Effects of Light Intensity and Quality on Physiological Changes in Winter Rice (Oryza Sativa L.)
Abstract
An investigation into the effects of low light (≈ mean PPFD 169–493 µmol m-2 s-1), red light (≈ mean PPFD 657–843 µmol m-2 s-1) and normal light (≈ mean PPFD 1061–1260 µmol m-2 s-1) on physiological changes including yield attributes and nutritional status at panicle initiation (PI), flowering and harvest stages in winter rice (genotype: Monoharsali) was carried out from 40 days after transplanting (DAT) to crop maturity under both pot and field conditions. There was a reduction in available PPFD at flowering as compared to the PI stage of the crop. The genotype exhibited significantly higher total dry matter (TDM) and lower leaf area index (LAI) values under normal or red light than under reduced light conditions. The bio-economical yield and harvest index (HI) under normal light exceeded those under the low light regime. Yield attributes such as number of panicles, number of filled grains per panicle, 1000-grain weight (test weight), high-density (HD) grains, potential 1000-grain weight, and sink capacity were superior under normal or red light compared to low light conditions. Higher total carbohydrate content in grains resulted in higher sink capacity under normal and red light illumination at the harvest stage. At the PI stage, leaf nitrogen content was reduced under low light conditions but increased significantly at the flowering stage. Grain protein content under normal and red light exceeded that under low light treatment. A positive correlation between most yield attributes and grain yield was observed under normal and red light conditions, while a negative correlation was found under low light conditions.
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Introduction
Rice (Oryza sativa L.) grown in winter season in tropical and subtropical regions are influenced adversely by the climatic conditions, which is characterized by overcast sky posing low sunshine hours during the vegetative to reproductive growth stages of the crop. Rice plant requires about 1500 bright sunshine (BSS) hours for the period from transplanting to maturity. Instead, prevalence of only about 800-900 BSS hours during August to December in places like Northeastern region of India hampers the physiological efficiencies, and ultimately the productivity of winter rice crop (Bharali et al., 1994). It’s because, solar radiation in tropics is one of the major climatic factors limiting grain yield in rice (Vergara et al., 1976).
Apart from light intensity, an illumination condition is determinant for proper growth and development of plant, which ensures interactions of all photoreactions (Voskresenenskya, 1979). Many vital biosynthetic processes of plant can be regulated by simple alteration of light quality. Accumulation of carbohydrate, particularly starch is a notable metabolic feature of plants photosynthesizing under red light during a long period. In plants under the influence of blue light, the carbon is preferentially utilized for biosynthesis of proteins (Pierson and Lowallik, 1964). The peculiarities in metabolism due to effects of light quality are accompanied by changes in chloroplast organization. The plants under red light form lesser active chloroplast than plants under blue light. Moreover, the plants photosynthesizing under red light have limited CO 2 exchange, week photophosphorylation and low activity of electron transport from water (Voskresenskaya, 1972). Low light at PI and flowering stages of crop causes physiological aberrations in rice including nutritional imbalance (Bharali et al., 1993). The present study attempted to compare the physiological performance of winter rice crop modulated by low light and red light in comparison to normal light in field and pot culture conditions.
Conclusion
The foregoing investigation into the effects of different light regimes on the rice crop revealed that low light and even red light reduce most of the physiological parameters in comparison to normal white light. So, it can be inferred that the most favorable illumination condition for proper growth and development of rice crop is the white light with the required bright sunshine hours.