Abstract: To study the effect of phosphorus (P) deficiency on leaf photosynthesis and carbohydrates partitioning and to determine whether the characteristics of leaf photosynthesis and carbohydrates partitioning are related to low P tolerance in rice plants, a hydroponic culture experiment supplied with either sufficient P (10 mg /L) or deficient P (0.5 mg /L) was conducted by using two rice genotypes different in their responses to low P stress. Results showed that the plant growth of Zhenongda 454 (low P tolerant genotype) was less affected by P deficiency compared with Sanyang’ai (low P sensitive genotype). Under P-deficient conditions, photosynthetic rates of Zhenongda 454 and Sanyang’ai were decreased by 16% and 35%, respectively, and Zhenongda 454 showed higher photosynthetic rate than Sanyang’ai. Phosphorus deficiency decreased the stomatal conductance for both genotypes, but had no significant influence on leaf internal CO₂ concentration (C_i), suggesting that the decrease in leaf photosynthetic rate of rice plants induced by P deficiency was not due to stomatal limitation. Phosphorus deficiency increased the concentration of soluble carbohydrates and sucrose in shoots and roots for both genotypes, and also markedly increased the allocation of soluble carbohydrates and sucrose to roots. Under deficient P supply, Zhenongda 454 had higher root/shoot soluble carbohydrates content ratio and root/shoot sucrose content ratio than Sanyang’ai. In addition, phosphorus deficiency increased the concentration of starch in roots for both genotypes, whereas had no effect on the content of starch in shoots or roots. Compared to genotype Sanyang’ai, the better tolerance to low-P stress of Zhenongda 454 can be explained by the fact that Zhenongda 454 maintains a higher photosynthetic rate and a greater ability to allocate carbohydrates to the roots under P deficiency.

Key words: carbohydrate, phosphorus deficiency, photosynthesis, rice