Effects of Different Selenium Concentrations on Agronomic Traits, Antioxidant Defense, and Leaf Metabolome in Blueberry (Vaccinium corymbosum L. ‘Brigitta’)

Selenium (Se) is an important micronutrient that is required in very small amounts and plays a significant role in enhancing plant growth, stress resistance, and fruit quality. In this study, we investigated the effects of different sodium selenite concentrations (CK, 0 mg/L; Se1, 0.50 mg/L; Se2, 1.00 mg/L, Se3, 2.00; and Se4, 3.00 mg/L) on the growth, nutrient absorption, antioxidant capacity, and leaf metabolome of blueberry (Vaccinium corymbosum L. ‘Brigitta’) in hydroponic culture. Our results showed that moderate Se concentration (1.00 mg/L, Se2) had noticeable enhancements in key traits like taller plants, thicker stems, a greater number of leaves, and stem fresh weight, with increases of 60.23%, 61.90%, 36.05%, and 87.97%, respectively, compared to the CK. In addition, the appropriate application of Se fertilizer (1.0 mg/L, Se2) can enhance the absorption of macronutrients by plants, with the total contents of nitrogen (N), phosphorus (P), and potassium (K) increasing by 48.11%, 15.85%, and 14.25%, respectively, compared to CK. In comparison to CK, the content and accumulation of total Se rose dramatically under the Se4 treatment, showing increases of 2300% and 2514%. The contents of chlorophyll and antioxidant enzyme activities were maximized at Se2, while excessive Se (Se4) led to oxidative damage, as indicated by elevated MDA, H2O2, and O2− levels. Moreover, metabolomic analysis revealed that moderate Se concentration (Se2) significantly altered metabolic pathways related to aminoacyl-tRNA biosynthesis, arachidonic acid metabolism, and ABC transporters, with downregulation of key metabolites in sugar and organic acid metabolism (e.g., α-D-glucose-6-phosphate, L-lactic acid, maleic acid). In contrast, high Se concentration (Se4) disrupted these pathways and promoted volatile compound accumulation. These findings demonstrate that moderate Se application enhances blueberry growth and quality by regulating nutrient uptake, antioxidant defense, and primary metabolism, whereas excessive Se induces metabolic imbalance and oxidative stress. Overall, moderate Se fertilizer (1.00 mg/L) can significantly enhance the growth and quality of blueberries, while excessive selenium may have adverse effects.