Sodium chloride improves photosynthesis and water status in the succulent xerophyte Zygophyllum xanthoxylum

Zygophyllum xanthoxylum, a C-3 woody species, is a succulent xerophyte that is well adapted to arid environments. Our previous investigations showed that Na+ has a positive effect on the growth of Z. xanthoxylum under drought conditions, which was closely related to high Na+ accumulation in leaves. To reveal the physiological mechanisms underlying how Na+ accumulation improves the drought resistance of Z. xanthoxylum, 3-week-old seedlings were treated with a series of additional external NaCl concentrations (5-150 mM) in sand culture experiments. Seedlings were also subjected to water deficit (30% of field water capacity) in the presence or absence of additional NaCl (50 mM) in pot experiments. The results indicated that 50 mM NaCl could mitigate deleterious impacts of water deficit on the growth of Z. xanthoxylum, by improving the relative water content, inducing a significant drop in leaf water potential and, concomitantly, increasing leaf turgor pressure and chlorophyll concentrations resulting in an enhancement of overall plant photosynthetic activity (i.e., photosynthetic rate and water use efficiency). Furthermore, NaCl (50 mM) could alleviate the inhibitory effect of water deficit on the activity of photosystem II in Z. xanthoxylum. The contribution of Na+ to the total osmotic potential varied from 8% in the control to 13% in plants subjected to water deficit and, surprisingly, to 28% in plants grown in the presence of 50 mM NaCl under water deficit; however, the contribution of K+ significantly decreased from 13 to 8%. These findings suggest that, under arid environments, Z. xanthoxylum is able to accumulate a high concentration of Na+ in its leaves and use it directly for osmotic adjustment, which was coupled with an improvement in leaf hydration and photosynthetic activity.