Effect of Divalent Cations on Na+Permeability ofChara corallinaand Freshwater GrownChara buckellii

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Abstract
The effect of elevated Na+ concentration on Na+ permeability (PNa) and Na+ influx in the presence of two levels of external divalent cations was determined in Chara corallina and freshwater-cultured Chara buckellii. When Na+ in the medium was increased from 1.0 to 70 mol m−3, Na+ influx increased in both species if Ca2+ was low (0.1 mol m−3). If Ca2+ was increased to 7.0 mol m−3 when Na+ was increased, Na+ influx remained at the low control level in C. corallina, and showed only a temporary increase in C. buckellii. Mg2+ was a better substitute for Ca2+ in C. buckellii than in C. corallina. Na+ permeability data suggest that when the external Ca2+ concentration is low, PNa does not increase in the presence of elevated NaCl; the increase in Na+ influx appears to be due to the increase in external Na+ concentration alone. Ca2 + supplementation appears to decrease PNa whereas supplemental Mg2+ has no effect. Na+ effluxes were computed from previously determined net fluxes and the influxes. It was found that for both species, fluxes in both directions were stimulated in response to all experimental treatments, but Na+ influx always exceeded efflux. This resulted in net Na+ accumulation in the vacuoles of both species. The results are discussed with reference to net flux and electrophysiological data obtained previously under identical conditions, as well as the comparative salinity tolerance of both species and the Na+/divalent cation ratio.