Exogenous plant growth regulators alleviate natural high temperature stress after anthesis in wheat (Triticum aestivum L.) by sustaining efficient photosynthesis and antioxidant defense responses.

INTRODUCTION: High temperature stress during the grain-filling period severely suppresses plant growth and yield formation in winter wheat. Plant growth regulators including 2,4-epibrassinolide (EBR) and melatonin (MT) have been reported to effectively enhance plant thermotolerance, representing a promising strategy to mitigate extreme high temperature stress and stabilize wheat yield.

METHODS: In this study, integrated agronomic, physiological and transcriptomic analyses were conducted to explore the regulatory effects of different exogenous plant growth regulators on thermotolerance and physiological characteristics of winter wheat during the grain-filling stage.

RESULTS AND DISCUSSION: Exogenous application of these regulators modulated the expression of core photosynthesis-related genes, increased the net photosynthetic rate of flag leaves, enhanced photosynthetic capacity, and alleviated heat-induced damage. Meanwhile, exogenous regulators altered the transcript levels of key antioxidant enzyme-coding genes, thereby boosting antioxidant enzyme activities, reducing the accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA), and relieving membrane lipid peroxidation. Coordinated regulation of photosynthetic and antioxidant metabolic pathways substantially elevates the thermotolerance of winter wheat against natural high temperature stress after anthesis. The Mix treatment (0.05 µmol L-1 2,4-epibrassinolide + 50 µmol L-1 melatonin + 15 mmol L-1 KH2PO4) markedly optimized the physiological and biochemical status of heat-stressed wheat, alleviated biomass loss and increased grain yield. Collectively, this study improves the core regulatory mechanisms by which exogenous plant growth regulators mediate thermotolerance in winter wheat during grain filling, and provides new theoretical references and practical cultivation strategies for maintaining wheat yield and grain quality under high temperature stress.