Disentangling the contributions of osmotic and ionic effects of salinity on stomatal, mesophyll, biochemical and light limitations to photosynthesis

There are conflicting opinions on the relative importance of photosynthetic limitations under salinity. Quantitative limitation analysis of photosynthesis provides insight into the contributions of different photosynthetic limitations, but it has only been applied under saturating light conditions. Using experimental data and modelling approaches, we examined the influence of light intensity on photosynthetic limitations and quantified the osmotic and ionic effects of salinity on stomatal (L-S), mesophyll (L-M), biochemical (L-B) and light (L-L) limitations in cucumber (Cucumis sativusL.) under different light intensities. Non-linear dependencies of L-S, L-M and L-L to light intensity were found. Osmotic effects on L-S and L-M increased with the salt concentration in the nutrient solution (S-s) and the magnitude of L-M depended on light intensity. L-S increased with the Na+ concentration in the leaf water (S-l) and its magnitude depended on S-s. Biochemical capacity declined linearly with S-l but, surprisingly, the relationship between L-B and S-l was influenced by S-s. Our results suggest that (1) improvement of stomatal regulation under ionic stress would be the most effective way to alleviate salinity stress in cucumber and (2) osmotic stress may alleviate the ionic effects on L-B but aggravate the ionic effects on L-S. Interactions between osmotic and ionic effects on photosynthetic limitations were found. Osmotic stress of salinity has no effect on biochemical capacity but affects the magnitude of biochemical limitation. By applying quantitative limitation analysis at non-saturating light conditions, non-linear dependencies of stomatal, mesophyll and light limitations on light intensity were found. Our results strongly suggest that quantitative limitation analysis for saturating light conditions do not represent the photosynthetic limitation at canopy level.