α-Tocopherol mediates alleviation of salt stress effects in Glycine max through up-regulation of the antioxidant defense system and secondary metabolites

α-Tocopherol (αT) has been reported to overcome the adverse effects of many environmental stresses, including salinity. A pot experiment was performed to investigate the response of salt-stressed soybean plants to the foliar application using 50, 100, or 200 mg αT L^–1. The stress was applied by irrigation with three different dilutions of seawater [e.g., EC = 0.23 (tap water as a control), 3.13, or 6.25 dS m^–1]. The gradual increase in salinity level gradually increased net carbon dioxide (CO₂) assimilation, relative growth rate, antioxidant capacity, secondary metabolites (total phenolic compounds, flavonoids, and tannins), enzymatic activities, and DNA fragmentation due to the gradual increase in lipid peroxidation as a result of an increase in hydrogen peroxide (H₂O₂). On the contrary, yield components (number and weight of seeds plant^–1) and photosynthetic pigment (total chlorophyll and carotenoids) contents gradually decreased with increasing salinity level. However, leaf treatment with αT, especially at 200 mg αT L^‒1, preserved the highest antioxidant activities (antioxidant capacity, secondary metabolites, and enzymes), which were associated with the preservation of soybean yield and leaf pigments. These positive results occurred due to the reduction in lipid peroxidation as a result of a significant decrease in the level of H₂O₂. Therefore, our results recommend the use of 200 mg αT L^–1 as a commercial formula for soybeans grown under saline irrigation of no more than 6.25 dS m^–1.