Genotype-Specific Responses of Maize (Zea mays L.) to Exogenous Melatonin in Mitigating Flooding Stress

Abstract

Flooding stress severely constrains maize growth at the seedling stage. Here, we determined the mechanism and an effective dosage of foliar melatonin (MT) in alleviating flooding stress injury for three Guangxi maize genotypes (Guidan 668, Guidan 162 and Zhaofeng 505). Seedlings at the three-leaf stage were subjected to flooding simulated by maintaining a 2–3 cm water layer in pots for 7 days, and leaves were sprayed with MT at 0, 50, 100, 150 and 200 μM every day. Flooding significantly decreased plant height, root length, relative water content, dry matter accumulation and relative chlorophyll content across genotypes, whereas intensifying oxidative stress. MT mitigated these adverse effects in a concentration-dependent manner, with 100 μM MT consistently showing the most pronounced protection. Compared with flooded plants without MT, 100 μM MT reduced ROS accumulation and oxidative damage, enhanced antioxidant enzyme activities (POD, APX and CAT), increased osmolyte levels and activities of nitrogen utilisation enzymes (NR, GS and GDH), and partially restored hormone homeostasis, thereby improving seedling growth and physiological stability. The extent of MT-induced tolerance varied among genotypes, indicating genotype-dependent responsiveness under flooding. Overall, foliar application of 100 μM MT is an effective approach to enhance maize seedling flooding tolerance through coordinated regulation of redox balance, osmotic adjustment, nitrogen metabolism and hormonal stability.