT has a bitter flavor with distinctive herbal notes. Extensive analysis has identified 145 components in the essential oil obtained from the aerial parts of S. baicalensis. These components span various chemical classes, such as alkanes, carboxylic acids, fatty acids, monoterpenes/oxygenated monoterpenes, sesquiterpenes triterpenoids and Vitamins (Supplemental Table S1), which have demonstrated their efficacy in combatting bacteria, reducing inflammation and inhibiting tumor growth[2731]. Among these, major constituents include germacrene D (5.4%−39.3%), β-caryophyllene (29.0%), caryophyllene (18.9%), eugenol (18.4%), caryophyllene (15.2%), caryophyllene oxide (13.9%), (E)-β-caryophyllene (11.6%), 5-en-3-stigmasterol (11.3%), carvacrol (9.3%), thymol (7.5%), vitamin E (7.4%), neophytadiene (7.3%), γ-elemene (6.2%), 1-octen-3-ol (6.1%), allyl alcohol (5.5%), bicyclogermacrene (4.8%), myristicin (4.7%), acetophenone (4.6%), α-amyrin (4.6%), β-amyrin (4.4%), germacrene d-4-ol (4.3%), spathulenol (4.2%), β-pinene (4.1%), α-humulen (4.0%), 1-vinyl-1-methyl-2-(1-methylvinyl)-4-(1-methylethylidene)-cyclohexane (4.0%) are found in the aerial parts of S. baicalensis from different places[2731].

    Takeoka et al.[27] identified 64 components in volatile components of S. baicalensis flowers by solid-phase microextraction and analyzed them by GC and GC-MS. These flowers were collected at San Francisco State University (USA). Among the flower volatiles, the content of β-caryophyllene, germacrene D, δ-cadinene, γ-muurolene, and γ-cadinene were more than 3%. The essential oil obtained from the stem of S. baicalensis is mainly composed of diphenylamine, 2,2-methylenebis (6-tert-butyl-4-methylphenol), bornyl acetate, β-caryophyllene, germacrene D and 1-octen-3-ol.[32]. Gong et al.[28] analyzed and identified the specific chemical constituents of the aerial parts of S. baicalensis by using GC-MS technology and identified 37 compounds in total, such as allyl alcohol, acetophenone, caryophyllene, α-humulene, germacrene D, and γ-elemene. The plant material was collected in the Qinling Mountains in China. Lu et al.[29] found a big difference in essential oil components between the aerial and root of S. baicalensis from Kunming Botanical Garden, Yunnan Province (China). The aerial part of S. baicalensis mainly contained enols and sterols such as neophytadiene and vitamin E. However, it has the same compounds as the roots, such as nerolidol, hexadecanoic acid, 1,2-benzenedicarboxylic acid, squalene, stigmast-4-en-3-one, and partial alkanes. Recently, Wang et al.[31] found the essential oil level of the aerial parts of S. baicalensis was 0.09% (v/w, based on fresh weight) while its density was 0.93 g/mL, and obtained 31 components accounting for 97.64% of the crude essential oil, including sesquiterpenoid, monoterpenoids, phenylpropanoids, and others. It is also reported that the major components of the essential oil from the aerial parts of S. baicalensis were myristicin, eugenol, caryophyllene, caryophyllene oxide, germacrene D, spathulenol, and β-pinene, with eugenol as the most abundant. The sample of the aerial parts were harvested from Tangshan City (China). The composition of S. baicalensis essential oils varies according to the plant part used, geographical location, and growing conditions.

    Zgórka & Hajnos[33] identified the phenolic acid compounds of aerial parts of S. baicalensis by solid-phase extraction and high-speed countercurrent chromatography: p-coumaric acid, ferulic acid, p-hydroxybenzoic acid, and caffeic acid. Chirikova & Olennikov[34] found that the aerial part of S. baicalensis contains 11 kinds of saturated fatty acids and nine kinds of unsaturated fatty acids, among which the palmitic acid content is the highest. Chlorogenic acid, fernlic acid, protocatechuic acid, vanillic acid, rosmarinic acid, caffeic acid, p-hydroxybenzoic acid, and p-coumaric acid were also detected.

    Zhao [17] isolated four sterol compounds: β-sitosterol-3-O-β-D-glucoside, α-apinasterol, β-sitosterol, and four ester compounds: methoxyphaeophorbide, p-hydroxybenethyl ethanol hexadecanoic methyl ester, ethoxyphaeophorbide, and n-octadecanol, lutein from the aerial parts of S. baicalensis.

    It is reported that flavonoids and diterpenes are the two main groups of active constituents in the genus Scutellaria. However, only one diterpene (scutebaicalin) was identified in the stems and leaves of S. baicalensis[35].

    By atomic absorption spectrophotometry, Yuan et al.[36] determined the contents of 11 metal elements in different parts of S. baicalensis. It was found that the leaves and stems of S. baicalensis were rich in Mg, K, Cr, Ni, Co, Fe, Mn, and Pb. Meanwhile, Yan et al.[37] developed an inductively coupled plasma mass spectrometry method and determined 23 kinds of inorganic elements in the stems and leaves of S. baicalensis from eight regions. Although there were no differences in the types of inorganic elements in the stems and leaves of S. baicalensis from the different areas, the content of these elements varied significantly. Among these elements, Fe, Zn, Cu, Mn, Cr, Co, Ni, Sr, B, and Ni were essential human body elements. The content of Al (516.83 μg/g) and Fe (700.62 μg/g) was the highest, while the content of B (31.54 μg/g), Ti (23.10 μg/g), Mn (65.64 μg/g), Sr (62.27 μg/g), and Ba (89.68 μg/g) was relatively high.

    Olennikov et al.[38] studied the water-soluble polysaccharides from the aerial parts of S. baicalensis from Russia and found that the polysaccharides from S. baicalensis gradually accumulated before flowering and progressively decreased after flowering.

    Yan et al.[39] found that the stems and leaves of S. baicalensis were rich in amino acids, and there was no difference in the kinds of amino acids among different producing areas, but there was a significant difference in the contents of amino acids. The content of proline, threonine, glutamic acid, lysine, glutamine, and arginine was higher, and the content of methionine, hydroxyproline, and citrulline was low.

    Several studies have focused on the functional properties of HQT, with increasing attention given to the aerial parts of S. baicalensis as the main raw material for HQT production. S. baicalensis stems and leaves flavonoids (SSF) are considered the functional components of HQT. Modern pharmacology has shown that the flavonoids extracted from the stem and leaf of S. baicalensis have been found to possess anti-inflammatory, anti-bacterial, antiviral, antipyretic and analgesic, anti-tumor, hepatoprotective, antioxidant, hypoglycemic, hypolipidemic, detoxification, myocardial ischemia protection, brain injury protection, and immunomodulatory effects. However, few studies have been conducted on the individual flavonoid compounds in the total flavonoid extract from S. baicalensis. Therefore, this paper aims to summarize and supplement the current research on the functional properties of HQT and its primary raw material (S. baicalensis) extract.

    Injury and infection could lead to inflammation, which plays a key role in the accelerated pathogenesis of immune-mediated disease[40]. Tong et al.[41], Zhou et al.[42], and Zhao et al.[43] found that S. baicalensis stem-leaf total flavonoid (SSTF) could inhibit acute exudative inflammation caused by xylene, glacial acetic acid, and egg white and also have a significant inhibitory effect on chronic inflammatory of granulation tissue hyperplasia. Wang et al.[44] observed the effect of SSTF on the aerocyst synovitis of the rat model and found that it could reduce capillary permeability, reduce the aggregation of neutrophils and basophils in tissues, reduce histamine, bradykinin, and other substances that increase vascular permeability, which is conducive to the recovery of vascular permeability in inflammation. Studies have shown that the SSTF significantly inhibits specific and non-specific inflammatory responses and can regulate the body's cellular and humoral immune functions. The mechanism of action is closely related to the effective reduction of capillary permeability, inhibition of PGE2 and NO synthesis in vivo, reduction of TNF-α expression, and reduction of inflammatory exudation[45,46]. SSTF (200 mg/kg) could balance the CD+4 Tlymphocyte subsets Th1/Th2 cells and the related cytokines IL-10 and IFN-γ in the rheumatoid arthritis model[47]. SSTF (17.5, 35 and 70 mg/kg for 38 d) could significantly improve the impairment of relearning ability and retention ability on memory impairment and nerve inflammation in chronic cerebral ischemia rats, which might be due to the inhibition of the proliferation of astrocyte and balanced the expression of the inflammatory factors in the brain[48]. Besides, the extract of S. baicalensis stem-leaf shows anti-inflammation effects both in vitro and in vivo. In cultured macrophage cells (RAW 264.7), the extract of S. baicalensis stem-leaf showed a strong anti-inflammation effect, which inhibited the expression of IL-1β. Similarly, it suppressed the LPS-induced transcriptional activity mediated by NF-κB in fish aquaculture[49]. S. baicalensis stems and leaves (3, 6, and 12g /kg, gavage for 7 d, once per day) have anti-inflammatory effects on 2% carrageenan-induced acute pleurisy in rats, and the mechanism of action may be related to the reduction of the production of inflammatory factors and the down-regulation of TRPV1 signaling protein[50]. The combination of S. baicalensis stems- Polygonum cuspidatum (3.5, 7, and 14 g/kg, gavage for 7 d) has a protective effect on lipopolysaccharide-induced acute lung injury rats, and its mechanism may be related to down-regulating the expression of TRPV1 and inhibiting the levels of TNF-α and IL-1β in inflammatory cells[50]. It is also reported that S. baicalensis stem-leaf combined with Morus alba (4, 8, and 16 g/kg/d, gavage, 10 d) has a protective effect on rats with acute pneumonia induced by lipopolysaccharide, and the mechanism might be related to the reduction of inflammatory factors and the down-regulation of TRPV1 signaling pathway[51]. Besides, the network pharmacology showed that S. baicalensis stem-leaf could prevent and control COVID-19 by intervening in 30 targets and 127 pathways, potentially preventing and treating inflammation caused by COVID-19[52].

