H ₂ O ₂ accumulation shapes the differential recovery from nitrogen and magnesium deficiency in cucumber

Nitrogen (N) and magnesium (Mg) deficiencies involve distinct underlying damage mechanisms and recovery responses. In this study, the physiological and biochemical responses of hydroponically grown cucumber plants to N or Mg deficiency and their recovery after nutrient resupply were evaluated. Considering that these deficiencies induce oxidative stress, we evaluated alterations in the antioxidant system and the accumulation of hydrogen peroxide (H2O2). The results revealed that both N deficiency and Mg deficiency inhibited cucumber growth and biomass production, with more pronounced adverse effects observed in leaves under Mg-deficient conditions. N resupply effectively restored leaf morphology, gas exchange parameters, and photosynthetic efficiency. In contrast, Mg resupply resulted in only partial recovery. This limited recovery can be attributed to irreversible damage to the photosynthetic system caused by the inability of antioxidant enzymes to effectively scavenge excessive amounts of reactive oxygen species (ROS), indicating that Mg deficiency triggers H2O2-induced cellular damage. Furthermore, the application of the H2O2 scavenger DMTU (N,N'-dimethylthiourea) significantly reduced H2O2 levels under both deficiency conditions, particularly under Mg-deficient conditions. These findings demonstrate that N deficiency and Mg deficiency have distinct effects on antioxidant responses and underscore the pivotal role of H2O2 in plant responses to nutrient deficiency stress and subsequent recovery. These findings suggest that targeted modulation of H2O2 levels may serve as a promising strategy to increase crop resilience, offering a clear direction for future research.