Effects of Low Light on Agronomic and Physiological Characteristics of Rice Including Grain Yield and Quality
Rice is the primary food source for about 65% of the world’s population, which mainly grows as a rainy season crop in Southeast Asia and China, and is frequently exposed to poor light intensity at various stages of growth. Light intensity determines grain yield and quality (Seo and Chamura, 1980; Furuno et al, 1992; Wilson et al, 1992; Yao et al, 2000). Continuous cloudy days or rainfall during critical stages of growth, such as panicle differentiation or grain-filling stages, often induce great loss of grain yield and poor grain quality (Janardhan et al, 1980; Nayak and Minor, 1980; Praba et al, 2004). Low light stress has severely constrained rice yield in some rice-growing regions of the world, especially in Southeast Asia and China (Chaturvedi and Ingram, 1989; Ren et al, 2002). Sichuan, Yunnan and Guizhou provinces in Southwest China serve as staple rice cultivation regions but perennially suffer from low light stress because of their unique geographical positions. In these districts, cloudy or rainy days frequently occur with total solar radiations of 3 345–3 763 MJ/m2, and sunshine hours are often less than 1 200 h per year (Huang, 1998). Rice, cultivated in the adjoining districts of the Yangtze Valley, also experiences low light stress because continuous rainfall often occurs during the grain filling stage (Li and Zhang, 1995). Therefore, the poor light environment often severely hampers normal plant development and adversely affects rice yield and quality in major rice-growing regions of China and other countries.
Because low light conditions can damage rice production dramatically, light intensity has received attention from researchers worldwide increasingly (Chaturvedi and Ingram, 1989; Thangaraj and Sivasubramanian, 1990; Nakano, 2000; Liu et al, 2007). Numerous simulations analyzing the effects of low light on rice development as well as grain yield and quality have been performed (Li L et al, 1997; Kobata et al, 2000; Liu et al, 2006b; Fu et al, 2009). A lack of adequate light strongly influences not only the duration of growth but also some agronomic traits of rice. For example, low light results in a prolonged period of growth and also increases plant height and leaf area (Ren et al, 2002; Liu et al, 2009). Before the heading stage, low light gives rise to a pronounced decrease in fertile panicles of rice plants. After the heading stage, shading causes impairment of the net photosynthetic rate as well as lower dry matter accumulation and sink capacity in rice plants, and this significantly reduces the number of filled grains and 1000-grain weight, thereby leading to decreased grain yield (Sato, 1956; Kato, 1986; Deng et al, 2009; Liu et al, 2009). Low light after the heading stage also results in poor appearances of rice grain and milling qualities, including a high percentage of chalky grains and also a reduced head yield. This may be primarily attributed to an insufficient supply of assimilates and decrease activity of a soluble starch branching enzyme involved in starch synthesis in grains (Tashiro and Ebata, 1975; Miizuno et al, 1992; Li T G et al, 1997; Ren et al, 2003b). In this review, based on previous reports, we mainly highlighted the negative effects of low light on the development of rice as well as on grain yield and quality in rice. We also discussed the physiological mechanisms related to variations in grain yield and quality observed under low light conditions. These results can help rice researchers better understand the relationship between light intensity and rice production, and facilitate further research related to effective cultivation practices and breeding strategies for the improvement of rice grain yield and quality in regions prone to low light conditions.
REVIEW OF LITERATURE:
Ren et al, 2002; Ding et al, (2004) observed that Low light conditions result in significantly increased leaf length, leaf width, leaf area and growth duration, and the increases are enhanced with a reduction of light intensity.
Chonan, 1967 noted that mesophyll thickness and the number of cells per square millimeter in leaves decrease by 14.61% and 15.86%, respectively, when rice plants are grown under 20% of natural light.
Wang, 2011 observed that Chlorophyll a and b are important pigments involved in the absorption and transmission of solar energy, with part of chlorophyll a involved in converting solar energy into electrochemical energy.
