Selenium and Iodine Regulate Redox Homeostasis and Photosynthetic Efficiency in Blackberry Seedlings Under Freezing Stress

Freezing stress is one of the major abiotic factors limiting the establishment and growth of blackberry seedlings. Beneficial elements such as selenium (Se) and iodine (I) have been reported to enhance stress tolerance in several crops, but their combined regulatory roles under freezing stress remain unclear. This study evaluated the effects of foliar Se and I applications on the antioxidative metabolism and photosynthetic performance of ‘BRS Tupy’ blackberry seedlings subjected to freezing stress. A 3 × 3 factorial design included three Se sources (0, sodium selenate, and organic Se at 75 mg L⁻ 1 ) and three I doses (0, 25, and 50 mg L⁻ 1 ), applied individually and in combination. One week after foliar treatment, plants were exposed to 26/-2 °C (day/night) for three days. Freezing exposure impaired photosynthetic activity, promoted reactive oxygen species (ROS) accumulation, and increased lipid peroxidation. However, some treatments, particularly those combining Se and I, mitigated these effects, showing improved antioxidant responses, osmoprotectant accumulation, and photosynthetic performance. These responses suggest a coordinated regulation of redox homeostasis and metabolic adjustment under freezing stress. Foliar application of Se in combination with iodine may represent a promising strategy to improve freezing tolerance and metabolic resilience in blackberry seedlings.