Combined Selenium and Iodine Application Modulates Antioxidant and Photosynthetic Responses in Raspberry Seedlings Under Heat Stress

Heat stress limits raspberry cultivation in tropical and subtropical regions, yet strategies to mitigate its effects on seedlings are scarce. Selenium (Se) and iodine (I) individually enhance plant tolerance to abiotic stress, but whether their combined application produces complementary effects on the physiological responses of heat-stressed raspberry seedlings remains unexplored. This study addressed this gap by evaluating the physiological, biochemical, and photosynthetic responses of ‘Heritage’ raspberry seedlings to foliar applications of Se and I, alone and in combination, under high-temperature conditions. The experiment used a completely randomized design with ten treatments: one non-stressed control (0 mg L−1 Se and 0 mg L−1 I) and nine heat-stressed treatments. Among the stressed treatments, one received no selenium or iodine (0 mg L−1 Se and 0 mg L−1 I) and served as the stressed control. The remaining eight treatments received combinations of selenium (0, 25, and 50 mg L−1) and iodine (0, 50, and 100 mg L−1). One week after foliar application, seedlings were exposed to heat stress under controlled conditions of 40/18 °C (day/night) for three days. Physiological, biochemical, antioxidant, and chlorophyll fluorescence parameters were evaluated during and after the stress period. During heat stress, intermediate Se and I combinations helped maintain plant water status and photosynthetic performance, although the physiological and biochemical responses varied across treatments and sampling times. After the stress period, these treatments promoted osmotic adjustment and improved antioxidant regulation, contributing to the recovery of raspberry seedlings. Principal component analysis reinforced the association between intermediate Se and I combinations and mechanisms related to antioxidant regulation, osmotic adjustment, and photoprotection. Collectively, these findings indicate that co-application of Se and I triggers complementary physiological mechanisms of heat-stress acclimation in raspberry seedlings, suggesting a promising strategy for enhancing seedling tolerance to high-temperature stress.