Accumulation and volatilization of different chemical species of selenium by plants
Selenium (Se) removal from polluted waters and soils is especially complicated and highly expensive. Phytoremediation has been suggested as a low-cost, efficient technology for Se removal. Plants remove Se by uptake and accumulation in their tissues, and by volatilization into the atmosphere as a harmless gas. Unraveling the mechanisms of Se uptake and volatilization in plants may lead to ways of increasing the efficiency of the phytoremediation process. The objectives of this study were: (i) to determine the effect of different Se forms in the root substrate on the capacity of some plant species to take up and volatilize Se; (ii) to determine the chemical species of Se in different plant parts after the plants were supplied with various forms of Se; and (iii) to determine the influence of increasing sulfate levels on plant uptake, translocation, and volatilization of different Se species. Plants of broccoli (Brassica oleracea var. botrytis L.), Indian mustard (Brassica juncea L.), sugarbeet (Beta vulgaris L.) and rice (Oryza sativa L.) were grown hydroponically in growth chambers and treated for 1 week with 20 μM Se as Na2SeO4, Na2SeO3 or L-selenomethionine (SeMeth) and increasing sulfate levels. The data show that shoots of SeO4-supplied plants accumulated the greatest amount of Se, followed by those supplied with SeMeth then SeO3. In roots, the highest Se concentrations were attained when SeMeth was supplied, followed by SeO3, then SeO4. The rate of Se volatilization by plants followed the same pattern as that of Se accumulation in roots, but the differences were greater. Speciation analysis (X-ray absorption spectroscopy) showed that most of the Se taken up by SeO4-supplied plants remained unchanged, whereas plants supplied with SeO3 or SeMeth contained only SeMeth-like species. Increasing the sulfate level from 0.25 mM to 10 mM inhibited SeO3 and SeMeth uptake by 33% and 15–25%, respectively, as compared to an inhibition of 90% of SeO4 uptake. Similar results were observed with regard to sulfate effects on volatilization. We conclude that reduction from SeO4 to SeO3 appears to be a rate-limiting step in the production of volatile Se compounds by plants. Inhibitory effects of sulfate on the uptake and volatilization of Se may be reduced substantially if Se is supplied as, or converted to, SeO3 and/or SeMeth rather than SeO4.