Effects of planting density and topping time on the source-sink tributes and photosynthetic carbon production in short-fruiting branch cotton

Abstract Background Short fruiting branch cotton (SFBC) has normal main-stem growth. However, the growth of fruiting branches is compromised by a short sympodial internode of 2–5 cm, which is suitable for a high-density cropping system. Increasing plant population density (PPD) combined with late topping (LT) has been proven to increase yield for SFBC, but the underlying physiological mechanism remains unclear. An SFBC line, P4D, was employed to examine the dynamics of source-sink attributes and leaf carbon metabolism in the fourth main-stem leaf from the apex during peak flowering (PF) to boll opening (BO) stages in 2023 and 2024. The experiment involved a factorial design with two PPD levels (LD: 5.33 plants·m −2 , HD: 8 plants·m −2 ) and two topping times (ET: early topping for leaving ten sympodial branches; LT: late topping for leaving fifteen sympodial branches). Results Compared with the three other combinations, the combination of HD with LT (HDLT) produced higher lint and seed cotton yields by increasing biological yield and boll density. Averaged across the PF to BO stages and over two years, the greatest leaf area index (LAI) and reproductive biomass per unit ground area (RBGA) were recorded in the HDLT, with increases of 46.30% to 78.52% for LAI and 6.77% to 40.18% for RBGA compared with others. Additionally, the HDLT exhibited the lowest reproductive biomass per unit leaf area (RBLA), with a reduction of 15.20% to 42.74%. The above results showed that the HDLT was characterized by the greatest source and sink strengths as well as the strongest anti-senescence ability among the four treatments. Compared with the ET, the LT improved the activities of adenosine diphosphate glucose pyrophosphorylase and sucrose phosphate synthase and, in turn, increased leaf starch and total nonstructural carbohydrate (TNC) concentrations during the PF to BO stages in both years. Relative to the HD, the LD presented greater starch and TNC levels. Thus, the combination of LT and LD (LDLT) achieved the greatest starch and TNC concentrations, followed by the HDLT. Conclusion The higher cotton yield in HDLT is attainable through increasing both source and sink ability, delaying leaf senescence, and improving carbon production capacity. Increasing plant density combined with later topping for SFBC is a promising alternative for sustainable cotton production in Jiangxi province, China and other areas with similar ecology.