Plant availability of selenite and selenate as influenced by the competingions phosphate and sulfate

The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respe ctively. In solution culture, the concentration of phosphate is typically 1 00- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A nutrient soluti on study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L. cv. Evening Shade) and strawberry clover (Trifolium fragiferrum L. cv. O'Conner) at basal concentrations of SO4 (0.5 mM) and PO4 (2 mu M) similar to those found in soil solution. Conc entrations up to 5 mM SO4 and 200 mu M PO4 allowed quantitative comparison of the efficacy of the competing ions. In both species, selenite was more p hytotoxic than selenate, especially for shoot growth. Selenate was less tox ic, and tended to preferentially inhibit root growth. Translocation percent ages were much higher with selenate (greater than or equal to 84%) than wit h selenite (less than or equal to 47%). A 10-fold increase in sulfate decre ased uptake from selenate by > 90% in both species. In ryegrass, 10-fold in creases in phosphate caused 30% to 50% decreases in Se accumulation from se lenite, but in clover such decreases only occurred in the roots. Sulfate-se lenate antagonisms were thus stronger than phosphate-selenite antagonisms. Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate le vels of sulfate salinity can inhibit selenate uptake sufficiently to revers e the apparent relative availability of the two forms of Se.