Stay-Green QTLs Response in Adaptation to Post-Flowering Drought Depends on the Drought Severity.

Stay-green trait enhances sorghum adaptation to post-flowering drought. Six stay-green backcross introgression lines (BILs) carrying one or more stay-green QTLs (Stg1-4) and their parents were characterized under non-stress (W: 100% of soil field capacity (FC)) and two levels of post-flowering drought (W: 75% FC; W: 50% FC) in a controlled condition. We aimed to study the response and identify the drought threshold of these QTLs under different levels of post-flowering drought and find traits closely contributing to grain yield (GY) under different drought severity. W caused the highest reduction in BILs performance. From W to W, the GY of the recurrent parent reduced by 70%, whereas that of the BILs reduced by only 36%. W and W induce different behavior/response compared to W. Harvest index contributed to the GY under the three water regimes. For high GY under drought transpiration rate at the beginning of drought and mid-grain filling was important at W, whereas it was important at mid-grain filling and late-grain filling at W. Stay-green trait can be scored simply with the relative number of green leaves/plants under both irrigated and stress environments. QTL pyramiding might not always be necessary to stabilize or increase the GY under post-flowering drought. The stay-green QTLs increase GY under drought by manipulating water utilization depending on drought severity.