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The biosynthesis of active components in rice is influenced by rice varieties but also depends on cultivation management practices and their growth environment.
Effect of light on the accumulation of active components in rice
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Environmental conditions have a greater effect on protein content than genetic forces[53]. Both light intensity and light duration affect the synthesis and accumulation of active components in rice. Low light intensity in the early stage of rice growth is not conducive to the accumulation of glutelin in rice grains but favors the accumulation of amylose, while the opposite is true in the late stage of rice growth[54]. Low light intensity during the grain-filling period reduces the accumulation of total flavonoids in rice[55] and decreases Fe ions' movement in the transpiration stream and thereby the transport of Fe ions to rice grains[56]. An appropriate increase in light intensity is beneficial to the accumulation of flavonoids, anthocyanins, and Fe in rice, but the photostability of anthocyanins is poor, and too much light will cause oxidative degradation of anthocyanins[57]. Therefore, functional rice is best cultivated as mid-late rice, which would be conducive to accumulating active components in rice.
Effect of temperature on the accumulation of active components in rice
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The temperature has a great influence on the synthesis of active components in rice. An appropriate increase in the temperature is beneficial to the accumulation of γ-oryzanol[58] and flavonoids[59] in rice. A high temperature during the grain-filling period leads to an increase in glutelin content in rice[60], but an increase in temperature decreases the total phenolic content[61]. The results regarding the effect of temperature on the content of PA in rice were inconsistent. Su et al.[62] showed that high temperatures during the filling period would increase the PA content, while Goufo & Trindade[61] reported that the increase in temperature would reduce the PA content. This may be due to the different growth periods and durations of temperature stress on rice in the two studies. The synthesis of anthocyanins/proanthocyanidins in colored rice requires a suitable temperature. Within a certain range, lower temperatures favor the accumulation of anthocyanins/proanthocyanidins in rice[63]. Higher temperatures will lead to degradation, and the thermal stability of proanthocyanidins being higher than that of anthocyanins[64]. In addition, cold or heat stress facilitates GABA accumulation in rice grains[65]. Therefore, in actual production, colored rice and low-glutelin rice are best planted as late rice, and the planting time of other functional rice should be determined according to the response of its enriched active components to temperature changes.
Effect of water management on the accumulation of active components in rice
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Moderate water stress can significantly increase the content of glutelin[66] and GABA[67] in rice grains and promote the rapid transfer of assimilation into the grains, shorten the grain filling period, and reduce the RS content[68]. Drought stress can also induce the expression of the phytoene synthase (psy) gene and increase the carotenoid content in rice[69]. Soil moisture is an important medium in Zn diffusion to plant roots. In soil with low moisture content, rice roots have low available Zn, which is not conducive to enriching rice grains with Zn[70]. Results from studies on the effect of soil water content on Se accumulation in rice grains have been inconsistent. Li et al.[71] concluded that flooded cultivation could significantly increase the Se content in rice grains compared to dry cultivation. However, the results of Zhou et al.[72] showed that the selenium content in rice grains under aerobic and dry-wet alternative irrigation was 2.44 and 1.84 times higher than that under flood irrigation, respectively. This may be due to the forms of selenium contained in the soil and the degree of drought stress to the rice that differed between experiments[73]. In addition, it has been found that too much or too little water impacts the expression of genes related to anthocyanin synthesis in rice, which affects the accumulation of anthocyanins in rice[74]. Therefore, it is recommended to establish different irrigation systems for different functional rice during cultivation.
Effect of fertilizer application on the accumulation of active components in rice
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Both the amount and method of nitrogen application affect the accumulation of glutelin. Numerous studies have shown that both increased and delayed application of nitrogen fertilizer can increase the accumulation of lysine-rich glutelin to improve the nutritional quality of rice (Table 1). However, this improvement is not beneficial for kidney disease patients who cannot consume high glutelin rice. Nitrogen stress can down-regulate the expression of ANDs genes related to the anthocyanins biosynthesis pathway in grains, resulting in a decrease in anthocyanins synthesis[55]. Increased nitrogen fertilizer application can also increase the Fe, Zn, and Se content in rice[75,76]. However, some studies have found that increased nitrogen fertilizer application has no significant effect on the Fe content of rice[77], while other studies have shown that increased nitrogen fertilizer application will reduce the Fe content of rice[78]. This may be influenced by soil pH and the form of the applied nitrogen fertilizer. The lower the soil pH, the more favorable the reduction of Fe3+ to Fe2+, thus promoting the uptake of Fe by rice. Otherwise, the application of ammonium fertilizer can improve the availability of soil Fe and promote the absorption and utilization of Fe by rice. In contrast, nitrate fertilizer can inhibit the reduction of Fe3+ and reduce the absorption of Fe by rice[79].
