Stay green trait in grain sorghum: relationship between visual rating and leaf chlorophyll concentration

Post-flowering drought tolerance is referred to as the stay green trait in sorghum. Plants with stay green resist drought-induced premature plant sene scence. In breeding programmes, stay green is evaluated under limited irrig ation, post-flowering moisture-stress field conditions and visually scored at or soon after physiological grain maturity. The objective of this study was to investigate the relationship between the stay green rating and total leaf chlorophyll content. The parents B35 and Tx7000, and their 98 F-7 rec ombinant inbred lines were evaluated in replicated held trials under limite d (post-flowering stress) and full-irrigation (non-stress) conditions. Afte r scoring the stay green trait of stressed plants, total leaf chlorophyll c ontents were measured with a chlorophyll meter (SPAD values) and a spectrop hotometer method. The SPAD value had a significant linear relationship with total leaf chlorophyll (R-2 = 0.91) and with visual stay green rating (wit h R-2 = 0.82). Relative water content in top leaves of the stay green lines was about 81%, much higher than non-stay green lines (38%), indicating tha t the stay green lines kept the stalk transporting system functioning under severe drought conditions. The results indicate that visual stay green rat ings were a reliable indicator of leaf senescence and should be useful to s orghum breeders in evaluating progeny when breeding for drought tolerance.