Foliar applied zinc oxide nanoparticles alleviate severe water stress in wheat via improved leaf water status coordinated by osmotic, hormonal and antioxidant regulation

Drought stress critically limits crop production and threatens global food security. Despite significant efforts to alleviate drought stress through foliar applied zinc oxide nanoparticles (ZnO NPs), their stage-specific efficacy and underlying physiological mechanisms under varied soil moisture remain poorly understood. Therefore, we systematically investigated the effects of foliar application of ZnO NPs at 50 and 100 mg L -1 during the heading and grain filling stages on leaf photosynthetic, hydraulic, and physiological‑biochemical traits in wheat ( Triticum aestivum L.) under mild (W1, 65%–75% field capacity) and severe (W2, 50%–60% field capacity) deficit irrigation conditions. The results showed that under W2 condition the net photosynthetic rate, stomatal conductance, leaf water potential, plant hydraulic conductance decreased significantly, while abscisic acid (ABA), soluble sugars, and proline increased sharply during the heading stage of wheat. Foliar application of 100 mg L -1 ZnO NPs during this stage effectively alleviated drought‑induced photosynthetic inhibition, by reducing ABA content, normalizing osmotic potential via down-regulation of stress induced proline and soluble sugar over accumulation, and then promoting the stabilization of predawn leaf water potential. In contrast, during the grain filling stage, strategies relying solely on leaf protection exhibited limited potential for improving photosynthesis and drought resistance. Furthermore, under W2 conditions, the application of ZnO NPs significantly increased the zinc content in stems, leaves, and grains on a dry matter basis at maturity. These findings clarify that foliar ZnO NPs alleviate drought stress primarily during the heading stage by modulating ABA signaling, osmotic adjustment, and antioxidant defense, thereby improving leaf water status and photosynthesis. This study provides a physiological basis for the stage-specific application of ZnO NPs in wheat under water-limited conditions.