Differential Rootstock‐Mediated Regulation of Physiological and Hormonal Responses Enhances Apricot Resilience to Combined Drought and Heat Stress

Understanding the physiological and biochemical responses of apricot trees to combined drought and heat stress is crucial for improving orchard resilience under climate change scenarios. However, knowledge about how different rootstocks mediate these responses in apricot is limited. We investigated morphological, physiological, and biochemical responses of "Mikado" apricot grafted onto two rootstocks (Myrobalan 29C and Marianna 2624) under drought, heat shock, and combined stress conditions, followed by a recovery period. Rootstock genotype and stress treatment significantly influenced all measured parameters (p ≤ 0.01). Marianna 2624/"Mikado" consistently demonstrated superior stress tolerance, maintaining higher photosynthetic activity, better water relations, stronger antioxidant defences, and more effective osmoregulation (50% higher sugar accumulation) under combined stress compared to Myrobalan 29C/"Mikado." Recovery was also more pronounced in Marianna 2624/"Mikado," with better restoration of physiological functions and lower residual stress markers. Hormone regulation also differed markedly, with Marianna 2624/"Mikado" showing higher ABA accumulation during stress, but better recovery of growth hormones (IAA, GA, cytokinins) post-stress, suggesting more efficient stress signaling and recovery mechanisms. We conclude that rootstock selection significantly influences apricot resilience to combined abiotic stresses, with Marianna 2624 emerging as a superior choice for climate-resilient orchards. Relative differences in stress response mechanisms between rootstocks provide insights for breeding programs, while the quantitative stress response patterns identified offer valuable parameters for modelling apricot productivity under changing climatic conditions.