~ 1814 ~ Journal of Pharmacognosy and Phytochemistry 2019; 8(1): 1814-1821 E-ISSN: 2278-4136 P-ISSN: 2349-8234 JPP 2019; 8(1): 1814-1821 Received: xx-11-2018 Accepted: xx-12-2018 D Panda Division of Crop Physiology and Biochemistry, ICAR-National Rice Research Institute, Cuttack, Odisha, India M Biswal ICAR-National Rice Research Institute, Cuttack, Odisha, India L Behera ICAR-National Rice Research Institute, Cuttack, Odisha, India MJ Baig ICAR-National Rice Research Institute, Cuttack, Odisha, India P Dey ICAR-National Rice Research Institute, Cuttack, Odisha, India L Nayak ICAR-National Rice Research Institute, Cuttack, Odisha, India S Sharma ICAR-National Rice Research Institute, Cuttack, Odisha, India S Samantaray ICAR-National Rice Research Institute, Cuttack, Odisha, India U Ngangkham ICAR-National Rice Research Institute, Cuttack, Odisha, India A Kumar ICAR-National Rice Research Institute, Cuttack, Odisha, India Correspondence D Panda Division of Crop Physiology and Biochemistry, ICAR-National Rice Research Institute, Cuttack, Odisha, India Impact of low light stress on physiological, biochemical and agronomic attributes of rice D Panda, M Biswal, L Behera, MJ Baig, P Dey, L Nayak, S Sharma, S Samantaray, U Ngangkham and A Kumar Abstract Low light resulting from overcast skies during wet season is one of the important constraints in achieving high crop productivity. Field experiments were conducted to investigate the effect of low light stress on photosynthesis, chlorophyll accumulation, grain yield and yield components of rice. Five rice genotypes, Nirajo, Purnendu, Malliksalli, Megha Rice-1 and ASD-14 along with tolerant check, Swarnaprabha and susceptible check, IR-8 were grown under 100% and 75% of full sunlight intensity. Plants exposed to depleted radiation during reproductive stage were found to accumulate more of total chlorophyll but reduced Chl a/b ratio in the flag leaf and lesser net photosynthetic rate, stomatal conductance and rate of transpiration, which contributed to reduction in grain yield in all the genotypes Purnendu and Nirajo exhibited highest reduction in Chl a/b ratio and lesser reduction in starch accumulation in grains compared to others. Though there was significant reduction in grain yield of all genotypes, Purnendu and Nirajo exhibited relatively less grain yield loss which suggests there genetic competence to adapt to low light stress by retaining a reduced Chl a/b ratio and permitting insignificant curtailment in photosynthetic rate during reproductive stage under the subjected stress condition. Thus, they can be recommended for cultivation in regions prone to low light stress. Keywords: Carbohydrate, chlorophyll, low light, photosynthesis, Oryza sativa Introduction Rice (Oryza sativa L.) is the staple food of more than half of the world’s population. In India, rice is cultivated over an area of 43.39 million hectares with reported productivity of 2.4 t/ha during 2015-16. Rice production is influenced by various environmental factors such as solar radiation, temperature, water availability and soil fertility. Among them, light (solar radiation) is considered a critical environmental factor that influences the growth and development of plants affecting their morphology, anatomy, physiological/biochemical attributes and ultimately the flowering time and productivity [1] . Low light is known to have pronounced effect on the entire photo-morphogenesis of rice plant resulting in increased plant height, reduced tillers and panicle number plant -1 , grains panicle -1 which leads to reduced grain yield [2] . Globally, about 95% of rice is produced in the wet season when low solar radiation is prevalent. The availability of sub-optimal solar radiation during this growth season acts as an abiotic stress limiting the rice production to 30-60 % of the dry season. There is sufficient evidence that low light intensity caused by overcast skies is the major factor impairing grain yield in rice, which averages around 1.2 t ha -1 during the wet season. Rice plant requires about 1500 bright sunshine (BSS) hours during the period between transplantation to maturity. But only 800-900 BSS hours are available during the wet season, which adversely affects the physiology of rice resulting in low grain yield and poor grain quality In addition, low-light stress significantly increases plant height, leaf area and the leaf thickness [3] . Chlorophyll a (Chl a) and chlorophyll b (Chl b), the principal photosynthetic pigments, increased under low light intensity due to the presence of much larger grana stacks and higher stacking degree [4] . The concomitant increase in Chl b with a reduction in a/b ratio was more apparent under low light stress [5] . Carbohydrate accumulation and transportation are negatively affected under low light. Low light during grain filling stage was reported to cause decreased starch synthase activity in grains, resulting in poor grain filling and thus low rice yield [6] . Attempts have been made on global scale to enhance the productivity of rice crop grown in wet season by improving crop management and selecting tolerant varieties which perform better under low light conditions. Various physiological, phenological and biochemical parameters associated with low light tolerance are yet to be identified. Development of prospective low light tolerant rice cultivars