Complex Analysis of Physiological, Biochemical and Molecular Mechanisms of Drought Adaptation in Apple (Malus domestica Borkh.): A Review

Drought is one of the most critical abiotic stresses limiting apple (Malus domestica Borkh.) growth, productivity and fruit quality worldwide. Understanding the complex physiological, biochemical and molecular mechanisms underlying drought tolerance is essential for developing stress-resilient cultivars. The present review highlights integrated responses of apple varieties to drought stress, focusing on water relations, gas exchange, osmotic regulation, antioxidant defense and hormonal signaling. Physiologically, drought-tolerant genotypes maintain higher relative water content, stomatal conductance and water-use efficiency. Biochemically, accumulation of osmolytes (proline, soluble sugars, glycine betaine) and enhanced activity of antioxidant enzymes (SOD, CAT, POD, APX) mitigate oxidative damage. Molecular mechanisms involve activation of drought-responsive transcription factors (AREB/ ABF, DREB, NAC, MYB), signal transduction through ABA pathways and upregulation of stress-protective genes. Integration of these responses enables better nutrient and water uptake, osmotic adjustment and improved photosynthetic performance under water deficit. Future research should focus on genome editing, marker-assisted selection and omics-based approaches to accelerate the breeding of drought-resilient apple cultivars.