The issues related to the deepening problem of soil salinity constitute an important aspect of the protection of the natural environment globally. Therefore, new plant species and innovative solutions supporting the efficient cultivation of plants on saline lands are sought. The exogenous use of various phytoprotectants shows promising results in increasing plant resistance to environmental stresses. Quercetin is a strong antioxidant among phenolic compounds and plays a physiological and biochemical role in plants. The aim of the study was to assess the effect of an aqueous quercetin solution used in various concentrations (1.0%, 3.0% and 5.0%) on the efficiency of the photosynthetic apparatus of Andropogon gerardi (big bluestem) plants grown under salt stress (soil effect of sodium chloride (NaCl) at a concentration of 200 mM). The applications of the aqueous quercetin solution were carried out twice (on days 15 and 22 after plant emergence). Measurements of physiological processes (gas exchange, relative Chl content and Chl fluorescence) were performed four times on the first or second fully developed leaves: on the first and seventh days after each treatment. The foliar application of the aqueous quercetin solution significantly changed the relative chlorophyll content in the leaves (CCl), the chlorophyll fluorescence parameters (F_v/F_m, F_v/F₀ and PI) and the gas exchange parameters (P_N, E, g_s and C_i). Among the tested variants, compared to the control, the strongest stimulating effect on the course of physiological processes in the leaves of Andropogon gerardi was found for 3.0 and 5.0% of aqueous quercetin solution.