The detrimental effects of salinity can be partially alleviated by external calcium. Specific cellular mechanisms of such amelioration remain unclear. We addressed this issue by studying effects of Ca2+ on root nutrient uptake and plant physiological characteristics in hydroponically grown pea and barley.
Supplemental Ca2+ significantly reduced detrimental impact of salinity on plants. Neither photosynthetic characteristics (pigment content, Fv/Fm, Fo, Fm, Y, Fm' values), nor leaf growth were significantly different from control even at 100 mM NaCl levels when high (10 mM) Ca2+ was present. The supplemental Ca2+ also significantly ameliorated detrimental NaCl effects on plant development (leaf and tiller numbers; root hair density).
Onset of NaCl stress caused rapid and prolonged net efflux of H+, K+ and NH4+ from the root epidermis. This efflux was prevented, or even reversed, by the presence of high Ca2+ in the bath solution, even if added after several days of salt stress. The effect was Ca-specific and was not observed with 10 mM Mg. In root epidermal cells, high Ca2+ levels in the bath were able to restore NaCl-depolarised membrane potential to control (-120 to -130 mV). At the same time, no significant impact of Ca2+ on net Na+ uptake in plant roots was observed.
From the above data, it appears that K+ outward rectifying channels, but not voltage independent non-selective cation channels, are regulated by supplemental Ca2+ under saline conditions. This conclusion is further supported by nutrient analysis of leaf and root samples and cell sap osmolarity. We suggest that future research focus on mechanisms of xylem loading or Na+ compartmentation in leaf mesophyll cells, rather then on mechanisms of salt exclusion by roots.
