Effect of calcium on root development and root ion fluxes in salinised barley seedlings

The effects of various Na/Ca ratios on root growth, development, and ion acquisition patterns were studied in hydroponic experiments with barley (Hordeum vulgare L.) plants. In total, interactions between three different levels of salinity (1, 50 and 100 mM NaCl) and three different levels of Ca²⁺ (0.1, 1 and 10 mM) were studied (a full factorial experiment). Growth rate and biomass accumulation were significantly lower in salinised roots. In addition to reduction in extension growth, salinity also significantly affected plant developmental processes (for example reduced root hair density and root thickening). Supplemental Ca²⁺ significantly ameliorated those detrimental effects of salinity. Non-invasive, microelectrode ion-flux (MIFE) measurements showed that the onset of salt stress caused rapid and prolonged efflux of H⁺, K⁺ and NH₄⁺ from the root epidermis. This efflux could be significantly reversed, or completely prevented, by the presence of high Ca²⁺ concentration in the bath solution, even after several days of salt stress. Membrane potential measurements in root epidermal cells showed that high Ca²⁺ levels in the bath were able to restore (otherwise depolarised) membrane potential back to control level (-120 to -130 mV). At the same time, no significant impact of Ca²⁺ on net Na⁺ uptake in plant roots was found. Some limitations of the MIFE technique for study of Na⁺ uptake kinetics under saline conditions, as well as possible ionic mechanisms underlying the ameliorating Ca²⁺ effects on ion fluxes in roots of salt-stressed plants, are discussed.