Effects of Mepiquat Chloride and Plant Population Density on Leaf Photosynthesis and Carbohydrate Metabolism in Upland Cotton

Abstract

Background Mepiquat chloride (MC) application and increasing plant population density (PPD) are required for modern cotton production. However, their interactive effects on leaf physiology and carbohydrate metabolism remain obscure. This study aimed to determine how MC and PPD affect the leaf morpho-physiological charateristics, and thus final cotton yield. PPD of three levels (D1: 2.25 plants m-2, D2: 4.5 plants m-2, and D3: 6.75 plants m-2) and MC dosage of two levels (M0: 0 g ha-2, M1: 82.5 g ha-2) were combined to create six treatments. The dynamics of nonstructual carbohydrate concentration, carbon metabolism-related enzyme activity, and photosynthetic attributes in cotton leaves were examined during reproductive growth in 2019 and 2020. Results Among six treatments, the high PPD of 6.75 plants m-2 combined with MC application (MC1D3) exhibited the greatest seed cotton and biological yields. The sucrose, hexose, starch, and total nonstructural carbohydrate (TNC) concentrations peaked at first flowering (FF) stage and then declined down to a minimum at first boll opening (FBO) stage. Compared to the other treatments, MC1D3 improved starch, and TNC concentrations by 5.4–88.4%, 7.8–52.0% in 2019, and by 14.6–55.9%, 13.5–39.7% in 2020 at the FF stage, respectively; Additionally, The treatment produced higher transformation rates of starch and TNC during the FF to FBO stages, indicating greater carbon production and utilization efficiency with it. MC1D3 displayed the maximal SLW at the FBO stage, and the greatest Chl a, Chl b, and Chl a + b concentrations at the mid-late growth phase in both years. The Rubisco activity with MC1D3 was 2.6–53.2% higher at the flowering and boll setting stage in both years, and 2.4–52.7% higher at the FBO stage in 2020 than those in the other treatments. These results provided a explanation of higher leaf senescence-resistant ability in MC1D3. Conclusion Increasing PPD coupled with MC application improves cotton yield by enhancing leaf carbohydrate production and utilization efficiency and delaying leaf senescence.