Bioindicator potential of Ricinus communis to simulated rainfall containing potassium fluoride

Background: Fluoride pollution is a global problem because of its high phytotoxicity. Fluoride is released in air, water and soil through industrial processes, where it damages various plant species. Ricinus communis is widely distributed in Brazil, India and China and has been extensively used as a phytoremediation species in heavy metal-contaminated soils. However, few studies regarding the effect of air pollutants on R. communis have been published, and no information about the exposure of this species to fluoride is available. Therefore, the aim of the present study was to investigate the effects of fluoride on R. communis morphoanatomical and physiological responses using simulated rainfall containing potassium fluoride (KF).

Methods: Young plants at approximately 10 days after emergence were treated daily with KF using simulated rainfall at 0, 1.5, 3.0 and 4.5 mg L^-1, for 37 consecutive days. Chlorophyll a fluorescence, gas exchange, anatomical characteristics and fluoride accumulation in the roots and leaves were evaluated after this period.

Results: No visual or anatomical symptoms were observed for the first three treatments. Necrosis and chlorosis were visually evident after the 37th day of KF application at 4.5 mg L^-1, followed by changes in parenchyma tissues, cell collapse and phenolic compound accumulation at the end of the experiment. No damage was observed in terms of photosynthetic photochemical and biochemical stages. Maintenance of physiological characteristics in the presence of fluoride accumulation in roots and leaves were shown to be important fluoride biomarkers. These characteristics suggest that R. communis is tolerant to 1.5 and 3.0 mg L^-1 KF, and is anatomically sensitive at 4.5 mg L^-1 KF.

Keywords: Castor plant; Fluoride; Pollution; Tolerant plants.