Effects of sodium chloride on water potential components, hydraulic conductivity, gas exchange and leaf ultrastructure of Arbutus unedo plants

The purpose of this study was to evaluate the physiological and anatomical changes that occur in Arbutus unedo plants under saline conditions in order to understand the response of this species to salinity. A. unedo plants were grown in a greenhouse and submitted to three irrigation treatments using solutions containing 0, 52, and 105 mM NaCl with an electrical conductivity of 0.85 dS m−1 (control treatment), 5.45 dS m−1 (S.1) and 9.45 dS m−1 (S.2). After 16 weeks, the leaf water relations, root hydraulic conductivity, gas exchange, ion concentrations and leaf ultrastructure were determined. Salinity induced a significant decrease in total biomass, leaf area and plant height. The concentration of Cl− in leaves increased with increasing salinity and was higher than the corresponding concentration of Na+. Net photosynthesis (Pn) was reduced and the chloroplast ultrastructure was altered by salinity. Thylakoids were dilated and the number of plastoglobuli was greatly increased in both saline treatments compared with the control leaves. In addition, a reduction in the intercellular spaces of the lagunar mesophyll was observed in the saline treatments, affecting stomatal and mesophyll conductance to CO2. Root hydraulic resistance increased under saline conditions, affecting the water flow the root system. Pressure–volume analysis revealed osmotic adjustment values of 0.2 MPa at 52 mM and 0.5 MPa at 105 mM of NaCl, accompanied by 31 and 99% increases in the bulk tissue elastic modulus (ɛ, wall rigidity) and resulting in turgor loss at the same relative water content in control and at 5.45 dS m−1 and a higher relative water content at 9.45 dS m−1. Osmotic adjustment and a high ɛ together are seen as an effective means of counteracting the negative effects of salinity on the water balance of A. unedo plants.