Exogenous brassinosteroids improve the structure and quality properties of waxy maize starch under post-silking heat and drought stress.

Global climate change-induced heat and drought stresses severely reduce maize yield and starch quality. This study investigated the effects of post-silking heat-stress, drought-stress, and their combination on waxy maize starch physicochemical properties and examined the regulatory role of exogenous brassinosteroids. Compared to the control, combined stress, heat-stress, and drought-stress reduced yield by 44.6 %, 31.5 %, and 22.7 % in S5, and by 50.6 %, 29.5 %, and 20.0 % in Y7; starch content decreased by 15.4 %, 7.8 %, and 10.1 % in S5, and by 15.1 %, 9.2 %, and 9.4 % in Y7. Stresses disrupted starch granules, enlarged granule size, increased relative crystallinity, shortened amylopectin chains, and impaired gelatinization properties. The severity of these effects followed the order: combined stress > heat-stress > drought-stress. Heat-stress primarily affected granule morphology and relative crystallinity, while drought-stress mainly affected amylopectin chain length. Exogenous brassinosteroids increased pasting viscosity, reduced gelatinization enthalpy and rapidly digestible starch content, and alleviated the stress effects.