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An experiment was conducted to quantify the effect of rate of development of water deficit (Rψ) on the expression of maximum osmotic adjustment (OA) and desiccation tolerance (DT) in three grain sorghum lines, TAM422, Tx2813 and QL27. DT was determined as the lethal relative water content (RWCL) and lethal leaf water potential (ψL) of leaves, i.e. the value of these traits just prior to tissue death. Five rates of development of water deficit were generated by using different soil volumes in which the decrease in leaf water potential ranged from 0.093 MPa day−1 to 0.153 MPa day−1. The expression of maximum OA increased markedly with a reduced Rψ, for all three lines. In contrast, DT decreased only slightly with a reduced Rψ. The three lines differed for expression of maximum OA and DT. Both TAM422 and Tx2813 had higher maximum OA, higher RWCL and lower ψL than QL27. There was a significant line-by-Rψ interaction for the expression of OA and DT. The interaction was much smaller than the effect of lines for the expression of maximum OA. Therefore, the absolute level of OA for the three sorghum lines and the magnitude of the difference between them changed with the Rψ without affecting their ranking. As with maximum OA, the interaction for RWCL was smaller than the line effect. The RWCL of TAM422 and QL27 was stable across rates of development of water deficit. The results indicated that low Rψ increased the expression of OA and decreased ψL but had little effect on RWCL. While line-by-soil-volume interactions were observed, these were largely explained by the increase in magnitude of the variation for the traits as Rψ decreased and were not the result of changes in rank of the lines among soil volumes. Therefore, the current screening strategy is considered to be appropriate for examining genetic variation for OA and DT in grain sorghum. Opportunities exist for using smaller pots and soil volumes where large numbers of genotypes are examined. |
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