Genetic control of post-flowering drought tolerance (stay green) in sorghum

Sorghum exhibits two distinct responses to drought stress. One response is when drought stress occurs during the head development stage of growth (pre-flowering), and the other is when stress occurs during the grain filling stage of growth (post-flowering) . The term ''stay green" has been used to describe sorghum lines which possesss late season (postflowering) drought tolerance. The inheritance and physiological mechanisms involved in the stay green trait are not well understood. A stay green sorghum inbred line, B35 (post-flowering drought tolerant) was crossed with a non-stay green inbred line, Tx7000 (post-flowering drought susceptible) to form an F1 hybrid which was selfed to F2 population and also backcrossed to each of the parental lines to form BC1P1 and BC1P2 populations in order to determine the inheritance of the stay green trait. The two inbred lines, their F1, F2 and backcrosses were planted in the field and under a rainout shelter. A randomized complete block design was used with three replications in the rainout shelter and four replications in the field. Moisture stress was allowed to develop in both trials during the grain filling period. Stress in the rainout shelter was somewhat more severe than in the field,but the stress was sufficient in both locations to induce premature death in most Tx7000 plants. Stay green was evaluated by using a visual leaf and plant death rating score on an individual plant basis at or soon after physiological maturity. Population means, variances, and frequency distributions provided the basic data from which conclusions were drawn. Gene action appeared to be mainly dominant. A minimum of 1 or 2 genes, primarily dominant in their effect, appear to control the stay green characteristic. Broad-sense heritability estimates were 0.61 and 0.80 for the experiments grown in the rainout shelter and field, respectively. The narrow-sense heritability estimate obtained from the experiment grown in the rainout shelter was 0.61, as well, indicating that it is relatively easy to select for the stay green trait under similar conditions as the ones the trial in the rainout shelter was conducted.