EFFECTS OF SALINITY AND TURGOR ON CALCIUM INFLUX IN CHARA

Measurements were made of the influx of Ca-45 into internodal cells of Chara corallina in solutions containing high concentrations of NaCl. Increasing salinity in the range 4-100 mol m-3 NaCl resulted in a doub ling of Ca2+ influx at the plasmalemma. A time-course of Ca2+ influx i n 50 mol m-3 NaCl, 0.5 mol m-3 CaCl2 showed that while influx at the p lasmalemma increased only 1-5-fold, influx to the vacuole increased by up to 15-fold. This was interpreted as being due to inhibition of act ive Ca2+ efflux from the cell. The stimulation of Ca2+ influx by incre asing salinity appeared to be principally a response to reduced turgor since similar stimulations were obtained when turgor was reduced by N aCl, Na2SO4 Or mannitol. When cells were plasmolysed Ca2+ influx incre ased 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+ inf lux. Paradoxically, survival can also be extended by increasing extern al Ca2+ which leads to a higher influx. Therefore, it seems unlikely t hat the ameliorative effect of Ca2+ on the sensitivity of plants to hi gh NaCl is mediated by Ca2+ entry across the plasmalemma. It seems mor e likely that the principal role of Ca2+ under these conditions is exe rted externally through the control of membrane voltage and permeabili ty.