Crop-Level Screening and Physiological Evaluation Reveal Drought-Protective Effects of Plant-Derived Extracts in Cucumber Seedlings

Drought stress severely limits crop productivity by impairing water balance, photosynthesis, and oxidative homeostasis. In this study, plant-derived extracts from Psidium guajava, Aloe vera, Allium sativum, and Medicago sativa were evaluated as biostimulants for improving drought tolerance in barley, rice, maize, and cucumber seedlings. Crop-level screening showed pronounced interspecific differences, with cucumber exhibiting the highest recovery of shoot fresh weight (79% of the non-stressed control at 0.1% extract concentration, compared to 39% under drought stress alone). Consequently, cucumber seedlings were selected for detailed physiological and biochemical analyses. Under drought stress, extract-treated plants maintained higher relative water content, preserved PSII efficiency (Fv/Fm), and retained chlorophyll and carotenoid contents compared with drought-stressed controls. Drought-induced oxidative damage, reflected by increased hydrogen peroxide and malondialdehyde, was significantly reduced by extract application. These protective effects were associated with higher activities of key antioxidant enzymes and increased accumulation of ascorbate and glutathione. Integrated analyses using oxidative stress indices and z-score heatmaps revealed coordinated improvements across multiple physiological and biochemical parameters, particularly under prolonged stress. Collectively, the results suggest that selected plant extracts enhance drought tolerance by supporting water status, antioxidant capacity, and photosynthetic stability, supporting their potential use as sustainable biostimulants in drought-resilient crop production systems.