Seed Priming with Gibberellic Acid and Ethephon Improved Rice Germination under Drought Stress via Reducing Oxidative and Cellular Damage

Gibberellic acid (GA) and ethephon (ETH) can stimulate seed germination and seedling growth. However, uncertainties persist concerning the efficacy of GA or ETH priming in mitigating drought stress effects. This study aims to elucidate the underlying mechanisms of drought tolerance in rice seedlings induced by GA or ETH priming. Rice seeds were primed by water, GA, and ETH and sown under polyethylene glycol (PEG-6000)-induced stress. We set non-primed seeds under normal conditions as a control to investigate the effects of GA or ETH on seed germination and seedling growth under drought stress. Drought resistance mechanisms were characterized by osmoregulation, homeostasis of reactive oxygen species (ROS), antioxidants and transmission electron microscopy analysis. Drought significantly inhibited germination and seedling development. Nevertheless, GA or ETH priming enhanced rice emergence and establishment under water-deficient condition. Furthermore, hormonal priming enhanced chlorophyll content and osmolyte accumulation, ROS detoxification and antioxidant enzyme activity in rice seedlings (shoots and roots) during drought stress. Ultrastructural observations revealed GA or ETH priming's protective role in leaf mesophyll integrity, chloroplast preservation, and mitigation of drought-induced damage in root tip cells. Nonetheless, distinct defense responses and mitigation mechanisms emerged post-GA or ETH priming. While ETH treatment affected shoot fresh weight and root volume, GA priming excelled in germination attributes, seedling length, soluble sugar, H2O2 reduction, electrolyte leakage, and ascorbate peroxidase activity under drought. GA and ETH priming promoted rice seed germination and seedling establishment under drought conditions by maintaining root conformation, attenuating oxidative stress, and maintaining cellular ultrastructure.