Quantitative, chemically specific imaging of selenium transformation in plants

Quantitative, chemically specific images of biological systems would be inv aluable in unraveling the bioinorganic chemistry of biological tissues. Her e we report the spatial distribution and chemical forms of selenium in Astr agalus bisulcatus (two-grooved poison or milk vetch), a plant capable of ac cumulating up to 0.65% of its shoot dry biomass as Se in its natural habita t. By selectively tuning incident x-ray energies close to the Se K-absorpti on edge, we have collected quantitative. 100-mu m-resolution images of the spatial distribution, concentration, and chemical form of Se in intact root and shoot tissues. To our knowledge, this is the first report of quantitat ive concentration-imaging of specific chemical forms. Plants exposed to 5 m u M selenate for 28 days contained predominantly selenate in the mature lea f tissue at a concentration of 0.3-0.6 mM, whereas the young leaves and the roots contained organoselenium almost exclusively, indicating that the abi lity to biotransform selenate is either inducible or developmentally specif ic. While the concentration of organoselenium in the majority of the root t issue was much lower than that of the youngest leaves (0.2-0.3 compared wit h 3-4 mM). isolated areas on the extremities of the roots contained concent rations of organoselenium an order of magnitude greater than the rest of th e root. These imaging results were corroborated by spatially resolved x-ray absorption near-edge spectra collected from selected 100 x 100 mu m(2) reg ions of the same tissues.