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

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/C_i 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.