Abstract

Nitrogen availability regulates cucumber growth, oxidative balance, and powdery mildew resistance. This study evaluated three nitrogen gradients—50% nitrogen deficiency (T1), 100% normal nitrogen supply (T2), and 150% excess nitrogen (T3)—under powdery mildew inoculation (B0) and healthy non-inoculated control (B1) conditions, simultaneously measuring organ biomass, oxidative stress markers, antioxidant enzymes, disease-resistant PPO activity, and sugar metabolism indicators. Powdery mildew infection significantly suppressed cucumber biomass accumulation, elevated malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents, while triggering rises in superoxide dismutase (SOD), catalase (CAT), polyphenol oxidase (PPO), soluble sugar and sucrose levels. Under pathogen stress (B0), normal nitrogen (B0T2) outperformed both deficient and excess nitrogen treatments comprehensively: compared with nitrogen-deficient B0T1, leaf, stem and root fresh biomass of B0T2 increased by 40.3%, 43.8% and 54.9%, while their dry weights rose by 43.1%, 43.0% and 56.7%; all biomass indicators of B0T2 were over 20% higher than excess-nitrogen B0T3. For oxidative damage, B0T2 had 25.4% lower MDA than B0T1 and 21.9% lower MDA than B0T3, alongside 13.7% and 6.4% reductions in H2O2 relative to B0T1 and B0T3, respectively. In terms of antioxidant capacity, SOD activity of B0T2 was 8.7% higher than B0T1 and 30.6% higher than B0T3, with CAT activity increased by 45.0% vs. B0T1 and 23.9% vs. B0T3. As for defense and carbohydrate metabolites, B0T2’s PPO activity was 12.8% and 27.8% higher than B0T1 and B0T3; soluble sugar content grew by 31.8% and 9.3%, and sucrose concentration rose by 48.0% and 15.6% correspondingly. Nitrogen deficiency aggravated growth inhibition and disease injury, whereas excess nitrogen failed to further boost disease resistance and produced inferior biomass compared with optimal nitrogen supply. These quantitative findings demonstrate that appropriate nitrogen management strengthens cucumber powdery mildew resistance by mitigating oxidative lipid peroxidation, enhancing ROS-scavenging antioxidant enzyme activity, activating defensive PPO, and optimizing soluble sugar and sucrose accumulation to coordinate vegetative growth and stress defense.

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