Effects of salinity on sodium content and photosynthetic responses of riceseedlings 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 expande d leaf. During 1-week treatment at 6 and 12 dS m(-1) salinity levels (equiv alent to about 60 and 120 mmol/L NaCl, respectively), the salt-sensitive cu ltivars Hitomebore, IR28 and Banket, but not the salt-tolerant cultivar Pok kali, showed significant increases in Na+/K+ content with increasing salini zation. Salinity caused a substantial reduction in carbon assimilation rate and stomatal conductance in all cultivars. Chlorophyll fluorescence measur ed in intact leaves showed that the potential photochemical efficiency of P SII (maximum quantum yield F-V/F-M was almost unaffected by salinity, where as the overall or actual efficiency of photochemical energy conversion tact ual quantum yield Delta F/F-M) declined with increasing salinity in all cul tivars except Pokkali. The total non-photochemical quenching increased sign ificantly with increasing salinity level in the salt-sensitive cultivars bu t 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.