Photosynthetic limitations in olive cultivars with different sensitivity to salt stress

Olive (Olea europea L) is one of the most valuable and widespread fruit trees in the Mediterranean area. To breed olive for resistance to salinity, an environmental constraint typical of the Mediterranean, is an important goal. The photosynthetic limitations associated with salt stress caused by irrigation with saline (200 mm ) water were assessed with simultaneous gas‐exchange and fluorescence field measurements in six olive cultivars. Cultivars were found to possess inherently different photosynthesis when non‐stressed. When exposed to salt stress, cultivars with inherently high photosynthesis showed the highest photosynthetic reductions. There was no relationship between salt accumulation and photosynthesis reduction in either young or old leaves. Thus photosynthetic sensitivity to salt did not depend on salt exclusion or compartmentalization in the old leaves of the olive cultivars investigated. Salt reduced the photochemical efficiency, but this reduction was also not associated with photosynthesis reduction. Salt caused a reduction of stomatal and mesophyll conductance, especially in cultivars with inherently high photosynthesis. Mesophyll conductance was generally strongly associated with photosynthesis, but not in salt‐stressed leaves with a mesophyll conductance higher than 50 mmol m^?2 s^?1. The combined reduction of stomatal and mesophyll conductances in salt‐stressed leaves increased the CO₂ draw‐down between ambient air and the chloroplasts. The CO₂ draw‐down was strongly associated with photosynthesis reduction of salt‐stressed leaves but also with the variable photosynthesis of controls. The relationship between photosynthesis and CO₂ draw‐down remained unchanged in most of the cultivars, suggesting no or small changes in Rubisco activity of salt‐stressed leaves. The present results indicate that the low chloroplast CO₂ concentration set by both low stomatal and mesophyll conductances were the main limitations of photosynthesis in salt‐stressed olive as well as in cultivars with inherently low photosynthesis. It is consequently suggested that, independently of the apparent sensitivity of photosynthesis to salt, this effect may be relieved if conductances to CO₂ diffusion are restored.