    Based on the studies, S. baicalensis stem-leaf extract shows promising anti-inflammatory properties. These effects are likely mediated through a combination of factors, including the modulation of immune responses, reduction of inflammatory mediators, and potential interactions with signaling pathways like TRPV1. However, it's important to note that while these studies provide valuable insights, further research, including clinical trials, is needed to establish the full extent of its benefits and its potential for therapeutic applications in humans.

    Scutellaria baicalensis stem and leaf aqueous extract exhibit different degrees of inhibition of 36 strains from 13 kinds of bacteria, such as Staphylococcus aureus, Staphylococcus, Streptococcus pneumoniae, alpha-hemolytic streptococcus, beta-hemolytic streptococcus and Escherichia coli. This shows that anti-bacterial activity against Staphylococcus aureus is strong (MIC50 0.94 g/L, MBC 0.94 g/L). In vivo (217 mg/kg), it protects against the death of mice infected by Staphylococcus aureus and shows a certain dose-dependence[53]. Zhang et al.[54] found that the stem and leaves of S. baicalensis against Staphylococcus aureus and Shigella dysenteriae with MIC values of 1 and 4 mg/mL, respectively. Besides, it is reported that the water extract of the aerial part of S. baicalensis could inhibit the growth of several common pathogenic bacteria in aquacultures, such as Aeromonas hydrophila, Edwardsiella tarda, Vibrio alginolyticus and V. harveyi[49].

    Zhao et al.[55] found that the active part of the stem and leaf of S. baicalensis could inhibit the cytopathic effect caused by 10 kinds of viruses such as Coxsackie B virus, influenza virus, parainfluenza virus, adenovirus, respiratory syncytial virus, and herpes simplex virus. It is suggested that the active parts of the stem and leaf of S. baicalensis can be used for the prevention and treatment of influenza virus, parainfluenza virus, coxsackievirus, and other related infectious diseases.

    These findings indicate that S. baicalensis stem and leaf extract possess anti-bacterial and antiviral properties, making it a potentially valuable natural resource for combating infections caused by various pathogens. However, while these results are promising, further research, including clinical trials, would be necessary to fully establish the effectiveness and safety of using S. baicalensis extract for preventing or treating bacterial and viral infections, including COVID-19.

    In a series of studies, Tong et al.[41] demonstrated that SSTF at a dosage of 20 mg/kg significantly reduced body temperature in rats with fever induced by subcutaneous injection of a 10% dry yeast suspension. Zhang et al.[56] conducted research on the antipyretic effect of scutellarin, an extract from S. baicalensis stems and leaves, in febrile rabbits and observed an antipyretic substantial impact induced by pyrogen. Yang et al.[57] conducted several animal experiments, where they discovered that intraperitoneal injection of effective doses of SSTF (42.2 and 84.4 mg/kg) exhibited a specific inhibitory effect on infectious fever in experimental animals. Moreover, intraperitoneal injection of appropriate SSTF doses (30.1, 60.3, and 120.6 mg/kg), as found in Yang et al.'s experiments[58], effectively inhibited the pain response in experimental animals. Furthermore, Zhao et al.[59] noted that SSTF exhibited a certain inhibitory effect on the pain response in experimental animals subjected to chemical and thermal stimulation.

    These findings suggest that S. baicalensis stem and leaf extract may have antipyretic and analgesic effects, particularly its total flavonoid component. These effects could be beneficial for managing fever and providing pain relief. However, as with any natural remedy, further research, including controlled clinical trials in humans, is necessary to fully understand the effectiveness, safety, and optimal dosing of S. baicalensis extract for these purposes.

    Amyloid protein (Aβ) has been widely recognized as the initiator of Alzheimer's disease (AD)[60]. The SSTF can improve cognitive function and delay the process of dementia. SSTF has been found to exert neuroprotective effects in AD animal models through various mechanisms. Ye et al.[61] demonstrated that oral administration of SSTF (50 mg/kg) could effectively improve cognitive function and reduce neuronal injury in Aβ25-35-3s -induced memory deficit rats. The underlying mechanisms may involve inhibiting oxidative stress and decreasing gliosis[62,63]. Furthermore, SSTF was shown to reduce Aβ-induced neuronal apoptosis by regulating apoptosis-related proteins Bax and Bcl-2[64]. Subsequent studies further validated the neuroprotective effects of SSTF. Cheng et al.[65] found that SSTF treatment inhibited neuronal apoptosis and modulated mitochondrial apoptosis pathway in composited Aβ rats. Ding & Shang[66] found that the SSF improves neuroprotection and memory impairment in rats due to its inhibition of hyperphosphorylation of multilocus Tau protein in rat brains. SSTF has also been found to exert neurogenesis-promoting effects by regulating BDNF-ERK-CREB signaling[12] and activating the PI3K-AKT-CREB pathway[67].

    In further studies, Ding et al.[11] proposed that the effect of SSF on promoting neurogenesis and improving memory impairment may be related to the regulation of abnormal expression of Grb2, SOS1, Ras, ERK, and BDNF molecules in the BDNF-ERK-CREB signaling pathway. Zhang et al.[68] found that SSF (25, 50, and 100 mg/kg) could significantly modulate okadaic-induced neuronal damage in rats, which provides a basis for evaluating SSF as a means to reduce tau hyperphosphorylation and Aβ expression in Alzheimer's disease. Cao et al.[69] found that SSTF (100 mg/kg, 60 d) may alleviate tau hyperphosphorylation-induced neurotoxicity by coordinating the activity of kinases and phosphatase after a stroke in a vascular dementia rat model. Gao et al.[70] demonstrated that the stems and leaves of S. baicalensis (SSF, 25, 50, and 100 mg/kg/d, 43 d) could inhibit the hyperphosphorylation of tau in rats' cerebral cortex and hippocampus induced by microinjection of okadaic acid, which may be related to the activities of protein kinase CDK5, PKA and GSK3β. Furthermore, Liu et al.[67] demonstrated that SSF (35, 70, and 140 mg/kg/d, 43 d) improved composited Aβ-induced memory impairment and neurogenesis disorder in rats through activated the PI3K-AKT-CREB signaling pathway and up-regulated the mRNA and protein expression of TRKB, PI3K, AKT, CREB and IGF2. More recently, a new study demonstrated that SSF (35, 70, and 140 mg/kg) alleviated myelin sheath degeneration in composited Aβ rats, potentially modulating sphingomyelin metabolism[71]. Collectively, these findings suggest that SSTF holds therapeutic potential for AD by targeting multiple Alzheimer's pathogenesis-related processes.

    Li et al.[72] confirmed that SSTF (5 mg/kg) could improve the behaviors and the numbers of dopaminergic neurons in the substantia nigra in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's disease in mice, and these beneficial activities appear to be associated with the reduction of the level of serum malondialdehyde.

    These studies suggest that S. baicalensis stem and leaf extract have the potential to exert neuroprotective effects in various neurodegenerative conditions, including Alzheimer's disease and Parkinson's disease. These effects could be attributed to its ability to modulate oxidative stress, apoptosis, signaling pathways, and protein hyperphosphorylation. However, as with any potential therapeutic agent, further research is needed to establish the full extent of its benefits, optimal dosages, and mechanisms of action, as well as its potential applications in human patients.

