Various problems arise in plants that are exposed to stress factors causing cellular damage. Plants have developed various adaptive mechanisms against deficiency or excess of different plant nutrients, including boron deficiency or toxicity. Oxidative damage can be prevented by increasing stress tolerance in plants through these mechanisms. In addition, various elements play protective roles in the plant by increasing the biochemical activities responsible for the functioning of these mechanisms. Selenium, one of them, is an important trace element for humans, animals and plants. In this study, three genotypes [Tokak-Hamidiye (Barley), and Yellow Bride (Soy)] with two different structures (Monocotyl-Dicotyl) were used. In addition to deficient (0 mM) and toxic (1 mM) boron concentrations, 2 different doses of selenium were applied to the genotypes (0.1 ppm and 0.5 ppm). It was determined that the growth parameters of plants were suppressed in both B-deficient and toxic conditions. In addition, the relative water content of plants decreased in all plants under B toxicity conditions. On the other hand, increases were observed in both the growth values and the relative water content of the plant compared to the control conditions of the stressed groups with external selenium applications. Due to selenium applications, radical accumulation was suppressed and these radicals were also swept away with the increased activity of important antioxidant enzymes such as catalase, peroxidase and ascorbate peroxidase. It was determined that superoxide dismutase (SOD) activity was suppressed in Tokak and Hamidiye genotypes under boron deficiency and toxic conditions, but increased in Sarı bride. As a result of this study, it is seen that Selenium increases stress tolerance in plants with different characteristics under boron deficiency and toxic conditions.