LEAF WATER RELATIONS, OSMOTIC ADJUSTMENT, CELL-MEMBRANE STABILITY, EPICUTICULAR WAX LOAD AND GROWTH AS AFFECTED BY INCREASING WATER DEFICITS IN SORGHUM

A field experiment was conducted with a non-irrigated water stress treatment and an irrigated control using four sorghum (Sorghum bicolor L. Moench) cultivars. We investigated the effects of water deficits on leaf water relations, osmotic adjustment, stomatal conductance, cuticular conductance, cell membrane stability (CMS) measured by the polyethylene glycol (PEG) test, epicuticular wax load (EWL), cytoplasmic lipid content, solute concentration in cell sap, and growth. Osmotic adjustment was observed under water deficit conditions. Lower osmotic potential enabled plants to maintain turgor and decreased the sensitivity of turgor-dependent processes. Sugar and K were identified as the major solutes contributing to osmotic potential in sorghum. Sugar and K concentrations in cell sap increased by 37-4% and 27%, respectively, under water deficit conditions in favour of decreasing osmotic potential. Stomatal conductance and cuticular conductance were lower in the non-irrigated plants. A wide range in CMS among four cultivars was observed. CMS increased with increasing water deficits. EWL increased on leaves of water deficient plants and was positively correlated with cuticular conductance and CMS. Membrane phospholipid content increased in water-stressed plants. CMS as measured by the PEG test, was influenced by EWL, cuticular thickness, and osmotic concentration of leaf tissues. The cultivars which maintained higher CMS, higher EWL, lower cuticular conductance, higher turgor and higher osmotic adjustment under water deficit conditions were identified as drought tolerant.