    In recent years, ischemic cerebrovascular disease has seriously threatened human health. Cerebral ischemia is one of the leading causes of death. It can occur in focal or global ischemia, with most cases associated with ischemic stroke[73]. Neuronal protection against oxidative damage has been proposed as a potential therapeutic strategy to avoid damage during ischemic stroke[74]. It is reported that SSTF can reduce neuronal apoptosis and free radical damage caused by heart and brain ischemia. Zhao et al.[75] have found that the pretreatment of SSTF (100 mg/kg/d) can protect the ischemia-reperfusion myocardium by enhancing the activity of the anti-oxidative enzyme, inhibiting lipid peroxidation and attenuating the oxygen-free radicals-mediated damage to the myocardium in rats. In further studies, Zhao et al.[76] proposed that SSTF (50, 100, or 200 mg/kg/d, 7 d) pretreatment could alleviate the neuronal damage incurred by ischemia-reperfusion, demonstrating a neuroprotective effect in focal ischemia-reperfusion rat model, which may involve the prohibition of the apoptosis of the neurons. Yu et al.[77] confirmed that SSTF (17.5, 35, and 70 mg/kg/d, 7 d) could attenuate cardiomyocyte apoptosis during ischemia reperfusion injury by down-regulating the protein expression of the JAK2 gene. Qin et al.[78] found that SSF (17.5, 35, and 70 mg/kg/d, 38 d) can decrease the expression of the NMDAR in hippocampus, and increase the expression of VEGF in the cerebral cortex of chronic cerebral ischemia rats.

    Focal cerebral ischemia-reperfusion can result in neuronal loss but strongly promotes activation and proliferation of hippocampal glial cells. Losing hippocampal neurons is considered one of the basic pathological mechanisms of cognitive impairment[79]. Zhao et al.[79] found that the pretreatment with SSTF (100 and 200 mg/kg) could improve neurological function after focal cerebral ischemia-reperfusion injury, with preventive and protective effects. Shang et al. found flavonoids from S. baicalensis (35−140 mg/kg) could attenuate neuron injury and improve learning and memory behavior in rats with cerebral ischemia/reperfusion[80]. In further studies, Kong et al.[81] found that the mechanisms of the protective effects on the brain against cerebral ischemia/reperfusion injury of SSTF may involve decreasing the content of brain water, increasing microvascular recanalization, reducing the apoptosis of hippocampal neurons, and attenuating free radical damage. Bai et al.[82] proposed that SSTF (100 mg/kg/d, 7 d) could protect the neurological function in rats following I/R injury by alleviating the damage to the ultrastructure of cerebral cortex neurons and synapse. Yan et al.[83] found that SSTF (100 mg/kg/d, 7 d) pretreatment can exert preventive, protective effects on cerebral tissue by relieving brain edema, decreasing neural damage, promoting microvascular repatency, and increasing enzyme activity. It has been reported that the SSTF may protect neurons and their synaptic structures in multiple ways, but whether this mechanism enhances the resistance of neurons to damage or increases the repair function remains to be further explored.

    Essential hypertension is a common chronic cardiovascular disease, which can lead to multiple target organ damage, such as heart, brain, and blood vessels. It is a risk factor for coronary heart disease, heart failure, and other cardiovascular diseases. It is reported that SSTF (17.5, 35.0, and 70 0 mg/kg, 8 weeks) can inhibit myocardial remodeling in primary hypertensive rats, and the medium dose exerts the best inhibitory effect, and the mechanism may be related to inflammatory response induced by inhibiting the NF-κB signaling pathway[84].

    S. baicalensis stem and leaf extract have the potential to provide neuroprotective effects in conditions related to ischemic cerebrovascular disease and hypertension. Its ability to modulate oxidative stress, inflammation, and apoptotic pathways appears to contribute to its beneficial effects. However, further research is needed to fully understand the mechanisms and optimal usage of SSTF for these therapeutic purposes.

    Aging is associated with the deterioration of physiological function and the decline of cognitive ability[85]. It is reported that the alcohol extracts from roots, stems, leaves, and flowers of S. baicalensis (400 mg/kg, 7 weeks) could regulate the content of differential metabolites in urine samples of D-gal-induced aging-model rats to different degrees and play a certain role in improving the metabolic disorders of aging rats[7]. A further study investigated the anti-aging effects and potential mechanisms of S. baicalensis leaves and flower extract. S. baicalensis leaves (400 and 800 mg/kg, 7 weeks) have an anti-aging effect, which can improve the acquired alopecia, slow response, and other characteristics of aging rats, increase the spontaneous activity of aging rats, and reduce the damage of lipid peroxidation and glycosylation induced by D-galactose[86]. The S. baicalensis flowers extract (400 and 800 mg/kg, 7 weeks) could effectively reverse the cognitive decline and oxidative stress injury and alleviate liver pathological abnormalities in the D -galactose-induced aging rats, which are involved in the glutamine-glutamate metabolic pathway[85].

    S. baicalensis extracts, particularly those from leaves and flowers, may have anti-aging properties by regulating metabolic disorders, improving cognitive function, reducing oxidative stress, and alleviating aging-related physiological abnormalities. However, further research is necessary to fully understand the mechanisms underlying these effects and to determine the potential of these extracts for human applications in addressing age-related issues.

    The SSTF (200 mg/kg d, 35 d) can reduce the joint damage of collagen-induced arthritis mice and balance the CD+4 T lymphocyte subsets Th17 and Treg cells[87]. Besides, on the multiple sclerosis model, the SSTF (100, 200 mg/kg d, 16 d) displayed a protective effect on experimental autoimmune encephalomyelitis rats through a balance of the CD+4 Tlymphocyte subsets Th17 and Treg cells[88]. Zhang et al.[89] have found that SSTF attenuated EAE disease severity, accompanied by enhanced Treg frequency and level of Treg-associated cytokines (IL-10 and TGF-β), as well as downregulated Th17 frequency and expression of Th17-related cytokines (IL-17 and IL-23).

    It is reported that the essential oils from the aerial parts of S. baicalensis showed toxicity against booklice (Liposcelis bostrychophila) with an LC50 of 141.37 μg/cm2. The components of myristate, caryophyllene, eugenol, and caryophyllene oxide displayed dramatic toxicity against the L. bostrychophila, with LC50 values of 290.34, 104.32, 85.75, and 21.13 μg/cm2, respectively[31].

    Guo & Xu[90] have found that SSTF could inhibit the proliferation of Hela cells obviously (p< 0.01). Tang et al.[10] have found that SSTF could play an anti-colon cancer role by up-regulated the expressions of Cleaved Caspase-3 and the ratio of Bax/Bcl-2 (p < 0.05 and 0.01) and significantly down-regulated the expressions of MMP-2 and MMP-9 in HCT116 cells (p < 0.05 and 0.01). Recently, Shen et al.[6] studied the chemopreventive effects of HQT against AOM-induced preneoplastic colonic aberrant crypt foci in rats and found HQT inhibits AOM-induced aberrant crypt foci formation by modulating the gut microbiota composition, inhibiting inflammation and improving metabolomic disorders.

    Cardiovascular disease is one of the most important threats to human health. Hyperlipidemia is a major risk factor for atherosclerosis, which can cause various cardiovascular and cerebrovascular diseases. Different doses of SSTF (50, 100, and 200 mg/kg) can effectively reduce the body weight increase of rats with hypertriglyceridemia, reduce the serum levels of triglyceride(TG), total cholesterol(TC), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C), which indicate that SSTF has the effect of regulating blood lipid[91].

    Oxidative stress is important in developing tissue damage in several human diseases[92]. The antioxidant capacities of separated organs (flower, leaf, stem, and root) of S. baicalensis were conducted by DPPH, ABTS+, and RP methods, respectively. The results showed that the antioxidant activity of the root (66.9 ± 0.3, 121.6 ± 0.5, and 80.2 ± 0.4 μg/mL) was the highest, followed by the leaf (68.4 ± 1.3, 128.2 ± 2.1, and 135.8 ± 2.0 μg/mL), stem (127.8 ± 3.1, 199.2 ± 1.7, and 208.2 ± 8.3 μg/mL) and flower (129.8 ± 6.3, 285.7 ± 4.7, and 380.3 ± 14.2 μg/mL)[93]. The antioxidant activities of the extracts of the aerial part of S. baicalensis and separated organs were conducted by DPPH assay and reducing power method with the antioxidant ability 7.73–8.83 mg TE/g DW and 51.48–306.09 mg TE/g DW, respectively. The content of total polyphenols and total flavonoids were significantly positively correlated with the reducing power[94]. Liu et al.[95] found that the flavonoids extracted from the stems and leaves of S. baicalensis (SSF, 18.98, 37.36, and 75.92 μg/mL) could protect rat cortical neurons against H2O2-induced oxidative injury in a dose-dependent manner. Cao et al. found that SSTF could alleviate the damage of human umbilical vascular endothelial cells injured by H2O2 and reduce their apoptosis, which may be related to the increasing level of Bcl-2[96].

