Isoosmotic regulation of cotton and peanut at saline concentrations of k and na.

Peanut (Arachis hypogaea L.) and cotton (Gossypium hirsutum) plants were grown for 4 weeks in saline, isoosmotic rooting substrates with different proportions of K and Na. Isoosmotic media did not affect growth (except at the highest external K concentrations) or estimates of intracellular osmotic pressure in expanding leaves (i.e. osmotic pressure of leaf sap and intracellular osmotic pressure as calculated from pressure-volume curves). In expanded leaves, an increase in the proportion of external K increased sap osmotic pressure. The sum of [K+Na+Cl] in the sap of expanding and expanded leaves accounted for the effect of isoosmotic media on the concentration of osmolytes with high electrical conductance, so the difference between sap osmotic pressure and [K+Na+Cl] accounted for the concetration of osmolytes with low conductance. In expanding leaves, an increase in the proportion of external K increased [K+Na+Cl] and decreased the concentration of osmolytes with low conductance. In expanded leaves, an increase in the proportion of external K increased [K+Na+Cl] to approximately the same extent as sap osmotic pressure. Isoosmotic regulation was apparent in expanding leaves but not evident in expanded leaves. This suggests a turgor homeostat which can influence the concentration of organic solutes in expanding leaves but cannot control the import of inorganic solutes from a rooting medium nor the total production of organic solutes in plants with a low sink:source ratio.