Water–nitrogen synergy shapes maize yield through leaf antioxidant defense, grain carbon–nitrogen metabolism, and hormonal regulation

Synergistic water-nitrogen (N) management is vital for high maize (Zea mays L.) yields, but the integrated physiological mechanisms driving yield formation remain unclear. A 2-year field study with 3 irrigation levels and 4 N rates revealed that high maize yields were maintained under mild drought combined with medium-to-high N via distinct pathways. Water-N synergy enhanced leaf antioxidant capacity, with N increasing peroxidase (POD) activity and reducing malondialdehyde, thereby mitigating oxidative stress, delaying chlorophyll and photosynthesis (An) decline, and sustaining assimilates such as soluble sugars (SS) and free amino acids (FAA). In grains, mild drought raised SS by 3.0% but reduced sucrose synthase (SuSy) and ADP-glucose pyrophosphorylase (AGPase) activities by 13.3% and 20.7%, respectively, lowering starch (ST) by 9.7%. Severe drought drastically reduced assimilate input, enzyme activities, and ST (-37.3%). N metabolism was also impaired, with lower FAA and protein (PRO) linked to lower glutamine synthetase and glutamate synthase activities. Hormonal balance was critical: zeatin + zeatin riboside (Z + ZR) and indole-3-acetic acid (IAA) promoted grain weight and correlated positively with carbon-metabolizing enzymes, while severe drought increased gibberellin A3 (GA3). In a multivariate analysis, SuSy, AGPase, IAA, Z + ZR, and GA3 explained 82.32% of ST variation, and the interaction between N metabolism enzymes and hormonal ratios explained 92.0% of PRO variation. Carbohydrate metabolism, N metabolism, and hormone balance accounted for 44%, 19%, and 7% of the variation in 100-grain weight, respectively, while their interactions explained an additional 19%. This study establishes a physiological network of water-N synergy, highlighting antioxidant enhancement and hormone-metabolism interactions, that provides a theoretical basis for precision water-N management in maize production.