Abstract: 【Objective and Method】Removal of vegetative branches (VB) has been a traditional practice for field management in cotton production in China，but current studies indicate that retention of VB is of great worth for improving cotton yield. In order to determine the feasibility of VB retention in cotton plants and the corresponding mechanisms, three hybrid cotton varieties were grown in a fertile land at considerably low population density (3.0 plants/m2) in Linqing in 2004 and 2005. Yield and yield components, leaf area index, leaf chlorophyll content, boll load per leaf area and leaf photosynthetic rate (Pn) of cotton plants either with or without VB were examined each year. 【Results】Retention of VB reduced average boll weight, but significantly increased boll numbers per ground area and thus enhanced lint yield by 9.8% and 6.2% in 2004 and 2005 relative to removal of VB, respectively. Green leaf percentage and average chlorophyll content in canopy leaves in VB-retained plants were significantly increased, but chlorophyll content and Pn in functional leaves on the main stem of plants with VB was considerably decreased, compared to those of VB-removed plants. Retention of VB resulted in significantly strengthened leaf source before peak boll-setting and extended boll sinks at late stages of plant，but “flow” (transportation) capability of cotton plants seemed to be inhibited by VB retention.【Conclusion】It is concluded that VB retention for Bt hybrid cotton planted at a low density can be an effective practice for increasing yield. Yield increases could be attributed to the enhanced leaf sources before peak boll-setting and increased boll sinks at boll-setting by retention of VB. Retention of VB dose not substantially alleviate premature senescence of leaf canopy.

Key words: Bt hybrid cotton, Vegetative branches, Sink-source relation, Leaf senescence, Lint yield