Brassinosteroid Enhances Thermotolerance by Improving Photosynthesis and Membrane Stability Under Reproductive‐Stage Heat Stress in KDML105 Rice ( Oryza sativa L.)

ABSTRACT Brassinosteroids (BRs) are plant steroid hormones known to enhance tolerance to abiotic stress, including heat stress (HT). This study investigated the effects of foliar application of BRs (0, 0.05, 0.1, and 0.5 nM) on the physiological performance and yield of the tropical photosensitive rice cultivar KDML105 at the reproductive stage under HT (42°C for 7 days). Experiments were conducted at booting and 50% flowering stages under controlled HT in greenhouse and naturally HT exposure in field conditions. Heat stress significantly impaired physiological performance by increasing electrolyte leakage (EL) and malondialdehyde (MDA), indicating reduced membrane stability. It also reduced photosynthetic pigments, PSII efficiency, stomatal conductance and transpiration rate, leading to a decrease in assimilate production and grain yield. Brassinosteroids application improved physiological performance by maintaining membrane stability, preserving photosynthetic pigments and enhancing PSII efficiency and CO 2 gas exchange. These factors may be associated with better assimilate translocation and grain filling, resulting in higher yield under HT. Brassinosteroids mainly enhanced reproductive traits, including total number of grains/panicle, number of filled grains/panicle, and grain yield. At the 50% flowering stage, greater heat sensitivity was observed compared to the booting stage, while 0.5 nM BR showed the best physiological improvement. These results suggest that BRs enhance thermotolerance in KDML105 rice by protecting photosynthetic function and membrane stability during the reproductive stage.