Physiological traits of sodium toxicity and salt tolerance

Abstract

Worldwide up to 20 % of the irrigated arable land is already salt-affected; that portion is still expanding. The decline in crop productivity in salt-affected soils is caused by several factors, including salt-induced mineral perturbations in plants, e.g. K ⁺ and Ca ²⁺ deficiencies. The salt-induced growth inhibition is especially pronounced in leaves as compared to roots. It is well known that salinity causes toxic symptoms, especially in older leaves after long-term exposure. In addition, a relationship has been shown between high Na ⁺ concentrations in older leaves and the death of those leaves. Therefore, Na ⁺ accumulation in leaves, particularly in the leaf apoplast, could be responsible for Na+ toxicity in corn leaves. Furthermore, lower Na ⁺ concentrations in leaves of a more salt tolerant corn cultivar were found as compared to higher Na ⁺ concentrations in a salt sensitive corn cultivar. Thus, it is also of interest if the more tolerant corn cultivar exhibits a lower Na ⁺ concentration in the leaf apoplast under salt stress. In this context, more salt tolerant (cotton) and salt sensitive crops (rice) were also used in order to compare the results with corn. Mineral ion analyses were carried out in the apoplast, symplast and in whole leaves after salt treatment. The determination of Na ⁺, K ⁺ and Ca ²⁺ in the leaves was carried out by ion chromatography. In addition, Na ⁺ sensitive fluorescent dyes were used to detect the Na ⁺ concentration in the leaf apoplast by in vivo fluorescence ratio imaging. The Na ⁺ concentration in the leaf apoplast of a salt sensitive corn cultivar (Pioneer 3751) significantly increased with higher Na ⁺ supply. In contrast, a lower apoplastic Na ⁺ concentration was found in leaves of a more salt tolerant corn cultivar (Pioneer 3769). In comparison to corn, higher Na ⁺ concentrations were found in leaves of more salt tolerant cotton during salinity, whereas similar low Na+ concentrations were found in the leaf apoplast of salt sensitive rice plants. Nevertheless, the apoplastic Na ⁺ concentrations did not exceed about 20 mM at a NaCl concentration of 150 mM in the medium. Therefore, the Na+ concentration in the leaf apoplast was not high enough to be responsible for the decline in leaf growth. The K ⁺ and Ca ²⁺ concentrations in whole leaves decreased with salt treatment, while K ⁺ increased and Ca ²⁺ remained constant in the leaf apoplast under salt stress.