Abstract:     Increasing leaf photosynthesis per area (A) is of great importance to achieve yield further improvement. The aim of this study was to exploit varietal difference in A and its correlation with specific leaf weight (SLW). Twelve rice cultivars, including 6 indica and 6 japonica varieties, were pot-grown under two N treatments, low N (LN) and sufficient N (SN). Leaf photosynthesis and related parameters were measured at tillering stage. Compared with LN treatment, A, stomatal conductance (g_s), mesophyll conductance (g_m), leaf N content (Narea), and chlorophyll content were significantly improved under SN treatment, while SLW and photosynthetic N use efficiency (PNUE) were generally decreased. Varietal difference in A was positively related to both gs and gm, but not related to Narea. This resulted in a low PNUE in high Narea leaves. Varietal difference in PNUE was generally negatively related to SLW. Response of PNUE to N supply varied among different rice cultivars, and interestingly, the decrease in PNUE under SN was negatively related to the decrease in SLW. With a higher N_area, japonica rice cultivars did not show a higher A than indica rice cultivars because of possession of high-SLW leaves. Therefore, varietal difference in A was not related to N_area, and SLW can substantially interfere with the correlation between A and N_area. These findings may provide useful information for rice breeders to maximize A and PNUE, rather than over reliance on N_area as an indicator of photosynthetic performance.

Key words: specific leaf weight ,  leaf nitrogen content ,  leaf photosynthesis ,  mesophyll conductance ,  photosynthetic nitrogen use efficiency ,  stomatal conductance