Ionic relations of Phillyrea latifolia L. plants during NaCl stress and relief from stress

Two experiments were conducted on Phillyrea latifolia L. plants exposed to increasing NaCl concentrations at the root zone in hydroponic culture. Growth, ion fluxes within the plant, and rates of excretion by glandular trichomes were measured during both salinity stress and relief periods. The reduction in relative growth rate (RGR) of plants treated with 100 and 200 mM external NaCl was more marked when RGR was calculated on a fresh weight (FW) basis than on a dry weight (DW) basis. The RGR of previously salt-treated plants, expressed on a FW basis, did not differ from that of the controls over 5 weeks of relief from stress, whereas RGR calculated on a DW basis did not fully recover. Fluxes of Na+ and K+ appeared highly regulated. Sodium transport to the leaf of 100 mM salt-treated plants equalled that of the controls, whereas Na+ transport to the leaf was higher in 200 mM salt-treated plants. Selectivity ratio for K+ and Na+ transport to the leaf was increased by salt treatments. The exclusion ability for Cl- was markedly lower than that for Na+ at 300 and 400 mM external NaCl. The excretion of Na+ and Cl- by glandular trichomes was very low at all external NaCl concentrations and substantially higher in basal leaves than in apical leaves. These results indicate that in P. latifolia, mechanisms of salt tolerance operate by excluding sodium and maintaining high selectivity for uptake and transport of K+, whereas the excretion of toxic ions is of minor significance.