Metabolite-mediated enhancement of drought resilience in north-western Himalayan apple rootstocks through synergistic modulation of physiological performance

Metabolite-based interventions represent a promising strategy to mitigate water deficit-induced damage in perennial fruit crops such as apple (Malus × domestica Borkh.). This study evaluated the efficacy of two plant-derived metabolites**, melatonin (MT) and myo-inositol (MI), ** in enhancing drought resilience across five apple rootstocks (MM 111, MM 106, Bud 118, MM 116 and M9) under controlled greenhouse conditions. A two-phase experimental framework was used. In Phase I, dose optimization was performed under 40% field capacity using Response Index (RI) and Comprehensive Response Index (CRI) to identify the most effective metabolite combination. Among six treatments, T3 (100 µM MT + 100 µM MI) had the lowest CRI values, indicating improved physiological performance under moisture deficit. In Phase II, optimized treatments were evaluated under drought (40% FC) and irrigated (100% FC) regimes. Drought stress significantly impaired plant physiological functioning, reducing relative water content, chlorophyll stability, and gas exchange parameters. However, the combined application of MT + MI (T8: 100 µM each) significantly alleviated these effects by enhancing water retention, maintaining membrane stability, and improving photosynthetic efficiency. The most pronounced positive responses were observed in Bud 118 and MM 111, showing a strong genotype × treatment interaction. Notably, MT + MI also improved physiological performance under irrigated conditions, suggesting broader benefits for overall plant health. Each treatment was evaluated using three replications across five rootstocks, ensuring statistical reliability. Collectively, these results demonstrate the synergistic role of melatonin and myo-inositol in reinforcing drought tolerance. While the findings are based on controlled greenhouse conditions, they provide a mechanistic basis for future field validation and development of metabolite-assisted strategies for drought resilience in apple.