Lead accumulation in the aquatic fern Azolla filiculoides

In this study, we characterized lead (Pb²⁺) accumulation and storage by the aquatic fern Azolla filiculoides. Lead precipitates were detected in the vacuoles of mesophyll cells of Azolla plants cultured for 6 d in rich growth medium containing 20 mg l^-1 Pb²⁺. Energy dispersive spectroscopy (EDS) analysis of the relative element content of leaves collected from these plants revealed a 100% increase in the levels of P, S, Na and Ca and a 40% decrease in Mg and Cl compared to the untreated plants. Both Azolla whole plants and isolated apoplasts were incubated for 6 d in 20 mg l^-1 Pb²⁺. Lead content in the whole plant composed 0.37%, 2.3% and 1.8% of the dry weight after 2, 4 and 6 d of growth, respectively, while the isolated Azolla apoplast contained 0.125%, 1.22% and 1.4% Pb²⁺, respectively. Lead content in Azolla whole plant increase by 200%, 100% and 22% after 2, 4 and 6 d of growth, respectively, when compared to Azolla apoplast. Dark, electron dense deposits of lead were observed in light and transmission electron microscope in leaf cells treated with lead. All the observed lead deposits were localized in vacuoles while larger lead deposits were found in mature leaves than in young leaves. No lead deposits were found in cells of the cyanobiont Anabaena when the plants were exposed to similar conditions. Activity and content of V-H⁺-ATPase were studied in Azolla plants grown in the presence of 20, 40 and 80 mg l^-1 of lead for a period of 4 d. Activity of V-H⁺-ATPase was increased by 190%, 210% and 220%, respectively, but the content of V-H⁺-ATPase was reduced by all lead concentrations.