Influence of rate of development of water deficit on the expression of maximum osmotic adjustment and desiccation tolerance in three grain sorghum lines

An experiment was conducted to quantify the effect of rate of development of water deficit (R(Psi)) 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 (RWC(L)) and lethal leaf water potential (Psi(L)) 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 R(Psi), for all three lines. In contrast, DT decreased only slightly with a reduced R(Psi). The three lines differed for expression of maximum OA and DT. Both TAM422 and Tx2813 had higher maximum OA, higher RWC(L) and lower Psi(L) than QL27. There was a significant line-by-R(Psi) 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 R(Psi) without affecting their ranking. As with maximum OA, the interaction for RWC(L) was smaller than the Line effect. The RWC(L) of TAM422 and QL27 was stable across rates of development of water deficit. The results indicated that low R(Psi), increased the expression of OA and decreased Psi(L) but had little effect on RWC(L). While line-by-soil-volume interactions were observed, these were largely explained by the increase in magnitude of the variation for the traits as R(Psi) 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.