Heat and Water-deficit Stress Responses of Tomato Seedlings Grafted onto Wild-type Solanum pennellii

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Wild tomato species Solanum pennellii has shown tolerance to water-deficit and heat stress, but its performance as a rootstock, particularly under combined stress, has been largely unexplored. This study evaluated physiological and biochemical responses of tomato seedlings grafted onto S. pennellii under single and combined water-deficit and heat stress. Tomato cultivars Celebrity and Arkansas Traveler were used as scions, and self-grafted plants served as controls. Grafted seedlings were transplanted into pots and exposed to heat (38/30 °C, day/night) and/or water-deficit stress (20% to 60% of field capacity) for 19 days in growth chambers. Compared with self-grafted seedlings, S. pennellii rootstock partially improved tolerance to water-deficit stress, with or without heat stress, as indicated by increased F v /F m (2.5%) and leaf water potential (52.9%), and reduced leaf reactive oxygen species (16.5%). In contrast, under heat stress, S. pennellii –grafted seedlings showed heat-sensitive responses, expressing lower shoot fresh weight, stem diameter, soil plant analysis development (SPAD), F v /F m , and leaf water potential in a cultivar-dependent manner. Leaf antioxidant traits in S. pennellii –grafted seedlings, including proline, total phenolics, and total antioxidant capacity (ferric reducing antioxidant power), also showed cultivar-specific responses but were generally reduced across stress conditions, which could indicate either lower oxidative burden or reduced protective capacity, depending on the physiological context. However, these physiological responses were not consistently reflected in seedling growth parameters. Overall, S. pennellii rootstock conferred partial, cultivar-specific tolerance to water-deficit and combined stress, while its utility to improve heat tolerance appears limited, suggesting greater potential as breeding material for water-deficit resilience than for heat stress tolerance.