Removal of early fruiting branches impacts leaf senescence and yield by altering the sink/source ratio of field-grown cotton

Previous research shows that removal of early fruiting branches (FB) alters the sink/source ratio and delays leaf senescence in cotton. However, how the altered sink/source ratio regulates leaf senescence and yield formation is still poorly understood. In this study, 2 or 4 early FB were removed from two near isogenic cotton lines, late- and early-senescence lines, while plants with intact FB served as controls. The leaf chlorophyll content (Chl) and photosynthetic (Pn) rate in the late-senescence line were higher than those in the early-senescence line, but the malondialdehyde (MDA) accumulation and boll load per leaf area (BLLA) in the late-senescence line were lower than those in the early-senescence line. Removal of FB decreased the BLLA and MDA accumulation but increased the Pn rate, Chl concentration, and the expression of GhLHCB gene in both cotton lines, suggesting that FB removal reduced sink/source ratio and suppressed leaf senescence. However, it decreased ABA and JA contents by decreasing their biosynthesis related genes and increasing the expression of ABA catabolic related genes. Although FB removal increased the iP + iPA content, it had little effects on Z + ZR or DHZ + DHZR contents. The results suggest that the delayed leaf senescence following the decreased sink/source ratio associated with FB removal might be attributed to the increased iP + iPA and reduced ABA and JA contents, as well as the differentially expressed biosynthesis and catabolic related genes. Seedcotton yield increased by 6.3-10.2% following the removal of 2 FB but decreased by 5.7-11.7% following the removal of 4 FB in the early-senescence line; in the late-senescence line, the removal (whether of 2 or 4 FB) significantly decreased yield. Removal of 2 or 4 FB in the late-senescence line and 4 FB in early-senescence line significantly suppressed earliness. The overall results suggested that removal of early fruiting branches would practically regulate the sink/source relationship and suppress leaf senescence in cotton. Delaying leaf senescence would not necessarily increase cotton yield unless when earliness is not affected or when normal maturity is achieved.