ABSTRACIPeanut (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 pressurevolume curves). In expanded leaves, an increase in the proportion of external K increased sap osmotic pressure. The sum of [K+Na+ClJ 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+ClJ accounted for the concentration of osmolytes with low conductance. In expanding leaves, an increase in the proportion of external K increased [K+Na+CIl and decreased the concentration of osmolytes with low conductance. In expanded leaves, an increase in the proportion of external K increased [K+Na+Cli 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.In response to salinity or water deficit, the tissues of some higher plants undergo complete osmotic adjustment, i.e., accumulate solutes such that A7r2 and cell turgor are restored to their original values (24). Osmotic adjustment is considered to be a form of adaptation to salt stress and water deficit (10), but it is not known if tissues of nonhalophytic angiosperms contain a turgor homeostat (9, 23), as do many species of halophytic algae, which can detect changes in cell turgor and subsequently signal turgor restoration (5). Low growth rate and high solute availability will increase ri and turgor in the absence ofa turgor regulatory mechanism (5,23 (23).The capacity of some higher plants to maintain a particular Wi and cell turgor by the use of different solutes a the same ir0, defined to be isoosmotic regulation (27), has been cited as evidence for a mechanism of osmotic and turgor homeostasis (5, 14, 16). Different isoosmotic media can have a negligible effect on growth (1, 18) so that the effect of isoosmotic media on the composition of a tissue can be determined in the absence of interactions between growth rate and solute transport. However, past claims of isoosmotic regulation lack an assessment of the response of iri to the total amount of external solutes absorbed by a tissue. There are several cases in which -xi was affected by the composition of isoosmotic media (13,21,23) and in some cases ofisoosmotic regulation the differences in the total concentration of inorganic osmolytes in a tissue were not enough to significantly c...
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