Assessment of the Impact of the Application of a Quercetin—Copper Complex on the Course of Physiological and Biochemical Processes in Wheat Plants (Triticum aestivum L.) Growing under Saline Conditions

Salt stress is one of the main stressors limiting plant growth and yield. As a result of salt stress, unfavorable changes in the photosynthesis process take place, leading to a decrease in plant productivity. Therefore, it is necessary to use biologically active substances that reduce the effects of this stress. An example of such a substance is quercetin, classified as a flavonoid, which plays an important role in alleviating the effects of salt stress, mainly by the inactivation of reactive oxygen species (ROS) and by improvement of the photosynthesis process. A study was made of the effect of the quercetin–copper complex (Q-Cu (II)), which has a stronger antioxidant effect than pure quercetin. By means of a pot experiment, the influence of solutions of the Q-Cu (II) complex (100 mg∙L(−1) [Q1], 500 mg∙L(−1) [Q2] and 1000 mg∙L(−1) [Q3]) on the physiological and biochemical processes occurring in wheat plants subjected to salt stress was investigated. The plants were given two sprays of Q-Cu (II) solution, and their physiological parameters were examined both 1 and 7 days after each application of this solution. The level of ROS and the activity of antioxidant enzymes (catalase [CAT], superoxide dismutase [SOD] and guaiacol peroxidase [GPOX]) were also determined. It has been shown that spraying with Q2 and Q3 solutions improves the chlorophyll content, the values of chlorophyll fluorescence parameters (the photochemical efficiency of PS II [F(v)/F(m)], the maximum quantum yield of primary photochemistry [F(v)/F(0)], and the performance index of PS II [PI]), and gas exchange (net photosynthetic rate [P(n)], stomatal conductance [g(s)], transpiration rate [E] and intercellular CO(2) concentration [C(i)]). As a result of the application of Q2 and Q3 solutions, the level of ROS and the activity of the antioxidant enzymes tested decreased, which means that these concentrations are most effective in counteracting the effects of salt stress.