Effects of salinity on sodium content and photosynthetic responses of rice seedlings differing in salt tolerance

The effects of salinity on four rice (Oryza sativa L.) cultivars differing in salt tolerance were investigated with respect to gas-exchange parameters, chlorophyll fluorescence and Na+/K+ content in the youngest fully expanded leaf. During 1-week treatment at 6 and 12 dS m(-1) salinity levels (equivalent to about 60 and 120 mmol/L NaCl, respectively), the salt-sensitive cultivars Hitomebore, IR28 and Banket, but not the salt-tolerant cultivar Pokkali, showed significant increases in Na+/K+ content with increasing salinization. Salinity caused a substantial reduction in carbon assimilation rate and stomatal conductance in all cultivars. Chlorophyll fluorescence measured in intact leaves showed that the potential photochemical efficiency of PSII (maximum quantum yield F-V/F-M was almost unaffected by salinity, whereas the overall or actual efficiency of photochemical energy conversion tactual quantum yield Delta F/F-M) declined with increasing salinity in all cultivars except Pokkali. The total non-photochemical quenching increased significantly with increasing salinity level in the salt-sensitive cultivars but not in the tolerant Pokkali. These results suggest that salt sensitivity in rice is associated with increased shoot Na+ accumulation, decreased PSII photochemical efficiency, and enhanced non-photochemical quenching.