Drought stress (DS) challenges sustainable agriculture production by limiting crop growth and development. The objective of the study was to evaluate the effect of melatonin-priming on enzymatic and non-enzymatic antioxidant defense mechanisms and its relation with leaf ultrastructure and stomatal traits in maize (Zea mays L) seedlings under DS (PEG-6000). DS drastically decreased seed germination, plant growth, and leaf chlorophyll content due to excessive reactive oxygen species (ROS) production. Melatonin-priming significantly (P < 0.05) increased seed germination, root length, shoot length, fresh seedling weight, proline content, total soluble protein content, sugar content, chlorophyll content, and stomatal aperture size by 101%, 30%, 133%, 51%, 22%, 59%, 54%, 20%, and 424%, compared to no priming (NP) under DS, respectively. Similarly, priming improved leaf ultrastructure and reduced the amount of chlorophyll loss and oxidative damage in maize seedlings. Melatonin seed priming with 500 µM melatonin (M2) greatly increased superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione, and ascorbate (AsA) activity, by 65%, 63%, 94%, 41%, and 55% compared to NP under DS and by 0.26%, 8%, 33%, 42%, and 15% under no-stress (NS), respectively. Melatonin-priming also reduced malondialdehyde content, electrolyte leakage, hydrogen peroxide (H₂O₂) content, and superoxide anion (⁠ $O2−$⁠) content by 26%, 31%, 31%, and 33% compared to NP under DS and by 8%, 18%, 10%, and 11% under NS, respectively. In response to DS, melatonin-priming also stabilized the chloroplast structure, sustained cell expansion, protected cell walls, and greatly improved stomatal traits, including stomatal number, length, and width. Our results suggest that melatonin-priming improves drought tolerance in maize seedlings by alleviating the negative effect of ROS.

Nitrogen fertilization and irrigation patterns have been extensively studied for common maize (Zea mays L.), but there is limited published work for Zhengda 619, especially in subtropical areas. Nitrogen (N) fertilizer and irrigation play an important role in crop growth and yield improvements. The study aimed to investigate the yield, growth, chlorophyll content, reactive oxygen species (ROS) and enzyme activities of hybrid maize (Zhengda 619) under greenhouse conditions. Individual plants grown in plastic pots were subjected to two irrigation types—low irrigation (LW; 60% field capacity) and high irrigation water (HW; 80% field capacity)—and five N rates. Our results demonstrate that the LW irrigation increased dry matter, kernel yield, leaf chlorophyll, total root length, root diameter, root volume, and root surface area, as well as soil enzymes and plant antioxidant enzymes, while it lowered malondialdehyde (MDA), proline, and ROS. Moreover, most of the above parameters increased with increasing N application rates up to N3 under LW irrigation due to the increased N availability to the plant and soil enzymes. It is concluded that increasing N rates could improve soil enzyme activities as well as plant antioxidant enzymes and decrease ROS, ultimately resulting in a higher kernel yield under LW irrigation.