Comparison of Susceptibility to Photoinhibition and Energy Partitioning of Absorbed Light in Photosystem II in Flag Leaves of Two Rice (Oryza sativa L.) Cultivars that Differ in Their Responses to Nitrogen-Defi ciency

The energy partitioning in photosystem II (PSII) and the susceptibility to photoinhibition in PSII were investigated in flag leaves of two rice cultivars, Shirobeniya (a traditional japonica) and Akenohoshi (an improved japonica-indica intermediate) grown under standard-nitrogen (N) (SN) and low-N (LN) conditions. N-deficiency resulted in significant decreases in total dry weight, net photosynthetic rate (PN), the energy flux via carboxylation (Jc), and content of ribulose-1,5- bisphosphate carboxylase / oxygenase (Rubisco) in flag leaves in the two cultivars, and these parameters of Shirobeniya were lower than those in Akenohoshi under the LN condition. In the two cultivars, the energy flux via alternative electron flow was significantly increased by N-deficiency, which was accompanied by enhanced activity of superoxide dismutase (SOD). Although under the LN condition no cultivar differences were found in Ja and SOD, ascorbate peroxidase activity in Shirobeniya was lower than that in Akenohoshi. N-deficiency resulted in more significant increases in the susceptibility to photoinhibition (the degree of decrease in maximum quantum yield of PSII), hydrogen peroxide (H2O2) content and malondialdehyde content after exposure to high irradiance in Shirobeniya than those in Akenohoshi. These results indicated that the increased susceptibility to photoinhibition in the LN plants of Shirobeniya was mainly due to oxidative damages to chloroplasts, resulting from lower carboxylation and H2O2- scavenging capacities. Therefore, both carboxylation and H2O2-scavenging capacities could be important factors in determining the cultivar difference in the productivity of rice under LN conditions.