Stay-green QTLs effects on water extraction, transpiration efficiency and seed yield depend on recipient parent background

A stay-green phenotype enhances the adaptation of sorghum to terminal drought conditions, although the underlying physiological mechanisms leading to the expression of stay-green remain unclear. Differences in tillering and leaf area at anthesis, transpiration efficiency (TE), water extraction, harvest index (HI) and yield under both terminal drought and fully-irrigated conditions were assessed in 29 introgression lines (IL) developed targeting stay-green QTLs Stg1, Stg2, Stg3, Stg4, StgA, and StgB in S35 background, and 16 IL developed targeting Stg1, Stg3, Stg4, and StgB in R16 background. Transpiration efficiency was increased by StgB in the R16 background, whereas there was no effect on this trait in the S35 background. Water extraction was increased by Stg1 in the S35 background, whereas there was no effect in the R16 background. StgB modified the proportion of water extracted before and after anthesis in the S35 background. While tillering and leaf area at anthesis were decreased by Stg1 and Stg3 in the S35 background, there was no such effect in R16. By contrast, yield data under fully-irrigated conditions showed higher tiller grain yield in Stg1, Stg2, and Stg3 ILs. While yield differences were mostly explained by harvest index (HI) variation, the substantial yield variation unexplained by HI was closely related to TE in the S35 background (R2 = 0.29), and more so in the R16 background (R2 = 0.72), while it was closely related to total water extracted in the S35 background (R2 = 0.41), but not in the R16 background. These data indicate the potential for several stay-green QTLs to affect traits related to plant water use and capture. However, they also show that these effects depend on the interaction between genetic background and individual QTLs.