Comparative physiological analysis of salinity tolerance in rice

Comparative physiological analysis was performed to understand the differences in the mechanisms of salinity tolerance in two rice (Oryza sativa L.) cultivars (the tolerant cultivar CFX18 and the susceptible cultivar Juma67). It was found that growth was significantly decreased in Juma67, but not in CFX18, during 10 d of salinity treatment. Under high salinity conditions, CFX18 maintained a better physiological status as determined by higher leaf water potential and a lower electrolyte leakage ratio compared to Juma67. Analysis of photosynthesis-intercellular carbon dioxide (CO2) concentration response curves revealed decreases in both carboxylation efficiency and maximum photosynthetic CO2 fixation rate in Juma67 in response to salinity stress, suggesting reduced photosynthetic capacity in this cultivar. In addition, it was observed that under high salinity conditions, Juma67 accumulated 3.5 times more sodium ions (Na+) in the leaves compared to CFX18. To corroborate this observation, quantitative expression analysis was conducted to examine changes in the transcript levels of OsHKT1;5, which is responsible for the regulation of Na+ accumulation in shoots. It was found that salinity stress repressed expression of the OsHKT1;5 gene in the leaves of Juma67, but not CFX18. These results suggest that CFX18 and Juma67 differ in their profiles of Na+ accumulation under salinity stress because of differences in OsHKT1;5 expression. Thus, the mechanism of salinity tolerance seen in CFX18 relies on regulating Na+ accumulation in leaves and this trait would be useful for improving salinity tolerance in rice.