Synergistic effects of salicylic acid based plant growth regulators on growth, photosynthetic performance, and antioxidant defense of Polianthes tuberosa L. under drought stress

Drought stress is a major limitation to sustainable floriculture in arid regions, severely affecting plant growth and flower quality. To mitigate these effects, this study evaluated foliar application of salicylic acid (SA; 300 ppm) applied alone and in combination with indole-3-acetic acid (IAA; 200 ppm), ascorbic acid (AA; 300 ppm), and chitosan (CS; 300 ppm) on the growth, photosynthetic performance, and antioxidant defense of Polianthes tuberosa L. under graded drought conditions [90%, 70%, and 40% water-holding capacity (WHC)]. The experiment was arranged in a factorial randomized complete block design with five replications. Drought stress was imposed for 8 weeks until first floret opening, while foliar treatments were applied four times (25, 40, 55, and 70 days after planting) starting at the three-leaf stage (BBCH 13). Results showed that severe drought (40% WHC) stress significantly reduced shoot biomass by ~ 60%, root biomass by 65-70%, and root surface area by ~ 45%, compared with well-watered conditions (90% WHC), indicating strong growth inhibition. A significant drought × PGR interaction confirmed that treatment responses were stress dependent. Among treatments, SA + IAA most effectively improved growth, biomass accumulation, and reproductive traits across drought levels, whereas SA + CS improved plant water status and reduced electrolyte leakage. In contrast, SA + AA better maintained chlorophyll fluorescence parameters and photosynthetic efficiency under drought stress. These responses were supported by increased antioxidant enzyme activity (SOD, CAT, POD), higher phenolic content, and soluble proteins, contributing to improved drought tolerance. Overall, SA-based combinations improved drought tolerance in a trait-specific manner, with SA + IAA promoting growth, SA + AA supporting photosynthetic performance, and SA + CS improving water status and membrane stability under water-limited conditions. These findings suggest that SA-based growth regulator combinations may enhance drought resilience and sustain growth in Polianthes tuberosa under water-limited conditions.