Quantitative trait loci for the stay green trait in sorghum (Soughum bicolor L. Moench): consistency across genetic backgrounds and environments

Stay green in sorghum (Sorghum bicolor L. Moench) is characterized by the p lant's ability to tolerate post-flowering drought stress, thereby delaying the premature leaf and plant death. It contributes to normal grain filling and reduces the incidence of stalk lodging and charcoal rot disease during the late stages of grain development. Breeding for improving post-flowering drought tolerance in sorghum hybrids remains an important objective of sor ghum breeders. Since evaluation of the stay green response is difficult and unreliable under field conditions, due to the timing and intensity of mois ture stress and large environmental interaction, progress in improving drou ght tolerance by conventional breeding methods has been slow. The objective of the present study was to determine the consistency of quantitative trai t loci (QTLs) controlling stay green in sorghum. We re-evaluated the Recomb inant Inbred Line (RIL)-mapping population from the cross B35 x Tx7000 in t wo locations over 2 years and compared it with earlier reports. Analysis us ing the combined stay green-rating means of seven environments and the expa nded molecular map reconfirmed all four stay green QTLs (Stg1, Stg2, Stg3 a nd Stg4) that were identified earlier by Xu et al. (2000). Similarly, compa rison of the stay green QTL locations with earlier reported results indicat ed that all four stay green QTLs showed consistency across different geneti c backgrounds. Examination of the stay green QTL profiles of the best and p oorest stay-green lines indicated that three stay green QTLs, Stg1, Stg2 an d Stg3, appear to be important for the expression of this trait when the pe rcent phenotypic variation, and the consistency in different backgrounds an d different environments, are considered. A significant epistatic interacti on involving Stg2 and a region on linkage group C was also identified for t he stay green and chlorophyll content. We concluded that Stg2 is the most i mportant QTL controlling stay green, explaining the maximum amount of pheno typic variation. This report further strengthens our view to target the Stg 2 QTL region for gene discovery in order to improve the basic understanding of the stay green phenomenon, which might be helpful in manipulating this trait not only in sorghum but also in other cereal crop species.