    Preventive treatment with SSTF (50, 100, and 200 mg/kg) could significantly inhibit the blood glucose increase induced by alloxan in mice, and SSTF treatment could reduce the blood glucose level in diabetic mice. Both the prevention group and the treatment group could increase the activity of serum superoxide dismutase and decrease the content of malondialdehyde[97]. Liu et al.[98] found that SSTF (75 and 150 mg/kg) could significantly reduce blood glucose and blood lipid and improve insulin resistance in type 2 diabetic rats with hyperlipidemia.

    Yang et al.[99] found that SSTF (35 mg/kg/d, 8 weeks) could resist hepatic fibrosis by inhibiting the expression of α-smooth muscle actin in Hepatic Stellate Cells. In vivo, it is reported that SSTF (50, 100, and 200 mg/kg) could significantly reduce alanine transaminase activities in serum, increase the expression of superoxide dismutase and reduce the content of malondialdehyde in acute hepatic injury mice induced by carbon tetrachloride and ethanol[100].

    These findings suggest that S. baicalensis stem and leaf extract hold promise in promoting various aspects of health, including immune modulation, antioxidant activity, anti-tumor effects, cardiovascular health, oxidative stress protection, and more. However, further research, including clinical studies, is necessary to better understand the full therapeutic potential and safety of these effects in human applications.

    Flavonoids are considered the main active components in HQT. As the active substance basis of HQT, the safety of SSTF has also been investigated. After 90 d of oral administration of SSTF (0.5, 1, and 2 g/kg) to rats, no abnormal changes were observed in all indexes, and no delayed toxic reactions or obvious toxic reactions were observed, indicating that the toxicity of SSTF is low[101]. The LD50 value of SSTF was 14.87 g/kg is equivalent to 68.5 times the maximum dose in the pharmacodynamic test of mice, and the experiment confirms the safety of oral administration of the SSTF. Intraperitoneal injection of SST showed certain toxicity in mice, with an LD50 value of 732.11 mg/kg[102]. Liu et al.[103] conducted a systematic safety assessment experiment on the aqueous extract of S. baicalensis stem and leaves based on the China National Standard 'Guidelines for the Safety Evaluation of Food Toxicology (GB15193-2014)'. The results indicated that S. baicalensis stem and leaves are non-toxic, non-teratogenic, and non-mutagenic. Acute toxicity tests in mice revealed a Maximum Tolerated Dose of 15.0 g/kg. A 90-d feeding trial showed no changes in toxicological damage in animals, even at a high dosage of 8.333 g/kg (equivalent to 100 times the recommended human daily intake), suggesting the safety and non-toxicity of consuming S. baicalensis stem and leaves. In addition, HQT has long been used in folklore, and no toxicity has been reported.

    These findings collectively indicate that HQT is generally safe for consumption. However, as with any herbal product, it's important to follow recommended dosages and consult healthcare professionals, especially for individuals with pre-existing health conditions or medications.

    In recent years, HQT, a non-Camellia tea with a long history in China, has attracted attention due to its diverse pharmacological activities. Among various Scutellaria species, S. baicalensis is the most extensively studied and cultivated source for HQT. The aerial parts, including flowers, stems, and leaves, serve as the principal source of HQT preparation. HQT are rich in flavonoids and volatile components with various beneficial effects. To date, about 295 compounds have been identified from HQT, including approximately 54 flavonoid compounds and 145 volatile components identified online. The current research on the activity of HQT primarily focuses on flavonoid compounds, with limited studies on the larger quantity of volatile oil compounds.

    Additionally, the processing and brewing techniques used to prepare HQT may influence the bioactivity of its flavonoid content, although few studies have investigated this. More research is necessary to optimize the processing and brewing techniques to maximize the health benefits of HQT. Comparative studies reveal that the aerial parts of S. baicalensis, while sharing similarities with the roots, contain varying flavonoid compositions. Limited research on S. scordifolia, S. amoena, and S. viscidula, indicates the presence of comparable flavonoid compounds in their aerial parts. Although individual flavonoids like baicalin, wogonin, and scutellarin have demonstrated various therapeutic properties, it is essential to consider the synergistic effects of these compounds when consumed together in the form of tea. These findings contribute to laying the groundwork for quality assessment of HQT and offer insights into potential health benefits.

    HQT is mainly derived from the aerial parts of S. baicalensis. Recent studies have increasingly recognized the pharmacological value of the aerial parts of S. baicalensis. Preliminary pharmacological studies have shown that the aerial parts of S. baicalensis may possess beneficial activities in antioxidant, anti-tumor, antiviral, anti-bacterial, protection of ischemia-reperfusion injured neural function, neuroprotective effects against brain injury, and blood lipid regulation. These findings suggest that the value of using HQT may be attributed to these pharmacological activities. Although HQT is generally safe for consumption, further investigation is required to understand its safety profile, particularly in special populations such as pregnant or lactating women, children, and individuals with pre-existing medical conditions. Additionally, potential interactions between the flavonoids in HQT and conventional medications must be examined to ensure their safe and effective use with pharmaceutical treatments. In addition, although the safe dose of HQT on rodents has been studied, the safe dose for humans has yet to be determined.

    In conclusion, these initial research results support the potential health benefits of HQT and encourage more in-depth studies on its raw materials. Further studies are necessary to elucidate the synergistic effects of the flavonoids in HQT, optimize the processing and brewing techniques for maximum bioactivity, and investigate the safety profile and potential interactions with conventional medications. A comprehensive understanding of HQT will contribute to developing evidence-based recommendations for promoting health and well-being.

    The authors confirm contribution to the paper as follows: Conceptualization and writing: Quan Y, Li Z, Meng X, Li P, Shen J; Figure and table modification: Quan Y, Li Z, Meng X, Li P; review and editing: Wang Y, He C, Shen J. All authors reviewed the results and approved the final version of the manuscript.

    The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

    This research was funded by the Shandong Provincial Natural Science Foundation, China (ZR2022QH147, ZR2022QH165) and the Innovation Team and Talents Cultivation Program of National Administration of Traditional Chinese Medicine (No. ZYYCXTD-D-202005).

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

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  • Cite this article

    Regmi A, Rueda-Kunz D, Liu H, Trevino J, Kathi S, et al. 2024. Comparing resource use efficiencies in hydroponic and aeroponic production systems. Technology in Horticulture 4: e005 doi: 10.48130/tihort-0024-0002
    Regmi A, Rueda-Kunz D, Liu H, Trevino J, Kathi S, et al. 2024. Comparing resource use efficiencies in hydroponic and aeroponic production systems. Technology in Horticulture 4: e005 doi: 10.48130/tihort-0024-0002

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Comparing resource use efficiencies in hydroponic and aeroponic production systems

Technology in Horticulture  4 Article number: e005  (2024)  |  Cite this article

Abstract: With depleting sources of fresh water, approaches must be taken to reduce the use of water in agricultural systems. Along with reduced usage, research should focus on using resources more efficiently. Hydroponic production systems offer higher resource use efficiency, such as water and fertilizer, compared to traditional agriculture systems. Notably, water usage can be reduced by more than 90% and fertilizer by 60% depending upon the system and crop. This review focuses on water and nutrient use efficiency of different crops in greenhouse production systems to further elucidate the current accomplishments and future needs in this research area. This is important because water and nutrient use efficiency is highly dependent upon multiple factors like type of crop, cultivars, environment, type of system used, nutrient concentration and form, flow rate of water, water depth, location, etc. Herein, nutrient film technique (NFT), deep-water culture, aeroponics, drip, and other systems were compared for different water and nutrient use efficiencies. Because different crops were used in these studies, direct comparison was limited, but we found that crop type and cultivars, NFT channel depth, and fertilization rate were among the most influential factors affecting nutrient use efficiency in hydroponic systems. Surprisingly, water use efficiency in aeroponic systems was greater when more nozzles were used. Aeroponic systems also showed greater water use efficiency when compared to NFT systems. Overall, this review highlights the resource use efficiency of different vegetable crops in hydroponic production systems and highlights opportunities for future research.

    • Water scarcity is one of the important challenges threatening global crop production. According to the World Health Organization (WHO), 55 million people face the consequences of drought each year[1]. Meeting the food production demands over the next 50 years requires implementing improved water and resource management practices. Hence, many researchers are investigating drought-related challenges in crop production and developing techniques to reduce water usage, and increase the water and resource use efficiency[2]. Many new growers are moving away from traditional agriculture systems that require high inputs to alternative growing systems with high resource use efficiency[3]. Moreover, focus has shifted towards circular systems that reduce and conserve inputs and resources without compromising the overall yield[3]. Greenhouse and controlled environment production methods give us an opportunity to regulate and optimize the growing conditions for plants, but these systems still require nutrient and water inputs for production of high-quality crops[4]. So, it is important to prioritize efficient resource management practices that improve yield and quality of crops while minimizing wastage.

