GENOTYPIC VARIATION OF OSMOTIC ADJUSTMENT AND DESICCATION TOLERANCE IN CONTRASTING SORGHUM INBRED LINES

Variation in maximum osmotic adjustment and desiccation tolerance were determined before anthesis for 21 lines of Sorghum bicolor (L.) Moench, selected on the basis of their capacity for osmotic adjustment and for their putative drought resistance. A reproducible and controlled imposition of gradual water stress was achieved by withholding water from plants grown in a controlled environment with a constant evaporative demand. Even though the rate of imposition of stress was lower in the second of two experiments, the maximum level of osmotic adjustment expressed by the fines was fairly consistent across both. The highest osmotic adjustment was 1.71 +/- 0.06 MPa for TAM422 in Exp. 2, while the lowest was 0.78 +/- 0.09 MPa for QL27 in Exp. 1. The difference in maximum osmotic adjustment between the highest and the lowest lines was 0.75 and 0.87 MPa, respectively, for Exp. 1 and 2. There was also variation in desiccation tolerance among the 20 lines in Exp. 2; 58% to 68% for lethal relative water content and - 3.1 to - 3.9 MPa for lethal leaf water potential. Mean values of lethal relative water content and lethal leaf water potential were, respectively, 62% and - 3.4 MPa. Maximum osmotic adjustment was inversely related to desiccation tolerance; lethal relative water content and lethal leaf water potential increased linearly as maximum osmotic adjustment increased. Thus lines with high osmotic adjustment died at a higher relative water content and lower leaf water potential, than those with low osmotic adjustment. Despite their reduced tolerance of desiccation, lines with high osmotic adjustment survived 10 days longer. In both experiments, a high level of repeatability of line mean discrimination was identified for osmotic adjustment (0.75 +/- 0.15 and 0.96 +/- 0.09, for Exps. 1 and 2, respectively). The repeatability on a line mean basis across experiments was also high (0.84 +/- 0.07), as was the genetic correlation between the line means in both experiments (0.86 +/- 0.08). The high levels of repeatability suggest that the screening procedure developed in this study is robust, and that it could be used to evaluate the inheritance of osmotic adjustment in breeding populations.