ADAPTATION TO WATER DEFICITS IN 3 GRAIN LEGUME SPECIES - MECHANISMS OF TURGOR MAINTENANCE

Adaptation to water deficit, in terms of maintenance of leaf turgor pressure, was studied in four grain legumes - two soybean cultivars (Glycine max cvs. Valder and Forrest) and one cultivar each of cowpea (Vigna unguiculata cv. Banjo) and black gram ( Vigna mungo cv. Regur). Experiments, in which high and low water deficit treatments were created by varying water supply, were conducted in both the field and glasshouse. Turgor pressures were derived from measurements of water and osmotic potentials. The species differed in the methods of maintaining turgor pressure. In cowpea and black gram, regulation of leaf water potential was predominant, while in the soybean cultivars solute accumulation played a more important role, reaching values of up to 0.5 MPa at a water potential of -1 MPa. Regulation of water potential seemed to be effected by reductions in leaf area and, presumably, low resistances to water flow, though stomatal conductance did not seem to be an important cause of regulation of water potential under conditions of protracted water deficits in the field. Changes in the water potential at zero turgor with ontogeny were associated with increases in background solutes in all species. Values varied from 0.1 MPa to 0.4 MPa. However, solute accumulation in response to water stress did not change with ontogeny or leaf position.