Transpiration efficiencies of indica and improved tropical japonica rice grown under irrigated conditions

There is limited information on the transpiration efficiency defined as the ratio of photosynthesis (A) to transpiration (T) of tropical japonica rice (Oryza sativa L.). In this study, transpiration efficiency (A/T) of seven tropical japonica lines developed by the International Rice Research Institute (IRRI) were compared with seven indica cultivars one week after flowering in 1993. The gas exchange rate and AIT of one genotype from each type were compared throughout the growing season in 1994. Both A and T were measured on topmost fully expanded leaves under saturating light with a portable photosynthesis system (LI-6200). Indica cultivars had higher T than the tropical japonica lines. The differences in A between the two types were relatively small and inconsistent across growth stages and years compared with the differences in T. The AIT was 25% and 30% higher for the tropical japonica than the indica type in 1993 and 1994, respectively. The differences in :T and AIT between the two types were not related to the differences in leaf N content or leaf water content. A lower carbon isotope (C-13) discrimination in a tropical japonica line than an indica cultivar confirmed that the improved tropical japonica lines had higher AIT than the indica cultivars.