Exogenous Regulators Enhance Physiological Recovery and Yield Compensation in Maize Following Mechanical Leaf Damage

To elucidate how exogenous regulators mitigate the impact of mechanical leaf damage on maize, field experiments were conducted on two sowing dates (S1, S2) using two cultivars (XY335, ZD958). Severe leaf damage at the six-leaf stage significantly reduced kernel number, ear number, and 100-kernel weight, causing yield losses of 21.9–48.9%. Foliar application of melatonin (MT), brassinolide (BR), and urea (UR) substantially alleviated these losses, increasing yield by 14.1–52.2% compared to damaged controls, with UR and BR being most effective, especially in ZD958. These regulators restored leaf area index (LAI) by promoting leaf width and delaying senescence, improved photosynthetic performance (Pn, Gs, Ci, and Tr), enhanced post-silking dry matter accumulation by up to 31%, and accelerated grain filling through increased maximum and mean filling rates. Structural equation modeling confirmed that kernel number and 100-kernel weight were the primary yield determinants. These findings reveal the physiological mechanisms underlying damage recovery and demonstrate the potential of targeted regulator applications—urea as a cost-effective option, brassinolide for improving kernel number under sustained stress, and melatonin for broad resilience. This study provides not only theoretical evidence but also a feasible strategy for mitigating yield loss in maize production under field conditions where leaf damage commonly occurs.