Physiological Control of Chloride Transport in Chara corallina: II. THE ROLE OF CHLORIDE AS A VACUOLAR OSMOTICUM.

The extent to which Cl(-) is replaceable as the major anionic constituent of the vacuole of Chara corallina was investigated. It was found that external Cl(-) is not essential in order for nongrowing cells to increase internal osmotic pressure. After growth of cells in low (9 micromolar) Cl(-), the vacuolar Cl(-) concentration is one-half that of cells grown at normal external Cl(-) concentration (850 micromolar). In contrast, both internal osmotic pressure and total concentration of the major cations, K(+) and Na(+), in the same cells were found to be only slightly sensitive to the external Cl(-) concentration. Thus, it is proposed that, at limiting external Cl(-) concentration, the cell is able to transport or synthesize another anion for vacuolar use rather than utilize a neutral solute.Although the total vacuolar (K(+) + Na(+)) concentration is relatively insensitive to low Cl(-) conditions, a large increase in K(+) was recorded. This increase in K(+) was offset by a correspondingly large fall in Na(+). This is unrelated to any mechanistic dependence of Na(+) on Cl(-) for entry into the cell but may indicate a control system acting on vacuolar sequestration of both Na(+) and Cl(-). Cells grown in low Cl(-) display an ability to take up Cl(-), at enhanced rates, from the medium. The enhancement of Cl(-) influx is maintained for several hours after external Cl(-) is raised to a high level and indicates the existence of a control on Cl(-) influx acting in addition to that of cytoplasmic Cl(-) concentration, which has a shorter decay time.The results are discussed in relation to similar work on higher plants.