      Water and nutrients can be used efficiently in greenhouse conditions as demonstrated by Ayarna et al.[5] and Verdoliva et al.[6]. Some of the more common techniques used in greenhouse production include drip irrigation, hydroponics and aeroponics[2,7]. Drip irrigation methods vary, but in general provide small amounts of water delivered on a regulated schedule to meet water demands of a crop. Hydroponics is defined as the technique of growing plants suspended in a nutrient rich water-based solution without using soil substrates[8] (Fig. 1). Alternatively, aeroponics is defined as growing of plants in air or mist environment without substrates where the plant roots are freely suspended in the air and are misted with nutrient solution periodically[9] (Fig. 2). However, there is no clear consensus on the definition and differences between hydroponics and aeroponics. Some classify vertical towers as a type of aeroponic system, instead of a type of nutrient film technique (NFT) system, even if mist systems are not used. Therefore, specific criteria must be established in research and academia to differentiate between hydroponics and aeroponics so that clearer guidelines and standards can be developed. The authors suggest that, while aeroponics is classified as a subcategory of hydroponics, it should be defined by the use of spray nozzles that spray roots with nutrient solution. Other guidelines may include a minimum or maximum droplet size, define the type of root suspension chamber, or provide criteria that exclude systems from being considered 'aeroponic'. Nonetheless, water and nutrient use efficiency of these systems remains a crucial factor in the success of both systems.

      Figure 1. 

      Illustration of two hydroponic systems. (a) Nutrient film technique. (b) Deep water culture. Figure created by Dario Rueda Kunz using BioRender.com.

      Figure 2. 

      Illustration of one type of aeroponic system with the reservoir outside the root chamber. Figure created by Dario Rueda Kunz using BioRender.com.

      Water use efficiency (WUE) can be defined as the amount of water used by plants or plant systems relative to the amount of biomass produced[10]. For instance, higher water use efficiency indicates that less water is required to achieve a desired level of crop productivity, indicating a more efficient utilization of available water resources. Because water is a critical resource for plant growth, its availability and quality significantly impact crop quality and yield. Water is a limited resource that faces multiple demands, including municipal requirements, population needs, and agricultural usage. Growers in controlled environment agriculture (CEA), particularly in areas that are prone to drought or water restrictions, need sustainable and efficient ways to produce crops for high profit margins[11]. One of the key components of CEA is that the microclimate can be more easily controlled which makes it easier to modify plant needs, crop requirements, and resources used[7]. While WUE has been extensively researched in field systems, there are fewer studies that have explored WUE in CEA systems. These studies have found differing results when comparing different production systems and crops or cultivars. For example, in an experiment conducted by Shtaya & Qubbaj[10], lettuce (Lactuca sativa) grown in the NFT system had higher WUE than plants grown in soil and a mixture of peat moss/perlite. Similarly, cultivar also impacts the WUE as shown by El-Nakhel et al.[12], where WUE was higher in red Salanova lettuce as compared to green Salanova lettuce. Hence it is important to identify the CEA systems and cultivars that have higher WUE.

      In addition to water, nutrients are also required for substrate culture hydroponic systems, such as soilless media (i.e. rockwool or coir) in CEA production because the substrate used contains little or no nutrients. Nutrient use efficiency (NUE) can be defined as the amount of nutrients such as nitrogen, phosphorus, and potassium used by the plants to produce biomass[13]. Most research that has been conducted focuses on nitrogen, as it is one of the major nutrients required by plants in the largest amounts[14]. While other nutrients are required, especially when absent from substrate, few studies on greenhouse crops regarding uptake or use efficiency have been conducted. Different production systems and factors also influence the NUE of crops. For example, cultivars impacted the NUE of tomato (Solanum lycopersicum) where higher NUE was found in Momotaro York as compared to Jaguar cultivars in study conducted by Ayarna et al.[5]. Many factors are known to affect the WUE and NUE in crops which include growing conditions and season, type of production systems used, and crop types. This further illustrates the necessity for more CEA research regarding WUE and NUE.

      Ultimately, CEA has emerged as a sustainable and efficient method for growing crops in controlled and artificial environments. It is comprised of multiple technologies and advancements which enable the grower to optimize growing conditions and resources along with maximizing the crop yield and quality[4]. However, water and nutrients are critical components in CEA that have potentially high costs, limited availability, or quality in many regions. In this review, we will discuss the current state of knowledge on WUE and NUE in aeroponic and hydroponic systems, their challenges, and opportunities, as well as different strategies and methods for improvement.

    • The articles for this review were collected from two databases: Google Scholar (https://scholar.google.com/) and the university library (Texas Tech University; www.depts.ttu.edu/library) which includes multiple scientific databases such as Agricola, Scopus and Web of Science. The search and screening were conducted following the systematic review guidelines outlined by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The search criteria included relevant keywords and phrases in the title such as: 'water use efficiency', 'WUE', 'nutrient use efficiency', 'NUE', 'nitrogen use efficiency', 'aeroponics', 'hydroponics', 'nutrient film technique', 'NFT', 'deep water', 'raft system' and combinations thereof. After articles were compiled, references of these articles were evaluated to identify additional relevant papers. Research article exclusion criteria were language (other than English), full-text unavailable, CEA articles that did not study water or NUE, and articles with confounding factors that may have influenced the interpretation of this review. The initial search resulted in 98 articles based on relevance of search terms to titles. The authors then independently read the abstracts of the articles and further refined the relevant articles to 36. Then, the authors went through each article to determine if either NUE or WUE was evaluated and found 20 articles to be relevant to this review. Among the included articles, several calculations for WUE and NUE were found. Equations 1−5 (below) show these calculations.

      Equations for WUE used in reviewed articles:

      WUE:YieldSolutionconsumed (1)
      WUE:YieldAppliedirrigation (2)
      WUE:YieldModelledwateruse (3)

      Equations for NUE used in reviewed articles:

      NUE:PlantdrymatterNSupply (4)
      NUE:PlantfreshweightNutrientsolutionused (5)

      The results of the search for individual factors WUE and NUE in different systems were presented in a tabular form. Each table has been individually constructed based on the needs of the given factor.

    • Water use efficiency has been studied extensively in agronomic systems; however, controlled environment system studies have been limited. Only eight separate articles were found that studied vegetable crop WUE in different hydroponic systems (Table 1). Of these, three experiments took place in NFT systems, five were in drip hydroponic systems, two were in deep water culture, and one was in a deep flow technique system (Table 1). The majority of all systems (five of eight studies) grew tomatoes[5,6,1517], and the remaining studied lettuce[10,12], coriander (Coriandrum sativum)[18], and cucumber (Cucumis sativus)[13]. The comparison between studies was limited by the different crop species, but also because of the different units and methods used in each study. While the units used varied by study, most authors reported WUE as g FW/L (FW = fresh weight) or kg/m3 (Table 1), which are interconvertible. After converting to the same units (Table 1), the WUE was averaged across all crops to determine variability amongst crop species in each system. One study could not be converted to g/L because the authors reported the WUE as yield (g) over transpired water[6]. This was due to the method of analysis (i.e., weighing pots), and thus is not directly comparable. NFT and drip systems were used in more than one study on tomatoes within these systems. When taking an average between different systems on tomato, WUE was different between the hydroponic systems and ranged between 24.5 g FW/L for drip system[5,6,15,17], 9.15 g FW/L for deep water culture[6,16] and 33 g FW/L for NFT[15,17]. These findings show that, of the hydroponic systems studied, NFT had higher WUE than other systems and methods on tomato. However, we suspect that these findings may change as more crops and system designs are studied for WUE.

      Table 1.  Studies that evaluated water use efficiency (WUE) in different hydroponic systems.

