Field-Based Evaluation of Heat Tolerance in Sweet Cherry Rootstocks Reveals Integrated Morphological and Physiological Adaptation Mechanisms

High summer temperatures increasingly constrain sweet cherry production, yet field-validated assessments of rootstock resilience remain scarce. To fill this gap, this study presents a pioneering multidimensional evaluation of five widely used sweet cherry rootstocks (Gisela 6, Gisela 12, Krymsk 5, Colt, and Lanting) under prolonged natural heat stress. Morphological traits, leaf anatomical characteristics, antioxidant enzyme activities (SOD, CAT, POD), lipid peroxidation (MDA), phytohormones (ABA and JA), and osmotic regulators were assessed. Traits with high coefficients of variation, including POD activity, ABA, JA, and soluble protein content, were identified as sensitive indicators of heat stress. Lanting exhibited the strongest heat tolerance, characterized by thicker leaves, fewer heat-induced lesions, and enhanced antioxidant capacity, whereas Gisela 6 showed severe leaf abscission, elevated MDA and ABA accumulation, and the weakest defense capacity. Correlation analysis indicated that root sucker number was positively associated with SOD activity and soluble sugar content, suggesting a potential role of whole-plant carbon allocation in mitigating oxidative stress. Using the Entropy Weight–TOPSIS model, we provided a robust ranking that identifies Lanting and Colt as superior heat-resilient genotypes. The results provide a field-validated framework that bridges the gap between controlled-environment theory and practical orchard management, offering critical guidance for expanding sweet cherry cultivation into high-temperature regions.