Zhu et al, 2008; Liu et al, 2009 observed that Differences exist in the chlorophyll content produced in response to low light among varieties.
Zhu et al, 2008 noted that when subjected to low light for 15 d (when treatment had commenced at the initial heading stage), varieties that are tolerant to low light exhibit higher chlorophyll b and lower chlorophyll a/b content in their leaves when compared with those perform poorly in low light.
Liu et al, 2009 showed that leaf chlorophyll a and b content during the grain- filling stage is markedly enhanced in low light tolerant varieties after being treated by low light from the transplanting to the booting stages, whereas the opposite is found in varieties that perform poorly in low light.
Ren et al, 2002 showed that these results suggest that tolerant varieties capture as much solar energy as possible under low light conditions through increased leaf area and higher chlorophyll b content, demonstrating the morphological and physiological responses of rice plants when they experience low light stress.
Meng et al, 2002; Yang et al, 2011 showed that Low light negatively affects stomatal conductance (fewer stomata are produced per square millimeter) while it results in enhanced concentrations of intercellular CO2 in rice leaves.
Sato and Kim, 1980 observed that the respiration rate decreases more than the net photosynthetic rate, thus resulting in a higher ratio of respiration to net photosynthetic rates under low light than under natural light.
Farquhar and Sharkey (1982) speculated that, under low light conditions, stomatal closure is the main constraint on photosynthesis if both stomatal conductance and intercellular CO2 concentration decrease. Nevertheless, they excluded the factors that lead to impaired photosynthesis when stomatal conductance declines with increased intercellular CO2 concentration. These results imply that a reduction of the net photosynthetic rate may not be strongly relevant to the promotion of stomatal closure and the decreased number of stomata produced under low light conditions.
Shi et al LIU Qi-hua, et al. Effects of Low Light on Agronomic and Physiological Characteristics of Rice 245 (2006) noted that the ribulose bisphosphate carboxylase (Rubisco) activity in chloroplasts declines dramatically under low light conditions.
Jiao and Li (2001) showed that light intensity alters the rates of non-photochemical quenching, electron transfer and quantum yield of PS II.
RESPONSES OF RICE GRAIN YIELD AND YIELD COMPONENTS TO LOW LIGHT
Low light treatment conducted from the transplanting to booting stages for the rice variety Xingfeng; b Low light treatment conducted from the heading to 10 d after heading stages for the rice variety Chuanxiang 9838; c Low light treatment conducted from the initial heading to maturity stages for the rice variety Jingxian 39.
Matsushima et al, 1953b observed that low light conditions during the panicle differentiation or grain-filling stages exert a greater adverse effect on rice grain yield than that during other growth stages, which better explains the greater loss in grain yield caused by low light during the reproductive stage.
Janardhan and Murty, 1980 noted that Under low light, nutrient source organs (leaves + culm + sheaths) cannot provide adequate amounts of assimilates to meet the requirements of tiller emergence and grain growth because of the impaired photosynthetic rate.
Ota et al, 1959; Nayak and Minor, 1980 noted that low light inhibits the translocation of assimilates from source organs to sink organs (grains).
Nayak et al, 1978; Nayak and Minor, 1980; Liu et al, 2012 showed that In spite of detrimental effects on carbohydrate accumulation and transportation caused by low light, tolerant varieties maintain their carbohydrate production levels, due to higher chlorophyll content and efficiency in photosynthesis, and stronger antioxidant ability, which makes them more adaptable to low light conditions.
RESPONSES OF GRAIN QUALITY TO LOW LIGHT
Cai and Luo, 1999; Zhu et al, 2008; Goto and Kumagai, 2009; Liu et al, 2009 observed that the photosynthetic capacity of source organs (leaves) recovers to a normal level when full light become available during the heading stage.
