Physiological Responses of Highbush Blueberry (Vaccinium corymbosum L.) to Combined Water Deficit and Aluminum Stress: The Role of Methyl Jasmonate in Enhancing Stress Resistance

Highbush blueberry (Vaccinium corymbosum L.) is widely cultivated in southern Chile on acidic Andisols, where aluminum (Al3+) toxicity and water deficit frequently occur simultaneously and limit plant performance. However, the integrated physiological responses to these stresses and the potential protective role of methyl jasmonate (MeJA) remain poorly understood. This study evaluated the physiological, biochemical, and hormonal responses of two cultivars with contrasting resistance, Legacy (Al-resistant) and Star (Al-sensitive), exposed to Al3+ stress, water deficit, and their combination, with or without MeJA application. Plants were grown in Andisol soil under greenhouse conditions and subjected to eight treatments, with measurements performed at 7, 14, and 21 days. Exposure to stress conditions resulted in decreased growth, reduced leaf water status, diminished photosynthetic performance, lower pigment stability, and decreased auxin concentration as estimated by Salkowski-reactive indolic compounds. Conversely, stress conditions led to increased aluminum (Al) accumulation, elevated proline levels, enhanced lipid peroxidation, and heightened antioxidant responses. Water deficit produced the strongest reductions in photosynthesis, about 48% in Legacy and 65% in Star, whereas Al accumulated mainly in the roots of Star (14-fold). The combined stress intensified physiological limitations and oxidative damage, particularly in the Star cultivar (4-fold), which showed stronger reductions in photosynthetic parameters, higher Al accumulation, and greater lipid peroxidation. In contrast, Legacy maintained more stable physiological performance. Exogenous MeJA mitigated stress effects by reducing Al accumulation (30–35%) and oxidative damage, improving photosynthetic performance (40–60%) and water status, and partially restoring auxin levels and growth in both cultivars, being more evident in the resistant cultivar Legacy. These results indicate that MeJA contributes to the regulation of physiological and antioxidant responses associated with resistance to combined Al toxicity and water deficit in highbush blueberry.