      SystemLocationCropTreatmentEquationWUEUnitConversion
      (g Fw/L)
      Main findingsRef.
      Drip recirculating hydroponicsKashiwanoha, JapanTomato (Solanum lycopersicum) cv. Momotaro YorkDifferent cultivarsFruit weight/total water uptake per plant0.015kg/kg15WUE was dependent on cultivar.[5]
      Tomato (Solanum lycopersicum) cv. Jaguar0.03333
      NFT systemNaples, ItalyLettuce (Lactuca sativa) cv. Red SalanovaDifferent cultivarsFW/Volume of water consumed92g/L92WUE was dependent on cultivar.[12]
      Lettuce (Lactuca sativa) cv. Green Salnova80.880.8
      Deep flow techniqueBahia, BrazilCoriander (Coriandrum sativum) cv. Tabocas & VerdaoTwo Nutrient solution depths: 0.02 and 0.03 m
      Three recirculation intervals: 0.25, 12 and 24 h
      FW/Total water consumed29.43 @ 15 min intervalg/L29.43Recirculation interval significantly affected WUE, with higher WUE at 15 min intervals.[18]
      25.93 @ 2 cm25.93A 2 cm depth of water had higher WUE.
      Closed loop- NFTBari, ItalyTomato (Solanum lycopersicum) cv. Diana, Jama (Beef steak)
      Naomi, Conchita (Cherry)
      NFTFruit yield/Water supplied31−45
      g/L38NFT had higher WUE.
      Cultivation system and tomato type were significant.
      WUE varied among different seasons.
      [15]
      Open loop- drip fertigationRockwool23−2524
      SoilSoil31−37
      34
      Deep water cultureGrossbeeren, GermanyTomato (Lycopersicon esculentum) Mill. Cv. CounterNutrient form and concentrationDW/water used1−3.5g/kg2.25Nutrient form and concentration affected WUE.[16]
      Drip systemPunjab, IndiaCucumber (Cucumis
      sativus) var. Kafka,
      Multistar & PBRK-4
      Three fertigation levels (100%, 85% and 70%)Irrigation WUE: Yield/Irrigation water appliedIrrigation WUE: 34.5−51.4kg/m342.95Both variety and nutrient solution significantly affected WUE.[13]
      Crop WUE: Yield/Crop Water useCrop WUE: 120.6−179.9150.25
      Closed system-NFTAhvaz, IranTomato (Solanum lycopersicum) cv. V4-22 & AmiraClosed systemFruit yield/cubic meter water appliedWater productivity: 33.7
      Water productivity biomass: 48.91
      kg/m327.91Water productivity was lower in the closed hydroponic system.[17]
      Open system-dripOpen systemWater productivity: 21.84.
      Water productivity biomass: 34.42
      41.88
      Drip systemWales, United KingdomTomato (Solanum lycopersicum) cv. Forticia F1DripFW fruit/L of transpired water0.0072−0.0099kg/L8.55Hydroponic systems had higher WUE than soil.[6]
      Deep water cultureDeep water0.0059−0.01249.15
      soilSoil0.0035−0.00443.95
      NFTTulkarm, Palestine-Lettuce (Lactuca sativa)Four growing methodsDW/kg water applied121kg/m3121WUE was higher in NFT.[10]
      FW – fresh weight, DW – dry weight, NFT – nutrient film technique.

      The literature was very diverse and revealed many factors that could affect WUE results (Table 1). WUE in hydroponic systems can be affected by cultivars[5,12], nutrient solution depth[18], circulation intervals[18], nutrient concentration[16] and fertigation levels[13]. For example, different cultivars of tomatoes showed different WUE in the literature. Ayarna et al., found that the tomato cultivar Jaguar doubled WUE in drip recirculating hydroponics (33 g FW/L), compared to Momotaro York, which had a WUE of 15 g FW/L. Similarly, Red Salanova, a lettuce cultivar, had higher WUE (92 g FW/L) than the Green Salanova cultivar (80 g FW/L)[5]. Furthermore, interactions between cultivar, species, or botanical variety and environment may also affect WUE. Valenzano et al.[15] found that beefsteak tomatoes showed a higher WUE than cherry tomatoes in both winter-spring and autumn-winter growing seasons. So far, these results indicate that physiological characteristics, not only between species, but cultivars will impact crop WUE.

      Crop WUE is also highly dependent upon climate and season, which is rarely consistent between various systems, studies, regions, and individual researchers (Table 1). The five tomato studies that were evaluated in this review were not directly comparable because seasons, locations, and systems differed. Singh et al.[13] studied Kafka, Multistar, and PBRK-4 cucumber cultivars and their interaction with different irrigation levels and found that WUE of Multistar was statistically higher (44.1 kg/m3) than PBRK-4 (40.3 kg/m3). Furthermore, da Silva et al.[18] examined the impact of cultivar, nutrient solution depth, and circulation intervals in NFT on coriander and determined that 0.25 h intervals and 0.02 m solution depth showed a significant influence on WUE at 20 and 25 d after transplant. However, the study did not address variations in WUE across different cultivars. Fertilizer form is also an important factor when considering WUE, because fertilization directly influences yields. Contrary to soil cultivation, in hydroponics, water is the only medium through which plants can absorb applied nutrients. This, in turn, affects fertilizer use, availability, and absorption of nutrients. Claussen[16] found that fertilizer form and rate significantly impacted WUE in tomato where treatments with ammonium had more varying impacts on WUE than nitrate. This further illustrates that WUE is affected by multiple factors that must be considered when evaluating efficiency of hydroponic systems.

    • Nutrient use efficiency is one of the most crucial factors to consider when growing plants in controlled environments. The literature search revealed three papers that studied the NUE (i.e. nitrogen) of crops grown in hydroponic systems. In the literature, most researchers have used NUE interchangeably with nitrogen use efficiency, as they primarily focused on nitrogen as the nutrient of interest. Therefore, this review will focus on nitrogen in NUE. Nitrogen is the mineral nutrient needed in the highest amounts for plant growth and development and makes up the highest amount of mineral nutrients present in plants[19]. As a result, adequate amounts of nutrients must be applied to the plants, but different growing conditions affect the efficiency of applied nutrients to be taken up and converted to plant biomass[20]. Specifically, factors like temperature, plant species and cultivars, water quality, flow rates, type of systems and ratio of nutrients applied have been found to affect the NUE[10, 11]. Three studies were found that determined the NUE in hydroponic production of various crops (Table 2). Of those, one study was conducted in NFT system[14] and two in drip system[5, 13]. The NUE varied among the different studies likely due to different equations for calculating NUE (Table 2, Eqns 1−3). Among the other factors that contribute to differences in NUE are the types of crops studied and the systems used (Table 3). In the NFT hydroponic system, NUE was affected by the flow rate. For example, Baiyin et al.[14], found that the NUE of Swiss Chard (Beta vulgaris L.) ranged from 20−25 g/g DW in NFT when different flow rates of nutrient solution (2 L/min, 4 L/min, 6 L/min and 8 L/min) were used. The Swiss Chard had a higher NUE of approximately 25 g/g DW at 8 L/min flow rate and lower NUE of 21 g/g DW at 2 L/min flow rate. Alternatively, NUE of tomato grown in drip system ranged from 112−221 g/g FW in a drip system in an experiment conducted by Ayarna et al.[5] . In this study, Ayarna et al.[5] compared two tomato cultivars Jaguar and Momotaro York and found that Jaguar had two times higher NUE than Momotaro York at 70 d after transplanting (221.1g/g FW and 111.9 g/g FW, respectively). Furthermore, in a study conducted by Singh et al.[13] on cucumber grown in a drip system, NUE of cultivar Multistar was statistically higher (266.7 g/plant) than PBRK-4 (243.1 g/plant). In the same study, NUE compared among different fertigation levels calculated based on the total nutrients applied ranged from 229−282 g/g FW[13]. However, the use of fresh weight and dry weight, and different equations to calculate NUE in these studies makes the comparison difficult. While we can provide a broad overview and perspective of NUE in different systems, we cannot directly compare the systems. However, in most of the studies, the primary factors affecting the NUE were cultivar, species and fertigation levels[5,13,14]. Additionally, the limited number of studies, variability in crops, systems, and environment limited the comparison between studies and made identification of primary factors that affect NUE in hydroponic systems difficult. Therefore, it is essential that future research include NUE as a component to compare system and plant performance.

      Table 2.  Studies conducted on nutrient use efficiency (NUE) in hydroponic systems.

      SystemLocationCropTreatmentEquationNUEUnitMain FindingsRef.
      NFT (flow of nutrients)Totori, JapanSwiss Chard (Beta vulgaris spp. Cicla)Different flow rates - 2, 4, 6 and 8 L/minDry wt./nutrient uptake of whole plant20–25g/g DWHighest NUE at 8 L/m flow rate[14]
      Drip recirculating hydroponicsKashiwanoha, JapanTomato (Solanum lycopersicum) cv. Momotaro YorkDifferent cultivarsRatio of fruit fresh FW/total nitrogen uptake per plant at first harvest111.9kg/FW kgJaguar had greater NUE than Momotaro York[5]
      Tomato (Solanum lycopersicum) cv. Jaguar221.1
      Drip systemPunjab, IndiaCucumber (Cucumis sativus) var. Kafka, Multistar & PBRK-4Three fertigation levels and three varietiesYield/nutrient applied229.6–281.8g/plantFertigation level affected NUE[13]
      NFT – nutrient film technique.