Table 1. Effect of nitrogen fertilizer application on glutelin content of rice.
Sample N level
(kg ha−1)Application time Glutelin content
(g 100 g−1)References Rough rice 0 5.67 [66] 270 Pre-transplanting : mid tillering : panicle initiation : spikelet differentiation = 2:1:1:1 6.92 300 Pre-transplanting : mid tillering : panicle initiation : spikelet differentiation = 5:2:2:1 6.88 Brown rice 0 5.35 [83] 90 Pre-transplanting : after transplanting = 4:1 6.01 Pre-transplanting : after transplanting = 1:1 6.60 180 Pre-transplanting : after transplanting = 4:1 6.53 Pre-transplanting : after transplanting = 1:1 7.29 270 Pre-transplanting : after transplanting = 4:1 7.00 Pre-transplanting : after transplanting = 1:1 7.66 Rough rice 0 5.59 [84] 187.5 Pre-transplanting : after transplanting = 4:1 6.47 Pre-transplanting : after transplanting = 1:1 6.64 300 Pre-transplanting : after transplanting = 4:1 7.02 Pre-transplanting : after transplanting = 1:1 7.14 Polished rice 0 3.88 [85] 90 Pre-transplanting : tillering : booting = 2:2:1 4.21 180 Pre-transplanting : tillering : booting = 2:2:1 4.43 270 Pre-transplanting : tillering : booting = 2:2:1 6.42 360 Pre-transplanting : tillering : booting = 2:2:1 4.87 Brown rice 0 9.05 [86] 120 Flowering 22.14 Appropriate application of phosphorus fertilizer is beneficial in promoting the translocation of Fe and Zn from leaves to rice grains, thus increasing the content in rice grains[80]. However, the excessive application of phosphate fertilizer will reduce the availability of Fe and Zn in soil, resulting in less uptake by the roots and a lower content in the rice grains[81]. The content of PA in rice increased with a higher phosphorus fertilizer application rate[80]. Increasing the phosphorus fertilizer application rate would increase the values of [PA]/[Fe] and [PA]/[Zn] and reduce the effectiveness of Fe and Zn in rice[80]. Currently, there are few studies on the effect of potassium fertilization on the synthesis of active components in rice. Available studies report that increased application of nitrogen fertilizer can increase the Zn content in rice[82]. Therefore, the research in this area needs to be strengthened.
Because the iron in soil mainly exists in the insoluble form Fe3+, the application of iron fertilizer has little effect on rice biofortification[87]. There are different opinions about the effect of Zn fertilizer application methods. Phattarakul et al.[88] believed that foliar spraying of Zn fertilizer could significantly improve the Zn content in rice grains. Jiang et al.[89] concluded that most of the Zn accumulated in rice grains were absorbed by the roots rather than from the reactivation of Zn in leaves. In contrast, Yuan et al.[90] suggested that soil application of Zn fertilizer had no significant effect on Zn content in rice grains. The different results may be affected by the form of zinc fertilizer applied and the soil conditions in the experimental sites. Studies have found that compared with the application of ZnEDTA and ZnO, zinc fertilizer in the form of ZnSO4 is most effective for increasing rice's Zn[70]. In addition, the application of zinc fertilizer reduces the concentration of PA in rice grains[70].
