Effects of drought stress and nitrogen application on anatomical structure and hydraulic traits of apple leaves

The mechanisms by which water, nitrogen, and their interaction affect leaf hydraulic traits of apple （Malus domestica Borkh.） remain unclear. In this study, Three-year-old 'Danxia' apple seedlings were subjected to a two-factor experiment with water and fertilizer manipulation to analyze the effects of drought, nitrogen application, and their interaction on leaf anatomical and hydraulic characteristics. The results showed that under well-watered conditions, the vessel diameter, vessel wall thickness, and vessel vulnerability index in leaf veins were significantly higher under medium and high nitrogen treatments than under low nitrogen treatment, whereas the vessel anti-collapse index was significantly lower under medium and high nitrogen than under low nitrogen. Under drought conditions, the vessel diameter, vessel wall thickness, vein-specific hydraulic conductivity, and vulnerability index were significantly lower under medium and high nitrogen than under low nitrogen, while the vessel anti-collapse index was significantly higher under medium and high nitrogen than under low nitrogen. These results indicate that the effects of nitrogen application on leaf hydraulic architecture and drought resistance of apple depend on water availability. Under well-watered conditions, nitrogen supply enhances leaf water transport efficiency and morphological water retention capacity. In contrast, under drought conditions, nitrogen application favors hydraulic safety but reduces morphological water retention capacity. Therefore, in apple nutrient management, nitrogen application should be controlled under drought stress to avoid reducing leaf drought resistance.