Potassium application alleviates the drought-induced reduction in photoassimilates synthesis and distribution within the middle and upper fruiting branches, enhancing subtending cotton boll weight

Abstract

Drought significantly reduces cotton boll yields across various fruiting branches (FBs). Potassium (K) application can partially mitigate the drought-induced damage by modifying the biosynthesis of photoassimilates in the leaf subtending to cotton boll (LSCB) and facilitating their transport to the subtending bolls, although its effects vary among FBs. The underlying mechanisms remain unclear. To investigate this, potting experiments were conducted at three soil relative water content (SRWC): 75 ± 5 % (W75), 60 ± 5 % (W60), and 45 ± 5 % (W45), along with K rates of 0 (K0), 150 (K150) and 300 (K300) kg K₂O ha⁻¹. Compared to W75, the W60 and W45 treatments reduced the photosynthesis of LSCBs in different FBs, adversely affecting carbohydrate accumulation in the subtending cotton bolls. K application can mitigate this negative impact, with the most pronounced effects observed in the middle and upper FBs. K application (K150 and K300) enhanced the net photosynthetic rate, stomatal conductance, maximum mass yield of PSII and chlorophyll content of LSCB in the middle and upper FBs compared to K0 under drought conditions. Additionally, K application significantly increased K content in LSCBs within the middle and upper FBs, which in turn elevated sucrose phosphate synthase (SPS), and sucrose synthase (SuSy) activities, reducing the conversion of sucrose into starch, ultimately facilitating carbohydrate exports to the subtending bolls. In summary, we propose a model that elucidates how K application mitigates drought damage by enhancing the exports of photoassimilates from the middle and upper FBs to their respective subtending cotton bolls.