Diallel analysis of the stay-green trait and its components in sorghum

Stay-green in sorghum [Sorghum bicolor (L.) Moench] is an important component of post-flowering drought tolerance. This research was conducted to describe (i) stay-green as post-flowering green leaf area duration (GLAD) and its components [green leaf area (GLA) at flowering, timing for onset of senescence, and senescence rate] and (ii) the expression of heterosis for stay-green in terms of heterosis for its components. The study was conducted during the 1992 to 1993 and 1993 to 1994 post-rainy season at ICRISAT headquarters near Hyderabad, India. It involved a nine-parent complete diallel, in two experiments differing in soil-water availability after flowering. Weekly estimations of % GLA were made on 36 leaves per plot. Relative (% days) and absolute (m(2) days) GLAD and their components were derived from a fitted logistic function. The variances in both relative and absolute GLAD were each fully (R(2) > 0.96) accounted for by their components. In spite of significant genotype x environment interactions for the component traits, the expression of heterosis for non-senescence as related to the stay-green 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) was partially dominant over a small relative GLAD. Because of the dominance of a large leaf area at flowering, the partial dominance in relative GLAD translated into overdominance for a large absolute GLAD. These results offer an opportunity for improving drought tolerance of sorghum in environments with post-flowering drought stress.