Ren et al, 2003b showed that When rice is grown under low light for 32 d (starting from the initial heading stage), brown rice, milled rice and head rice yields, as well as grain amylose content and gel consistency decrease while the percentage of chalky kernels and grain protein content increase (Table 2). These findings verify that low light conditions during the grain-filling stage result in poor appearance and milling qualities of rice grains.
Tashiro et al, 1980; Li et al, 2005, 2006 noted that Low light during the grain-filling stage results in a decreased supply of carbohydrates to grains as well as a decrease in starch synthase activity in grains, which directly inhibits grain filling and enhances the occurrence of chalky rice.
Li et al (2005) has demonstrated that increased activity of soluble starch branching enzyme impairs the accumulation of amylose in grains when grain amylose content is reduced under low light.
Ren et al, 2003b, c Note that grain protein content increases under low light although the amount of N imported from culm and sheaths to grains declines
SUMMARY AND CONCLUSION:
Numerous experimental data and related reports have confirmed that low light markedly affects agronomic and physiological traits of rice plants, hampering the underlying physiological metabolisms, including photosynthesis, respiration, antioxidant characteristic as well as the conversion and distribution of carbon and nitrogen (Sato, 1956; Li L et al, 1997; Ren et al, 2002; Zhu et al, 2008). Such changes eventually result in decreased rice grain yield and quality with a poor production of tillers, impaired capacity for panicles to differentiate, an abnormal grain-filling process, and sophisticated variability of activities of enzymes controlling starch grain synthesis (Nakano, 2000; Wang et al, 2001; Ren et al, 2003b; Li et al, 2005). Furthermore, responses of rice yield components and quality to low light differed substantially depending on diverse growth stage. That is, low light at the vegetable stage mainly decreases the number of productive panicles per unit area and eating quality, and that at the productive stage primarily results in the decreased number of grains per panicle, grain size and appearance, milling and eating qualities (Ren et al, 2003b; Liu et al, 2006a; Goto and Kumagai, 2009). The results suggest that when plants receive only low light during the reproductive stage, it generates a strong influence on rice grain yield and quality than those receive low light during the vegetative stage. Previous reports also indicate that when plants are grown under low light, they exhibit different effects on rice yield depending on the varieties used (Voleti and Singh, 1996; Liu et al, 2012). Low light tolerant varieties can maintain a more efficient photosynthetic rate and effective antioxidant capacity under low light, because they can maintain higher chlorophyll content and antioxidant enzyme activity level, thereby minimizing grain yield loss (Nayak et al, 1978; Liu et al, 2012). However, susceptible varieties suffer from a lack of the ability to adapt to low light conditions because of a significant difference in variety (Nayak et al, 1978; Voleti and Singh, 1996).
Currently, scientists have basically determined that the detrimental effects of low light are in relation to rice morphological and physiological characteristics and grain yield and quality. Few reports describe how to alleviate the negative influence of low light, so additional related research is needed in the future (Okamoto, 1970; Agarie et al, 1992). Based on previous reports and our research, we believe two important channels exist for improving rice grain yield and quality under low light conditions. First, varieties with high tolerance to low light should be planted and observed in regions with poor light conditions. Additional research is needed to determine whether some of the physiological traits mentioned above, such as chlorophyll content, can be used as indicators of tolerance to low light conditions for breeding work. Thilmony et al (2009) confirmed that the promoter of LP2 gene in rice is highly responsive to light. Hence, improving the inherent resistance of rice to low light by a molecular method is of great importance for breeding a tolerable cultivar that can thrive under low light stress. Second, related studies need to draft optimum agronomic measurements to cope with low light stress. Okamoto (1970) and Tamaki et al (1999) noted that the application of both silica and/or organic fertilizer can mitigate the damage caused by low light conditions during rice development. Hence, in the future, new research should focus on strengthening the resistance of rice plants to low light conditions by adopting suitable cultivation practices, including improving the supply and translocation efficiency of assimilate and analyzing the appropriate application of new fertilizers and commercial plant growth regulators to rice plants and fields.
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