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Gas exchange parameters, water relations and Na+/Cl‐ content were measured on leaves of one‐year‐old sweet orange (Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO3) were used to separate the effects of Cl− and Na+ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl− in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na+ concentrations of up to 478 mM in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca2+ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl− and Na+ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.

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