Contribution of leaf nitrogen to photosynthetic gas exchange in contrasting rice (Oryza sativa L.) cultivars during the grain-filling period

Photosynthetic parameters and leaf carbon isotope composition (delta C-13) in contrasting rice genotypes in relation to supplemental nitrogen (N) application and water management during the grain-filling period were compared. The changes in stomatal conductance (g (s)) and ratio of intercellular to ambient CO2 mole fraction (C (i)/C (a)) depended on the leaf nitrogen concentration (leaf N) in both 'Hinohikari' (temperate japonica genotype) and 'IR36' (indica genotype). In 'Hinohikari', delta C-13 reflects photosynthetic gas exchange during the grain-filling period, which is indicated by the significant response of delta C-13 to leaf N. In contrast, in 'IR36' delta C-13 did not depend on leaf N. This varietal difference in delta C-13 to leaf N can be attributed to a difference in the timing of leaf senescence. In 'IR36', leaf N and photosynthetic parameters decreased more rapidly, indicating earlier senescence and a shorter grain-filling period in comparison with 'Hinohikari'. The significant increase in shoot dry mass in 'Hinohikari' resulting from supplemental N application, compared with nonsignificant effect observed in 'IR36', suggests that the timing of senescence in relation to the grainfilling period has a preponderant influence on productivity.