      Table 3.  Studies conducted on water use efficiency (WUE) in aeroponic systems.

      SystemLocationCropTreatment (T)EquationUnitWUEMain findingsRef.
      Aeroponic towerKerman, IranCucumber
      (Cucumis sativus)
      Different flow rates and spray duration:
      125 mL/min, 10 min
      WUE = (YS )/∑WU (Total water consumed)kg (total yield)/m394.4Applying insufficient or excess water affected WUE more than changing the application duration. The optimum rate was 233.59 mL/min with an application time of 16.06 min.[21]
      125 mL/min, 15 min98.8
      125 mL/min, 20 min98.2
      250 mL/min, 10 min109.2
      250 mL/min, 15 min111.8
      250 mL/min, 20 min105.8
      375 mL/min, 10 min91.1
      375 mL/min, 15 min87.2
      375 mL/min, 20 min85
      Root chambersBenha, EgyptLettuce
      (Lactuca sativa)
      Different flow rates: 0.5 L/hWUE = CY/CWUkg (crop plant yield)/m3 (modelled water uptake)/plant3.87After the 50 d growing period, higher WUE with 0.5 L/h flow rate (3.87 kg/m3). Lowest at 1.5 L/h rate (3.02 kg/m3). A higher flow rate gave longer roots and reduced fresh biomass.[22]
      1 L/h3.47
      1.5 L/h3.02
      Aeroponic towerCairo, EgyptLettuce
      (Lactuca sativa)
      Number of sprinklers:
      1 nozzle Mini sprinkler
      WUE = (total fresh yield, kg)/(total applied
      irrigation water, m3)
      kg fresh yield/m3 water8Adding nozzles resulted in higher WUE in both crops because higher biomass production was achieved.[23]
      2 nozzle Mini sprinkler8.8
      4 nozzle mini sprinkler11.2
      Celery (Apium Graveolens)Number of sprinkler:
      1 nozzle mini sprinkler
      WUE = (total fresh yield, kg)/(total applied
      irrigation water, m3)
      kg fresh yield/ m3 water9.75[23]
      2 nozzle mini sprinkler12.4
      4 nozzle mini sprinkler12.8
      Root chambersKalamata, GreeceOnion
      (Allium cepa)
      Irrigation frequency:
      Daily 1 min ON/3 min OFF
      WUE = g (Bulb FW)/L (Nutrient solution consumed)g/L25.36Aeroponics outperformed an NFT system in WUE, plant biomass, bulb size, and total yield.[24]
      Root chambersChangchun, ChinaLettuce
      (Lactuca sativa L.
      var. ramosa Hort)
      Different nutrient solution
      and pH: T1: total nutrient solution, pH 5
      WUE = g (Total yield fresh weight)/L (Total water use). Total water use = sum of cumulative water uptake and water lossg FW/L29.68Half nutrient solutions with pH of 6 produced higher WUE. EC based fertilization method resulted in the highest WUE even in low pH.[25]
      T2: half nutrient solution, pH 541.88
      T3: adjusting EC, pH 5127.8
      T4: total nutrient solution, pH 630.81
      T5: half nutrient solution, pH 691.08
      T6: adjusting EC, pH 6142.91
      T7: total nutrient solution, pH 735.74
      T8: half nutrient solution, pH 763.57
      T9: adjusting EC, pH 7130.19
      EC − Electrical conductivity, CY − Crop yield, CWU − Crop water use.
    • Aeroponic systems studies regarding WUE are even more limited than the other hydroponic systems. Aeroponic systems vary widely in design and definition, which also made comparison of these systems challenging. While some consider systems that leave roots suspended in the air and circulate nutrient solution to be aeroponic systems, we limited our definition to systems that utilized nozzles to aerosolize nutrient solutions and spray roots. Following these guidelines, only five separate articles were found that studied vegetable crop WUE in aeroponic systems (Table 3). Three experiments were performed on lettuce[22,23,25], one on cucumber[21], one on celery (Apium graveolens)[23], and one on onion (Allium cepa)[24]. Half of these studies grew the crops in aeroponic towers, and the other half used root chamber designs (Table 3). The difference between these two designs were orientation of root chambers, and the location of the nutrient solution reservoir (directly attached to the system or in a separate reservoir). In this context, vertical aeroponic towers are oriented vertically and have attached reservoirs for circulating the nutrient solution[21,23]. Root chamber aeroponic units are oriented horizontally and typically have an external reservoir for circulating the nutrient solution[22,24,25]. The units used for WUE by the authors were either g FW/L or kg/m3, which are interconvertible as mentioned previously. After compiling all the experiments (Table 3), we averaged the WUE across all crops to determine variability amongst the crop species in these systems. The WUE ranged between 11 and 98 g/L amongst crops and 8 to 111.8 g/L for root chamber systems and 3.02 to 142.91 g/L for nutrient solution reservoirs (Table 2). This wide range of WUE demonstrates the variability in systems, crops, and how important standardization can be for comparison. For example, in an aeroponic tower, the average WUE for cucumber was 97.94 g/L[21] and celery was 11.65 g/L[23]. Furthermore, Jamshidi et al. found the duration of nutrient and water application as the main factors affecting the production of vegetables in aeroponics[21]. Alternatively, onion grown using root chambers in an aeroponic with floating and aggregate growing system, had an average bulb WUE of 25.36 g/L[23]. Hence, aeroponic systems were not as successful as other systems and the authors attribute this to the entanglement of new roots with older roots caused by the spray liquid[21]. This occurs depending on the position of the nebulizers applying the nutrient solution and frequency of irrigation[24]. Lettuce was extensively studied in different aeroponic construction configurations, with two experiments in root chambers[22,25] and one in an aeroponic tower[23]. However, the average WUE for all the lettuce experiments was not comparable since the authors used different equations to determine WUE. Instead, we separated the studies based on the equation used to compute WUE (Eqns 1−3). This was a key issue throughout all WUE studies evaluated, because consumed, applied, and calculated water use are different concepts which will impact WUE. When water use was modeled as a function of leaf area index and daily radiation as shown in Eqn 3, the WUE averaged 3.45 g/L[22]. On the other hand, WUE calculated using total applied irrigation water as shown in Eqn 2, had an average of 9.3 g/L[23]. The last study in our opinion, is more logical for a closed soilless production system, in which the authors use the used solution to compute WUE (Eqn 1) and had an average of 77.07 g/L[25]. This is important because changing nutrient solutions will affect WUE, because that volume of solution is being removed and therefore 'used', ultimately decreasing WUE because the total volume applied isn't necessarily consumed by the plant. The study by Chabite et al. shows that the method of replacing the nutrient solution (dilution and adjustment of the solution) and the pH of the solution are the most important factors that affect WUE[25]. Thus, the application or utilization of nutrient solutions in various methods might have a detrimental effect on WUE and its calculation. This further illustrates the importance of consistent measurements and methodologies when designing new studies and formulating new nutrient solutions.

    • The literature search indicated limited studies on NUE of aeroponic systems. In general, there are various applications of NUE, such as potassium use efficiency, phosphorus use efficiency, and so on. However, as mentioned above, similar to hydroponics production, nitrogen is the most frequently reported nutrient when it comes to NUE. To date, only five articles have been identified that specifically reported NUE results focusing on nitrogen. The typical equation for four out of five of these studies was Eqn 4, except for the water cycling lettuce study, that used Eqn 5 (Table 4). However, these studies primarily focused on other research aspects, while reporting NUE of their aeroponic crops as a secondary objective in their articles. Out of the five articles, three of them were authored by Tiwari et al.[2628] and focused specifically on seed potato cultivation. In these studies, Tiwari et al.[2628] conducted several potato growth trials to assess NUE of different varieties under varying levels of applied nitrogen. In the first study, Tiwariet al.[26] observed two potato varieties, namely Kufri gaurav and Kufri jyoti, and found that Kufri gaurav exhibited higher efficiency in nitrogen uptake. In this study, two nitrogen amounts were applied: a high amount, and a low amount. For Kufri gaurav, the high amount of nitrogen resulted in a NUE of approximately 0.15 g/g, while the low amount of nitrogen showed 2.1 g/g. However, for Kufri jyoti, the high amount of nitrogen showed a NUE at 0.2 g/g, while the low amount of nitrogen had a NUE of 1.6 g/g. In the subsequent study, Tiwari et al.[27] observed phenotyping of Kufri gaurav in more detail. They maintained the same rates of nitrogen as in the previous study and found that the low nitrogen supply yielded a greater NUE value of approximately 0.85 g/g while the high nitrogen showed a value approaching zero. In the third study by Tiwari et al.[28], 56 different seed potato varieties were compared and a NUE of 0.28 g/g was recorded for the lowest, and 2.95 g/g for the highest. Gaudin et al.[29] conducted similar research comparing modern corn varieties with earlier varieties grown in an aeroponic system. The results revealed that the modern corn variety had a NUE of 2.1 g/g for the high nitrogen treatment, and a 3.1 g/g for the low nitrogen treatment. Alternatively, the older teosinte corn showed an NUE of 3.6 g/g for the high nitrogen treatment, and a 5.5 g/g for the low nitrogen treatment. These findings demonstrate the importance of nutrient concentration in the solution and its impact on NUE. Furthermore, it was consistently proved that lower concentrations of nutrients in solution resulted in higher NUE, suggesting the potential for improving sustainability in aeroponic systems through nutrient management. In another study, Chabite et al. investigated how water cycling techniques in aeroponic nutrient reservoirs can influence NUE[25]. Chabite et al. grew nine lettuce plants in nine different treatments, involving various water cycling methods and pH levels[25]. The NUE for these plants ranged from 5.17 to 16.53 g/L. Notably, the treatment resulting in NUE of 16.53 g/L had a pH of 6 and replaced half of the nutrient solution every 10 d. These results highlight the influence of solution pH and management techniques, such as water cycling, on NUE in aeroponic systems.

