Effects of fluoride emissions on two tropical grasses: Chloris gayana and Panicum maximum cv. Colonião

In order to detect early effects of plant contamination by fluoride emission on two tropical grasses, Chloris gayana and Panicum maximum, previously cultivated under greenhouse conditions, were exposed to a single source of fluoride emission at a station at 1.1 km from an aluminum smelter in Ouro Preto, MG, Brazil. Controls were placed at a reference station 78 km from the fluoride source. During an 8-day period of exposure leaf injury, ionic permeability, photosynthetic rates, stomatal conductance, transpiration, chlorophyll a fluorescence and chlorophyll, soluble carbohydrates and fluoride contents were evaluated. Plants at the Ouro Preto station showed an increase in fluoride content, leaf injury and ionic permeability. Symptoms of injury by fluoride exposure were visible after 3–4 days in both species. High electrolyte leakage and correlation coefficients between the total ionic permeability and the fluoride content in leaves indicate a fluoride effect on the structural and/or functional integrity of the cellular membranes. Leaf fluoride injuries were quite different in the two species. In C. gayana necroses were limited to the leaf tips, while in P. maximum damages were observed in the whole leaf, suggesting a higher susceptibility of this latter species to fluoride. Nonetheless, neither grass showed statistical differences with respect to photosynthetic rates, stomatal conductance, transpiration, chlorophyll a fluorescence and chlorophyll content in leaves without any apparent fluoride injury. Plants at the Ouro Preto station showed a significant decrease in reducing sugar content between 3 and 5 days of exposure to fluoride, but thereafter reducing sugar content increased reaching the content of control plants. Fluoride exposed plants also showed a remarkable starch content reduction, remaining always much lower than those at the reference station.