Influence of Osmotic Adjustment on the Growth, Stomatal Conductance and Light Interception of Contrasting Sorghum Lines in a Harsh Environment

Abstract

Two drought resistant lines of sorghum, IS 13441 and IS 1347, with a high capacity for osmotic adjustment and two susceptible lines, IS 12739 and IS 12744, were subjected to drought in the field during a summer season in the semi-arid tropics in India. During this season there is little rain, air temperatures reach 42°C, and pan evaporation rates may reach 20 mm d^-1 providing a harsh environment for crop growth. Most of the osmotic adjustment occurred within 3 weeks after withholding water and at high predawn leaf water potentials, i.e. at values above - 1.0 MPa. As a result, resistant lines were able to maintain a positive turgor to lower leaf water potentials (- 2.8 MPa) than susceptible lines (- 2.0 MPa). Nevertheless, dry matter production was negligible in both resistant and susceptible lines when predawn leaf water potentials fell to - 0.55 MPa. Furthermore, throughout the drought period the leaf area of all water-stressed plants was similar when expressed relative to the control regardless of the level of osmotic adjustment. Resistant lines had similar stomatal response to leaf water potential as susceptible lines. Large changes occurred in stomatal conductance and leaf rolling soon after withholding water while there was considerable osmotic adjustment in the leaves. Leaf rolling coincided with a reduction in the ability of the plant to utilise radiation. Therefore, even if osmotic adjustment had delayed leaf rolling, the gain in productivity would be small. On the basis of these turgor related processes it can be concluded that there would be little advantage in selecting for plants with a higher capacity for osmotic adjustment in this harsh environment.