Growth, gas exchange and ion content in <i>Olea europaea</i> plants during salinity stress and subsequent relief

Olive ( Olea europaea L. cv. Frantoio) plants grown hydroponically in a glasshouse were supplied with half‐strength Hoagland solutions containing 0, 50, 100, and 200 m M NaCl for 4 weeks and subsequently supplied with the standard solution without NaCl to relieve salinity stress. Two complete stress‐relief cycles were repeated on the same plant material during one growing season. Growth was inhibited at all salt levels, but most growth parameters of plants treated with 50 or 100 m M NaCl returned to control levels after 4 weeks of relief. More severely stressed plants (200 m M NaCl) recovered to only 60% of the growth of the controls after 4 weeks. During relief, plants treated with 50 and 100 m M NaCl had net photosynthetic rates and stomatal conductances higher than the controls. Increasing the NaCl concentration of the external solution from 0 to 200 m M decreased both leaf pre‐dawn water potential (from ‐0.3 to ‐1.0 MPa) and osmotic potential (from ‐2.1 to ‐2.7 MPa). The sodium concentration in the leaves of plants treated with 200 m M NaCl reached maximum levels of 211 and 388 m M (expressed on a tissue water basis) at the end of the first salinity and relief periods, respectively. Leaf chloride concentrations were 359 and 223 m M at the same sampling dates. These data indicate that the inhibitory effects of salinization on growth and gas exchange of the salt‐tolerant olive cv. Frantoio can be readily reversed when salinity is relieved, despite the marked accumulation of potentially toxic ions (Na + . Cl) in the leaf.