Regulating sink-source ratio optimizes cotton defoliation efficiency during the early-to-mid flowering and boll stages

The sink-source relationship significantly influences chemical defoliation in cotton. However, studies on the effects of sink-source manipulations at different phenological stages remain limited. To address this, we designed four treatments with varying sink-source ratios at the initial flowering stage (IFS), flowering and boll stage (FABS), and full boll stage (FBS): conventional plant type (CK), 50% leaves of the fruit branches removal (1/2 L), 50% boll removal (1/2B) and 100% boll removal (0B). The results demonstrated that the sink-source ratio affects defoliation outcomes, with distinct effects depending on the treatment stage. At IFS and FABS, defoliation rates increased with higher sink-source ratios following defoliant application, with the 1/2 L treatment exhibiting the highest defoliation rate and the 0B treatment the lowest. Conversely, at FBS, the CK treatment showed the highest defoliation rate, while the 1/2 L treatment had the lowest. This discrepancy may be attributed to compensatory mechanisms at FBS, where leaf removal enhanced the physiological activity of leaves (e.g., higher IAA content, lower ABA content, and increased net photosynthetic rate (Pn)), reducing sensitivity to defoliants. In contrast, at IFS and FABS, the compensatory effects of leaf removal likely subsided by the time of defoliation, and the high boll load accelerated leaf senescence (e.g., lower IAA content, higher ABA content, and reduced Pn), enhancing defoliant sensitivity. Consequently, the 1/2 L treatment not only raised the defoliation rate by 0.5–6.2% and 0.9–1.5% at IFS and FABS, but more importantly, it substantially reduced residual leaves by 33.3–65.9% and 28.2–39.0%, respectively. Additionally, there was no significant difference in yield between the CK and the 1/2 L treatment during these two periods. Thus, moderately increasing the sink-source ratio during early and middle FABS optimizes defoliation efficiency. This study provides theoretical insights and practical guidance for optimizing sink-source dynamics to improve chemical defoliation in cotton. The IFS, FABS, and FBS correspond to the initial flowering, flowering and boll, and full boll stages, respectively. 1/2 L: 50% leaves of the fruit branches removal. • The effect of sink-source ratios on cotton defoliation varies depending on the stage of treatment. • Optimizing sink–source ratio at early-to-mid flowering and boll stage improves cotton defoliation without lint yield loss. • Carbohydrates and hormonal crosstalk jointly regulate leaf shedding in cotton.