Effects of salinity and turgor on calcium influx in Chara

Measurements were made of the influx of ⁴⁵Ca into internodal cells of Chara corallina in solutions containing high concentrations of NaCl. Increasing salinity in the range 4–100mol m^?3 NaCl resulted in a doubling of Ca²⁺ influx at the plasmalemma. A time-course of Ca²⁺ influx in 50 mol m^?3 NaCl, 0.5mol m^?3 CaCl₂ showed that while influx at the plasmalemma increased only 1.5-fold, influx to the vacuole increased by up to 15-fold. This was interpreted as being due to inhibition of active Ca²⁺ efflux from the cell. The stimulation of Ca²⁺ influx by increasing salinity appeared to be principally a response to reduced turgor since similar stimulations were obtained when turgor was reduced by NaCl, Na₂SO₄ or mannitol. When cells were plasmolysed Ca²⁺ influx increased by 10–20-fold. The increased permeability was relatively specific for Ca²⁺ and was inhibitable by La³⁺. Survival of cells in high salt conditions was increased by 30 mmol m^?3 La³⁺, which inhibited Ca²⁺ influx. Paradoxically, survival can also be extended by increasing external Ca²⁺ which leads to a higher influx. Therefore, it seems unlikely that the ameliorative effect of Ca²⁺ on the sensitivity of plants to high NaCl is mediated by Ca²⁺ entry across the plasmalemma. It seems more likely that the principal role of Ca²⁺ under these conditions is exerted externally through the control of membrane voltage and permeability.