The Effects of Salinity and Light on Photosynthesis, Respiration and Chlorophyll fluorescence in Salt-tolerant and Salt-sensitive Wheat

Intra-specific variation in responses of diurnal and long-term photosynthesis, stomatal conductance, chlorophyll fluorescence, and respiration in spring wheat (Triticum aestivum L.) to salinity was investigated using two tolerant cultivars (CR and Kharchia-65) and a sensitive one (Ghods). Plants were grown in sand culture in controlled environment at selected levels of salinity (5 as control, 100, 200, and 300 mol m-3 NaCl and CaCl2 in 5:1 molar ratios). Photosynthesis was markedly reduced in the saline conditions, but there were no significant differences observed amongst cultivars. Stomatal conductance of both upper and lower leaf surfaces was the main factor limiting photosynthesis in the presence of salinity. However, non-stomatal limitations as indicated by reduction in variable to maximum fluorescence (Fv/Fm) showed that plants might experience some degree of photoinhibition at the highest level of salinity. Gas exchange in control and 100 mol m-3 did not change throughout the day in either the salt-sensitive or the salt tolerant cultivars but in the last hours of the day photosynthesis in the sensitive cultivar was markedly reduced. Respiration remained unchanged up to 200 mol m-3 salinity but at 300 mol m-3 it decreased as compared with the lower salinity levels. The most remarkable change in respiration rate was that at seven hours after light when CO2 production was much higher than at the other times of the day. The results indicated that although photosynthesis is well correlated with stomatal conductance, wheat genotypes show different responses as regards other such aspects of photosynthesis, in different salt concentrations, as growth stage, time of the day and duration of salt exposure.