Interactions between growth, Cl− and Na+ uptake, and water relations of plants in saline environments. I. Slightly vacuolated cells

Abstract. Slightly vacuolated cells, i.e. microalgae and meristematic cells of vascular plants, maintain low Cl^? and Na⁺ concentrations even when exposed to a highly saline environment. The factors regulating the internal ion concentration are the relative rate of volume expansion, the membrane permeability to ions, the electrical potential, and the active ion fluxes. For ion species which are not actively transported, a formula is developed which relates the internal concentration to the rate of expansion of cell volume, the permeability of membranes to that ion, and the electrical potential. For example, when the external concentration of Cl^? is high, and Cl^? influx is probably mainly passive, the formula predicts that rapid growth keeps the internal Cl^? concentration lower than that in a non-growing cell with the same electrical potential; this effect is substantial if the plasmalemma has a low permeability to Cl^?. For ion species which are actively transported, the rate of pumping must be considered. For instance Na⁺ concentrations are kept low mainly by an efficient Na⁺ extrusion pump which works against the electric field across the membrane. The requirement for Na⁺ extrusion is related to the external Na⁺ concentration, the rate of expansion of cell volume, the membrane permeability, and the electrical potential. It is possible that microalgae have a more positive electrical potential than many other plant cells; if so, requirements for high rates of active Na⁺ extrusion will be lower. The required rates of Na⁺ extrusion are lower during rapid growth, provided that the permeability of the plasmalemma to Na⁺ is low. The energy required for the regulation of Cl^? and Na⁺ concentrations is low, especially in rapidly expanding cells where Na⁺ extrusion requires only 1–2% of the energy normally produced in respiration. The exclusion of these ions, however, must be accompanied by the synthesis of enough organic compounds to provide adequate osmotic solutes for the increases in volume accompanying growth. This process reduces the substrates available for respiration and synthesis of cell constituents, but the reduction is not prohibitively large—even for cells growing in 750 mol m^?3 NaCl, the carbohydrate accumulated as osmotic solute is only 10% of that consumed in respiration.