ACCUMULATION AND VOLATILIZATION OF DIFFERENT CHEMICAL-SPECIES OF SELENIUM BY PLANTS

Selenium (Se) removal from polluted waters and soils is especially com plicated and highly expensive. Phytoremediation has been suggested as a low-cost, efficient technology for Se removal. Plants remove Se by u ptake and accumulation in their tissues, and by volatilization into th e atmosphere as a harmless gas. Unraveling the mechanisms of Se uptake and volatilization in plants map lead to ways of increasing the effic iency of the phytoremediation process. The objectives of this study we re. (i) to determine the effect of different Se forms in the root subs trate on the capacity of some plant species to take up and volatilize Se; (ii) to determine the chemical species of Se in different plant pa rts after the plants were supplied with various forms of Se; and (iii) to determine the influence of increasing sulfate levels on plant upta ke, translocation, and volatilization of different Se species. Plants of broccoli (Brassica oleracea var, botrytis L.), Indian mustard (Bras sica juncea L.), sugarbeet (Bt tn vulgaris L.) and rice (Oryza sativa L.) were grown hydroponically in growth chambers and treated for I wee k with 20 mu M Se as Na2SeO4, Na2SeO3 or L-selenomethionine (SeMeth) a nd increasing sulfate levels. The data show that shoots of SeO4-suppli ed plants accumulated the greatest amount of Se, followed by those sup plied with SeMeth then SeO3. In roots, the highest Se concentrations w ere attained when SeMeth was supplied, followed by SeO3, then SeO4. Th e rate of Se volatilization by plants followed the same pattern as tha t of Se accumulation in roots, but the differences were greater. Speci ation analysis (X-ray absorption spectroscopy) showed that most of the Se taken up by SeO4-supplied plants remained unchanged, whereas plant s supplied with SeO3 or SeMeth contained only SeMeth-like species. Inc reasing the sulfate level from 0.25 mM to 10 mM inhibited SeO3 and SeM eth uptake by 33% and 15-25%, respectively, as compared to an inhibiti on of 90% of SeO4 uptake. Similar results were observed with regard to sulfate effects on volatilization. We conclude that reduction from Se O4 to SeO3 appears to be a rate-limiting step in the production of vol atile Se compounds by plants. Inhibitory effects of sulfate on the upt ake and volatilization of Se may be reduced substantially if Se is sup plied as. or converted to, SeO3 and/or SeMeth rather than SeO4.