Effects of high salinity irrigation on growth, gas-exchange, and photoprotection in date palms (Phoenix dactylifera L., cv. Medjool)

•CO2 assimilation in date palms is inhibited by salinity but photosynthetic efficiency and potential are preserved.•The main limitation to photosynthesis generated by salinity is through osmotically driven stomatal closure.•Toxic sodium ions are excluded from the leaves but the chloride ions are not toxic and are abundant in all plant tissues.•Excessive photochemical energy is diverted to alternative O2-related pathways in addition to thermal dissipation via NPQ. Date palms are widely cultivated in arid Mediterranean regions and require large quantities of water to produce commercial fruit yields. In these regions the plantations are commonly irrigated with low-quality water, which results in reduced growth and yields. To study the effect of using saline water for irrigation, date palm seedlings (cv. Medjool) were subjected to long-term irrigation treatments with water containing between 2 and 105mM NaCl. The effect of saline irrigation was determined according to leaf gas exchange, chlorophyll a fluorescence, growth parameters and the distribution of key minerals in different plant organs. High salinity decreased plant growth and increased Na+ accumulation in the roots and lower stem. However, Na+ ions were mostly excluded from the sensitive photosynthetic tissues of the leaf. Thus, the reduction in the CO2 assimilation rate was primarily attributed to a reduced stomatal conductance. Consistent with this finding, the photosynthetic response to variable intercellular CO2 concentrations (A/Ci curves) revealed no permanent damage to the photosynthetic apparatus and implicated developed photoprotective mechanisms. Independent of salinity treatment, 80% of the energy absorbed by the leaf was directed to non-photochemical quenching, as presented in electron-equivalent units. Functioning at full capacity, the non-photochemical mechanism could not compensate for all the excess irradiance. Thus, of the remaining absorbed energy, a significant portion was directed to photochemical O2 related processes, rather than CO2 prevented photoinhibition. The exclusion of toxic ions and O2-dependent energy dissipation maintained photosynthetic efficiency and supported survival under salt stress.