Enhancing drought stress tolerance in faba bean using phosphate nanoparticles

Drought stress is a major environmental constraint that limits the growth, productivity, and metabolic performance of faba bean ( Vicia faba L.) by disrupting nutrient uptake, impairing physiological processes, and inducing oxidative damage. This study investigated the effects of HA-NPs (phosphorus hydroxyapatite nanoparticles) applied through soil and foliar methods on mineral nutrition, biochemical attributes, and antioxidant defense mechanisms of faba bean plants under different drought stress levels. A comprehensive analysis of variance was conducted to evaluate the effects of drought, PHANPs concentrations, and their interactions on nutrient contents (N, P, Na, and K), photosynthetic pigments, protein levels, osmolyte accumulation, and antioxidant enzyme activities. Results showed that drought stress significantly ( P ≤ 0.001) reduced nitrogen, phosphorus, potassium, chlorophyll content, and protein levels, while increasing sodium accumulation, proline, soluble sugars, and antioxidant enzyme activities (catalase and peroxidase), indicating severe physiological and oxidative disturbances. In contrast, HA-NPs application significantly improved all measured parameters under both soil and foliar treatments. HA-NPs enhanced nutrient uptake and improved ionic balance by increasing N, P, and K concentrations while reducing Na accumulation, thereby improving plant nutritional status under drought conditions. Additionally, HA-NPs promoted chlorophyll retention, protein synthesis, and osmolyte accumulation, contributing to improved osmotic adjustment and metabolic stability. Moreover, HA-NPs modulated the antioxidant defense system by regulating catalase and peroxidase activities, suggesting reduced oxidative stress and improved ROS homeostasis in treated plants. The interaction between drought stress and HA-NPs was significant for most traits, indicating that HA-NPs effects were more pronounced under higher drought intensity. Although both application methods were effective, soil application generally resulted in more stable and consistent improvements across most parameters. Overall, the study demonstrates that HA-NPs effectively mitigate drought-induced damage in faba bean plants by enhancing nutrient acquisition, strengthening antioxidant defense systems, and improving physiological and biochemical resilience. These findings highlight the potential of HA-NPs as a promising nanobiostimulant for improving drought tolerance and sustaining productivity of faba bean under water-limited conditions.