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Salt stress represents one of the environmental challenges that drastically affect plant growth and yield. Evidence suggests that glycophytes and halophytes have a salt tolerance mechanisms working at the cellular level, and the plasma membrane (PM) is believed to be one facet of the cellular mechanisms. The responses of the PM transport proteins to salinity in contrasting species/cultivars were discussed. The review provides a comprehensive overview of the recent advances describing the crucial roles that the PM transport systems have in plant adaptation to salt. Several lines of evidence were presented to demonstrate the correlation between the PM transport proteins and adaptation of plants to high salinity. How alterations in these transport systems of the PM allow plants to cope with the salt stress was also addressed. Although inconsistencies exist in some of the information related to the responses of the PM transport proteins to salinity in different species/cultivars, their key roles in adaptation of plants to high salinity is obvious and evident, and cannot be precluded. Despite the promising results, detailed investigations at the cellular/molecular level are needed in some issues of the PM transport systems in response to salinity to further evaluate their implication in salt tolerance.

Summary Winter wheat seedlings were grown in Hoagland nutrient solution with or without 100 mmol/L NaCl added. Plasma membranes from root cells were prepared by aqueous polymer two phase partitioning and the stimulation of plasma membrane ATPase activity by Mg 2+ and Ca 2+ was investigated. The enzyme was activated by Mg 2+ or Ca 2+ and Mg 2+ -ATPase was further stimulated by K + , whereas no such stimulation was observed with Ca 2+ -ATPase. Ca 2+ inhibited Mg 2+ -ATPase activity. pH optima for Mg 2+ - and Ca 2+ -ATPase activities were 6 and 5.6, respectively. ATPase activated by Mg 2+ was inhibited by vanadate and DCCD, while Ca 2+ -ATPase was less sensitive. ATP was the preferred substrate but other nucleotide tri- and diphosphates were also hydrolyzed by the plasma membrane ATPases. NaCl stress during growth had no major effect on the plasma membrane polypeptide components. Simple Michaelis-Menten kinetics were found when Mg 2+ - or Ca 2+ -ATP was used as substrate for both control and salt stressed roots. NaCI markedly reduced the V max of Mg 2+ - and Ca 2+ -ATPase activities while it had little effect on the apparent K m for Mg 2+ - or Ca 2+ -ATP. NaCl-induced reduction of V max of ATPase activity correlated with NaCl-altered plasma, membrane lipid composition and the role of the lipid environment in ATPase activity is discussed.

Summary This research was undertaken to study ionic (NaCl) and osmotic (mannitol) effects on the plasma membrane properties of inner epidermal cells of onion bulb scales. The influence of Ca2+ in counteracting the Na-induced cellular alterations was also studied. Cell membrane permeability for individual intact cells and cell viability were determined by the plasmolytic method, and lipid partiality was evaluated. Na+, and not mannitol, a) increased membrane permeability to urea, methylurea and ethylurea, b) decreased membrane partiality to lipid permeators, and c) increased the number of cells showing swollen protoplasm and number of dead cells. These effects resulting from Na+ were prevented by Ca+ when it was added either simultaneously with Na+ or separately after Na+ exposure. The results suggested that NaCl disrupted the plasma membrane structure, which is most probably initiated by the loss of its Ca2+. The protoplasmic swelling is a symptom of salt injury resulting from membrane disruption and subsequent Na+ accumulation in the cytoplasm, which was a consequence of water uptake. A deleterious effect of salinity on the plasma membrane must be attributed to ion toxicity rather than to osmotic stress.