Optimizing Selenium Delivery in Grapevines: Foliar vs. Rhizosphere Fertilization Effects on Photosynthetic Efficiency, Fruit Metabolites, and VOCs of ‘Muscat Hamburg’ Grape (Vitis vinifera L.)

This study examined the effects of selenium (Se) fertilization, applied via foliar and rhizosphere methods, on the physiological and biochemical characteristics of ‘Muscat Hamburg’ grapes. Sodium selenite (Na2SeO3) treatments were administered at three concentrations (50, 100, and 150 ppm) during critical phenological stages. The results showed that Se at 50 ppm effectively increased the chlorophyll content and enhanced chlorophyll fluorescence parameters. Se significantly elevated total soluble solid content and reduced titratable acidity, thereby increasing the TSS/TA ratio. Foliar fertilization with 50 ppm Se enhanced cluster size without affecting berry dimensions, whereas rhizosphere fertilization increased both with increasing Se concentrations, albeit with negative impacts on berry size at higher concentrations. Se increased flavonoid content in grape peels, with rhizosphere fertilization exerting more pronounced effects. Se—via rhizosphere fertilization at 100 and 150 ppm—significantly influenced VOCs derived from fatty acid and isoprene metabolic pathways. Mantel’s test confirmed that foliar fertilization significantly increased chlorophyll content and fluorescence indices, while rhizosphere fertilization had more marked effects on flavonoid content, berry and cluster size, and VOCs, particularly through fatty acid metabolism. These findings suggest that Se can enhance grape quality, but optimal concentrations and fertilization methods must be carefully determined to avoid adverse effects.