Morphoanatomical and metabolic changes in Bixa orellana L. (Bixaceae) exposed to atmospheric fluorine

Fluorine (F) is a toxic element for plants and can cause damage even at low concentrations. Brazil has several anthropogenic sources of F; however, relatively few native species have been studied. Thus, the objective of this study was to evaluate the responses promoted by F in Bixa orellana, a species native to South America and economically important for producing bixin, a pigment used as a natural dye in the food, cosmetic, and textile industries. The plants were subjected to simulated rain containing 0, 10, 20, and 40 mg/L of F. Visual analyses were performed during the experiment, and at the end, gas exchanges, photosynthetic pigments, and F were quantified, and leaf samples were collected for light and scanning electron microscopy and cell death testing. The concentration of F in B. orellana increased according to the concentration used in each treatment. There was a reduction in CO₂ assimilation (A), stomatal conductance (gs), respiration (R_d), and photosynthetic pigment content in all plants exposed to the pollutant. Chloroses, necroses, and wrinkling occurred in the leaves of plants exposed to F. The stress also resulted in anatomical damage to the leaves, which showed atrophied epidermal cells and flaccid glandular trichomes. Laticifers and spongy and palisade parenchyma cells were plasmolyzed. Additionally, blue-stained cell groups indicating cell death were observed in the leaves of all treatments containing F. It is concluded that B. orellana is sensitive to F and responds quickly to the presence of this pollutant, making it a potential phytoindicator species for F in the atmosphere in possible biomonitoring programs.