Changes in leaf gas exchange and chlorophyll a fluorescence on soybean plants supplied with silicon and infected by Cercospora sojina

Frogeye spot, caused by Cercospora sojina, is one of the major diseases causing yield losses in soybean. Considering the potential of silicon (Si) to attenuate the physiological impairments imposed by pathogens infection, this study investigated its effect on leaf gas exchange and chlorophyll (Chl) a fluorescence parameters as well as on the concentrations of photosynthetic pigments and carbohydrate pools on soybean plants from cultivars Bossier and Conquista (susceptible and resistant to frogeye spot, respectively) grown in hydroponic culture containing 0 or 2 mM Si (−Si and +Si plants, respectively) that were noninoculated or inoculated with C. sojina. Plants from cultivar Bossier were more susceptible to frogeye spot compared to cultivar Conquista regardless of Si supply. Frogeye spot severity increased by Si supply regardless of the cultivar. There were no changes in the physiological parameters for noninoculated +Si plants. Even though the susceptibility of plants from cultivar Bossier to frogeye spot increased by Si supply, they showed lower values for the leaf gas exchange parameters, photochemical efficiency and concentration of photosynthetic pigments. The impairments imposed by C. sojina infection on the physiology of plants from cultivar Conquista were governed chiefly by reductions in stomatal conductance regardless of Si supply. The increase in hexose concentration for inoculated plants of the two cultivars was associated with their resistance to frogeye spot. Considering that the increased susceptibility of +Si plants from cultivar Conquista did not result in physiological impairments, it is plausible to rule out that the lower photosynthetic efficiency of plants from cultivar Bossier occurred due to a sharp reduction in the photosynthetically active leaf tissue.