Effect of salt stress on growth and cation compartmentation in leaves of two plant species differing in salt tolerance

Salinity can cause toxic symptoms, especially in mature leaves after long-term exposure. Thus, Na+ accumulation in leaves could be responsible for salt toxicity. The infiltration-centrifugation technique was employed for the isolation of apoplastic washing fluids (AWF) from leaves and the detection of Na+, K+ and Ca2+ was carried out by ion chromatography. The Na+ concentrations in the leaf apoplast of salt-sensitive corn and salt-tolerant cotton plants significantly increased with higher Na+ supply. Nevertheless, the apoplastic Na+ concentration did not exceed 10 and 30 mmol/L, respectively, after salt exposure up to 150 mmol/L in the medium in short- and long-term salt treatments. Higher Na+ concentrations were found in the leaf apoplast of salt-tolerant cotton in comparison to those of salt-sensitive corn, particularly in fully expanded leaves. No bound Na+ was found in the leaf apoplast. The Na+ concentration in the leaf apoplast did not reach high enough concentrations to be responsible for the decline in leaf growth under salinity. Supplemental Ca2+ did not affect Na+ concentration in the leaf apoplast under salinity. Apoplastic Ca2+ concentration remained constant, while K+ concentrations increased in the leaf apoplast under salinity. Our results do not support the hypothesis by Oertli (1968) who proposed that salt accumulation in the leaf apoplast could be responsible for the death of leaves in plants exposed to salinity.