      Table 4.  Studies conducted on nutrient use efficiency (NUE) in aeroponics.

      SystemLocationCropTreatmentEquationUnitsNUEMain findingsRef.
      Root chamberShimla, IndiaPotato (Solanum tuberosum var. Kufri Gaurav)T1: 0.75 mM N with 30 s ON and 5 min OFFNUE = Plant dry matter accumulation/crop N supplyg/mM~0.85Lower nitrogen applied resulted in higher NUE[26]
      T2: 7.5 mM N with 30 s ON and 5 min OFF~ 0
      Root chamberShimla, IndiaPotato (Solanum tuberosum var. Kufri Jyoti)T1: 0.75 mmol/L N with 30 s ON and 5 min OFFNUE = Plant dry matter accumulation/crop N supplyg/g~1.6Kufri Gaurav had the highest NUE when the 0.75 mmol/L supply of N was applied[27]
      T2: 7.5 mmol/L N with 30 s ON and 5 min OFF~0.20
      Potato (Solanum tuberosum var. Kufri Gaurav)T1: 0.75 mmol/L N with 30 s ON and 5 min OFF~2.1
      T2: 7.5 mmol/L N with 30 s ON and 5 min OFF~ 0.15
      Root chamberShimla, IndiaPotato (Solanum tuberosum)T: 56 seed potato varieties.
      2 mM N with 30 s ON and 5 min OFF
      NUE = Plant dry matter accumulation/crop N supplyg/g0.28 (Kufri Shailja) to 2.95 (Kufri Frysona)Kufri Frysona had the highest NUE of all potato seed varieties tested[28]
      Root chamberOntario,
      Canada
      Corn (Zea mays)T1: Low N with 10 s ON and 50 s OFFNUE = shoot dry weight/ N supplyg/mmol3.1The Teosinte variety of corn had higher NUE when using low N in an aeroponic system when compared to modern day corn using the same amount of N treatments.[29]
      T2: High N with 10 s ON and 50 s OFF2.1
      Teosinte (Zea mays spp. Parviglumis)T1: Low N with 10 s ON and 50 s OFF5.5
      T2: High N with 10 s ON and 50 s OFF3.6
      Root chamberChangchun, ChinaLettuce (Lactuca sativa L. var. ramosa Hort)T1: total nutrient solution, pH 5NUE = Total yield dry matter/Total nitrogen use (gDM/N)
      gDM N
      7.84T5 (half nutrient solution at pH 6) was found to have the greatest NUE.[25]
      T2: half nutrient solution, pH 55.17
      T3: adjusting EC, pH 511.12
      T4: total nutrient solution, pH 66.75
      T5: half nutrient solution, pH 616.53
      T6: adjusting EC, pH 613.82
      T7: total nutrient solution, pH 77.87
      T8: half nutrient solution, pH 78
      T9: adjusting EC, pH 710.68

      Furthermore, similar to hydroponics, the lack of consistency among studies regarding cultivars, environmental conditions, practices, and methodology poses challenges in determining the best practices for NUE in aeroponic systems. One particular difficulty arises from variations in reporting NUE, including differences in the formula used for NUE calculations. Four of the five studies reported NUE based on dry weight, while one study used fresh weight. Establishing standards or guidelines for calculating NUE could greatly benefit researchers by promoting consistency across all studies thereby facilitating relevant comparisons.

    • While there has been a limited amount of research on water and nutrient use efficiency in different CEA production systems, we have seen that these systems are conducive for efficient use of water and nutrients. Though discrepancies have been found between different research studies, the results are promising with regards to resource efficiency in controlled environment production. Improved resource use efficiencies can be attributed to reduced nutrient leaching, precisely controlled nutrient delivery, reduced competition with weeds for resources, low pest infestation rates, etc[30]. However, there is a high rate of variability between environments, cultivars, species, and production systems. All crops have different requirements and demands for water and nutrients, which highlights the necessity for more research on a wide variety of crops in different environments. There is much remaining to be learned with regards to water and nutrient use efficiency in CEA production systems, yet these technologies also hold much promise for the future.

      Studying crops in controlled environments using different hydroponic production systems is challenging due to the technology involved and equipment used. This equipment can be assembled in different ways, and there are few guidelines for standardization amongst systems. For example, aeroponic systems are not clearly defined in literature; with some researchers using aerosolized nutrient solution sprays and others using roots suspended in air as their qualifiers. While we have provided some suggestions for defining criteria, we encourage researchers to include more thorough details in the future when describing these systems to allow for a better understanding of the research being conducted and guidelines to be developed. Research is being conducted on CEA worldwide, but access and availability of supplies is variable. Therefore, studies should consider how their system could vary from those conducted elsewhere when reporting results. The authors realize that research should be accessible even if environmental conditions are not strictly controlled, and that the results are still significant which can contribute to the overall body of research. However, variability in systems can have environmental impacts as inefficient systems can generate more waste and have higher energy consumption. The sustainability of the controlled environment must be considered in order to optimize efficiency. Thus, care must be taken to determine where efficiencies can be improved. Further challenges will vary based on location, research facility, supplies, equipment, and many other factors, but consistent reporting of findings is essential for comparison and building scientific knowledge.

      Research examined in this review has shown that overall, aeroponic and other hydroponic systems can improve this efficiency, but results are inconsistent across systems and crops. This is partially due to the wide variety of systems used, individual characteristics of cultivars and crop species, environment, and growing conditions. We suggest that, in light of these findings, researchers should focus on evaluating crops using more standardized methods and units, so that the results are more comparable to others and help us more fully understand water and nutrient use efficiency of different crops grown in the different systems. While innovation and novel research is essential for the future of CEA, this can still be achieved by using consistent methods and equations. This review provides a baseline for determining and selecting the most suitable methods and techniques for efficient resource use in horticultural crop production.

    • The authors confirm contribution to the paper as follows: study conception and design: Simpson C, Regmi A, Rueda-Kunz D, Trevino J, Liu H; data collection: Simpson C, Regmi A, Rueda-Kunz D, Trevino J, Liu H, Kathi S; analysis and interpretation of results Simpson C, Regmi A, Rueda-Kunz D, Trevino J, Liu H, Kathi S; draft manuscript preparation: Simpson C, Regmi A, Rueda-Kunz D, Trevino J, Liu H, Kathi S. All authors reviewed the results and approved the final version of the manuscript.

    • The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

    • The authors would like to thank and acknowledge the Urban Horticulture and Sustainability Group at Texas Tech University for their insight and knowledge about the subject. The authors would also like to acknowledge Vikram Baliga for providing space for this project to be conceptualized and written. Finally, the authors would like to thank Kamron Newberry for the suggestion to write this review paper.

      • The authors declare that they have no conflict of interest. Catherine Simpson is the Editorial Board member of Technology in Horticulture who was blinded from reviewing or making decisions on the manuscript. The article was subject to the journal's standard procedures, with peer-review handled independently of this Editorial Board member and the research groups.

      • 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 (2)  Table (4) References (30)
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    Regmi A, Rueda-Kunz D, Liu H, Trevino J, Kathi S, et al. 2024. Comparing resource use efficiencies in hydroponic and aeroponic production systems. Technology in Horticulture 4: e005 doi: 10.48130/tihort-0024-0002
    Regmi A, Rueda-Kunz D, Liu H, Trevino J, Kathi S, et al. 2024. Comparing resource use efficiencies in hydroponic and aeroponic production systems. Technology in Horticulture 4: e005 doi: 10.48130/tihort-0024-0002

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