The K/Na replacement and function of antioxidant defence system in sugar beet (Beta vulgaris L.) cultivars

Salinity is one of the major constrains restricting crop productivity. Sugar beet is one crop species grown mainly on salinity-affected soils. In a preliminary experiment in this work, the effect of different NaCl concentrations was studied on the growth of six cultivars of sugar beet (Beta vulgaris L.) in hydroponic medium. After selection of two contrasting cultivars, growth, sodium accumulation and distribution among leaf fractions as well as the functional significance of the antioxidant defence system under mild (50 mM) and severe salinity (200 mM) were investigated. NaCl treatment exerted either a stimulatory or an inhibitory effect on shoot growth, depending on concentration and cultivar. Higher growth improvement under mild salinity was observed in cultivar IC simultaneous with higher tolerance to severe saline treatment. Although two tested cultivars did not differ in Na concentration of leaves, its distribution among leaf fractions (apoplasmic fluid, cell wall, and cell sap) responded differentially to salinity treatment. Under mild salinity, proportional Na in cell sap of cultivar IC was greater than that of cultivar 7233, leading to greater replacement of K by Na in the former cultivar. In leaves of 7233, in contrast, proline was the major osmoticum. Activity of H2O2-scavenging enzymes (APX, CAT, and POD) was induced by salinity, keeping H2O2 concentration low under mild but not severe salinity. Shoot and root concentration of superoxide radicals was related to differential response of cultivars to salinity, i.e. IC was much more protected against superoxide radicals because of higher SOD activity than was 7233. Activity of nitrate reductase was reduced by both salinity levels and caused reduction of total free amino acids and protein concentrations. Results suggested that the main cause of cultivar difference is higher replacement of K by Na and allocation of more Na to the symplasm under mild salinity, and an indigenously higher protection against superoxide radicals under severe salinity.