Summary
Shoot concentrations of Se associated with a 10% reduction in dry matter yield were established for perennial ryegrass (Loliumperenne L.) and white clover (Trifolium repens L.) grown in sand culture. Selenite– treated plants had lower shoot concentrations of Se than those treated with selenate; the critical toxic values associated with a 10% reduction in growth were 48 and 320μg Se g− 1 dry matter shoots for ryegrass plants, and 160 and 330μeg1 dry matter shoots for white clover plants, respectively. Differences in the mode of absorption of selenite and selenate, and in the distribution and chemical form of Se found in the plant after absorption, probably account for this disparity in toxicity. While both selenate and selenite increased the Se concentration in the tissues of the plants to high levels, a greater proportion of the absorbed Se was transported to the shoots of the selenate‐treated plants than of those treated with selenite. Increasing supplies of selenate were associated with a reduction in chlorophyll content. Contrary to the concept of a common uptake mechanism in the roots for selenate and sulphate, increasing supplies of selenate had a synergistic effect on S concentrations in the shoots rather than the expected antagonistic effect.
A strain of Bacillus megaterium isolated from soil has been found to oxidize elemental selenium in laboratory cultures to selenite and a trace of selenate (< 1 percent of the selenite). This observation represents an important but hitherto unreported oxidative step in the biological selenium cycle.
Selenite was adsorbed on an allophane clay from solutions of different concentrations at pH 5.0, at 30C, and under a N2 atmosphere, and the amounts of sulfate, silicate and hydroxyl ions released were measured. The results were compared with those from a similar study with phosphate on the same clay.The results indicate that at low concentrations both phosphate and selenite exchanged with adsorbed sulfate, adsorbed silicate, and aquo and hydroxo groups. About three times more phosphate than selenite was adsorbed, due mainly to phosphate displacing more aquo groups and thus making the surface less positive.At high concentrations, whereas the selenite adsorption reached a maximum, phosphate continued to be adsorbed. The latter was due to phosphate displacing structural silicate and probably also to disruption of hydrous oxide polymers. A two‐term Langmuir equation distinguished adsorption by surface ligand exchange from these other reactions at high concentration.A selenite desorption experiment showed that phosphate displaced all of the selenite adsorbed. Phosphate was adsorbed with greater strength, the selectivity coefficient, KSeP, being 2.2.
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