Enhancing physiological performance and antioxidant defense in Mexican lime (Citrus aurantifolia) under water-deficit: synergistic effects of PGPB and KNO₃ application

Drought stress severely restricts citrus seedling growth, necessitating sustainable strategies to mitigate water-deficit damage. This study investigated the synergistic effects of plant growth promoting bacteria (PGPB) and foliar application of potassium nitrate (KNO₃) on the physiological performance and antioxidant defense mechanisms of Mexican lime ( Citrus aurantifolia ) under water-deficit conditions. The experiment was conducted using a randomized complete block design (RCBD) with three irrigation levels (100%, 70%, and 40% of crop evapotranspiration, ETc), five bacterial treatments ( Enterobacter cloacae E1, Bacillus subtilis CS1, Enterobacter cloacae R33, a bacterial consortium (COB), and a non-inoculated control), and KNO 3 foliar spray (0 and 2% w/v). Physiological traits, osmotic adjustment indicators, and antioxidant responses were evaluated at 60, 120, and 180 days after treatment application. Water-deficit significantly reduced leaf relative water content (RWC) and increased electrolyte leakage (EL), indicating impaired plant water status. However, PGPB inoculation, particularly in combination with KNO 3 , effectively mitigated EL under both moderate and severe water-deficit conditions. Drought stress also triggered an increase in total soluble carbohydrates (TSC) and a concomitant decrease in starch content, reflecting osmotic adjustment. The combined PGPB + KNO 3 treatments further enhanced TSC accumulation and partially restored starch reserves, contributing to improved carbon metabolism stability under water-limited conditions. Under severe water-deficit (40% ETc), the combination of Bacillus subtilis CS1 and 2% KNO 3 resulted in a 49.8% increase in catalase (CAT) activity, an 83.4% increase in peroxidase (POD) activity, and a 9.8% increase in total phenolic content compared to the control. Likewise, Enterobacter cloacae R33 combined with KNO 3 reduced malondialdehyde (MDA) content by 24.1% and increased leaf protein levels. Total antioxidant capacity (TAC) increased by 6.9% in plants treated with KNO 3 and the COB under severe drought stress. Under moderate water-deficit (70% ETc), the combined application of KNO 3 and the COB produced the highest flavonoid content. Overall, these findings indicate that the integration of PGPB with KNO 3 substantially enhances antioxidant enzyme activities and physiological resilience in Mexican lime under water-deficit. This integrated approach represents a cost-effective and environmentally sustainable strategy for mitigating drought-induced damage and improving crop productivity.