DIALLEL ANALYSIS OF THE STAY-GREEN TRAIT AND ITS COMPONENTS IN SORGHUM

Stay-green in sorghum [Sorghum bicolor (L.) Moench] is an important co mponent of post-flowering drought tolerance. This research was conduct ed to describe (i) stay-green as post-flowering green leaf area durati on (GLAD) and its components [green leaf area (GLA) at flowering, timi ng for onset of senescence, and senescence rate] and (ii) the expressi on of heterosis for stay-green in terms of heterosis for its component s. The study was conducted during the 1992 to 1993 and 1993 to 1994 po st-rainy season at ICRISAT headquarters near Hyderabad, India. It invo lved a nine-parent complete diallel, in two experiments differing in s oil-water availability after flowering. Weekly estimations of % GLA we re made on 36 leaves per plot. Relative (% days) and absolute (m(2) da ys) GLAD and their components were derived from a fitted logistic func tion. The variances in both relative and absolute GLAD were each fully (R(2) > 0.96) accounted for by their components. In spite of signific ant genotype x environment interactions for the component traits, the expression of heterosis for non-senescence as related to the stay-gree n trait was stable across experiments. The inheritance of the onset of senescence was additive, but a slow senescence rate was dominant over a fast rate. Consequently, a large relative GLAD (slow senescence) wa s partially dominant over a small relative GLAD. Because of the domina nce of a large leaf area at flowering, the partial dominance in relati ve GLAD translated into overdominance for a large absolute GLAD. These results offer an opportunity for improving drought tolerance of sorgh um in environments with post-flowering drought stress.