The form of selenium fertilizer and the method and time of application will affect the accumulation of Se in rice grains. Regarding selenium, rice is a non-hyperaccumulative plant. A moderate application of selenium fertilizer can improve rice yield. However, the excessive application can be toxic to rice, and the difference between beneficial and harmful supply levels is slight[91]. Selenite is readily adsorbed by iron oxide or hydroxide in soil, and its effectiveness in the soil is much lower than selenite[92]. In addition, selenate can migrate to the roots and transfer to rice shoots through high-affinity sulfate transporters. In contrast, selenite is mainly assimilated into organic selenium in the roots and transferred to the shoots in smaller amounts[93]. Therefore, the biological effectiveness of Se is higher in selenate-applied soil than in selenite application[94] (Table 2). Zhang et al.[95] found that the concentration of Se in rice with soil application of 100 g Se ha-1 was only 76.8 μg·kg-1, while the concentration of Se in rice with foliar spray of 75 g Se ha-1 was as high as 410 μg·kg-1[73]. However, the level of organic selenium was lower in rough rice with foliar application of selenium fertilizer compared to soil application[96], while the bioavailability of organic selenium in humans was higher than inorganic selenium[97]. Deng et al.[73] found that the concentrations of total selenium and organic selenium in brown rice with selenium fertilizer applied at the full heading stage were 2-fold higher than those in brown rice with selenium fertilizer applied at the late tillering stage (Table 2). Although the application of exogenous selenium fertilizer can rapidly and effectively increase the Se content of rice (Table 2), it can easily lead to excessive Se content in rice and soil, which can have adverse effects on humans and the environment. Therefore, breeding Se-rich rice varieties is a safer and more reliable way to produce Se-rich rice. In summary, functional rice production should include the moderate application of nitrogen and phosphorus fertilizer and higher levels of potassium fertilizer, with consideration to the use of trace element fertilizers.
Table 2. Effect of selenium fertilizer application on the selenium content of rice.
Sample Se level (g Se ha−1) Selenium fertilizer forms Application method Se content (μg·g−1) References Rough rice 0 0.002 [98] 18 Selenite Foliar spray at full heading 0.411 Polished rice 0 0.071 [99] 20 Selenite Foliar spray at full heading 0.471 20 Selenate Foliar spray at full heading 0.640 Rough rice 75 Selenite Foliar spray at late tillering 0.440 [73] 75 Selenite Foliar spray at full heading 1.290 75 Selenate Foliar spray at late tillering 0.780 75 Selenate Foliar spray at full heading 2.710 Polished rice 0 0.027 [100] 15 Selenite Foliar spray at full heading 0.435 45 Selenite Foliar spray at full heading 0.890 60 Selenite Foliar spray at full heading 1.275 Effect of harvesting time on the accumulation of active components in rice
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The content of many active components in rough rice is constantly changing during the development of rice. It was found that the content of total flavonoids in brown rice increased continuously from flowering stage to dough stage and then decreased gradually[101]. The γ-oryzanol content in rice decreased by 13% from milk stage to dough stage, and then gradually increased to 60% higher than milk stage at full maturity[101]. The results of Shao et al.[102] showed that the anthocyanin content in rice reached its highest level at two weeks after flowering and then gradually decreased. At full ripeness, and the anthocyanins content in brown rice was only about 50% of the maximum level. The content of total phenolics in rice decreased with maturity from one week after flowering to the fully ripe stage, and the loss of total phenolics reached more than 47% by the fully ripe stage. In contrast, the content of total phenolics in black rice increased with maturity[102]. Moreover, RS content in rough rice decreases during rice maturation[68]. Therefore, the production process of functional rice should be timely and early harvested to obtain higher economic value.
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Functional rice contains many active components which are beneficial to maintaining human health and have high economic and social value with broad market prospects. However, the current development level of the functional rice industry is low. The development of the functional rice requires extensive use of traditional and modern polymerization breeding techniques to cultivate new functional rice varieties with endosperm that can be enriched with multiple active components and have broad-spectrum resistance to pests and diseases. It is also important to select suitable planting locations and times according to the response characteristics of different functional rice active components to environmental conditions.
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About this article
Cite this article
Jin Z, Nie L. 2023. Functional rice: a new direction for sustainable development of rice production. Tropical Plants 2:13 doi: 10.48130/TP-2023-0013
Functional rice: a new direction for sustainable development of rice production
- Received: 06 February 2023
- Accepted: 19 July 2023
- Published online: 21 August 2023
Abstract: Functional rice has a broad market prospect and represents one of the vital developmental directions for future rice production. This paper summarizes the types, breeding and cultivation technologies of functional rice, as well as prevention and control of pests and diseases. We conclude the following: (1) breeding for functional rice should focus on breeding rice varieties with an endosperm that is enriched with multiple active components and broad-spectrum resistance to pests and diseases; (2) moderate water stress and optimized fertilizer management practices of low nitrogen, low phosphorus, high potassium, high silicon, and moderate micronutrient fertilization, as well as timely and early harvest, are conducive to improving the yield and quality of functional rice. In addition, we stress the need to focus on the development and application of polymerization breeding technologies for the advancement of the functional rice industry, and future research in these areas should be reinforced.
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Key words:
- Functional rice /
- Active component /
- Health /
- Breeding /
- Cultivation