Thriving in adversity: Understanding how maize seeds respond to the challenge of combined cold and high humidity stress.

Extreme conditions, such as cold and high humidity in northeast China's high-latitude maize region, can hinder crop yield and stability during the vegetative stage. However, there is a paucity of research examining the effects of simultaneous cold and high humidity stress on plant responses. In this study, we characterized the acclimation of JD558 (cold- and high humidity-sensitive hybrid) and JD441 (cold- and high humidity-tolerant hybrid) to stress at sowing caused by cold (4 °C), high humidity (25%), and their combined stress for five days, using physiological measurements and metabolomics during the stress treatments and recovery stages. Cold, high humidity, and their combined stress prolonged seed development and restricted material transport, with high humidity harming seed survival more than cold. Combined stress exhibited a more significant inhibitory effect on growth than individual stress. Individual and combined stress reduced α-amylase activity, disrupted antioxidants levels, increased malondialdehyde content, disturbed the oxidative balance within seeds, and impeded seed growth and development. Most carboxylic acids and their derivatives were downregulated caused by combined stress. In JD558, sucrose, D-glucose, glucose-1-phosphate, and fructose-1,6-bisphosphate were downregulated, while these metabolites were upregulated in JD441, leading to a blockage of glycolysis in JD558. After eliminating stress, JD441 showed greater α-amylase activity and a smaller decrease in MDA levels, resulting in a smaller reduction in root growth and transport rate than JD558. In summary, the different responses of the cold and high humidity sensitive hybrid and the tolerant hybrid to combined stress are related to the recovery ability after stress elimination.