SULFATE UPTAKE AND XYLEM LOADING OF NONMYCORRHIZAL EXCISED ROOTS OF YOUNG FAGUS-SYLVATICA TREES

Non-mycorrhizal beech (Fagus sylvatica L.) trees were grown for 16-20 weeks on a 1/5 MS medium containing 0.35 mM sulfate, 1.0 mM nitrate an d 0.5 mM ammonium. In root chambers the rates of S-35-sulfate uptake a nd xylem loading and the relative xylem loading, i.e. the portion of s ulfate taken up that was loaded into the xylem, were determined. Rates of sulfate uptake from 5 mu M to 7 mM sulfate revealed kinetics with different states. Interpreted as multiphasic isotherm, three individua l phases were obtained, with V-max-values of 31 +/- 3, 87 +/- 15 and 2 03 +/- 144 nmol h(-1) g(-1) fw, and apparent K-M-values of 15 +/- 4 mu M, 550 +/- 200 mu M and 5.9 +/- 2.6 mM, respectively. The relative xy lem loading of sulfate remained constant over the range of sulfate con centrations investigated. The influence of sulfur and nitrogen nutriti on on sulfate uptake and xylem loading was investigated. Both processe s increased in response to decreasing sulfate concentrations. Incubati on with ammonium resulted in increased sulfate uptake as compared to m edia lacking ammonium. To obtain information on the regulation of sulf ate transport, the effects of organic sulfur compounds were studied. T he amino acids L-cysteine and L-methionine, but not the tripeptide glu tathione, inhibited the uptake of sulfate. The relative xylem loading was uneffected by treatment with L-methionine while high concentration s of glutathione as well as high and low L-cysteine concentrations inc reased xylem loading of sulfate as compared to controls. Apparently, s ulfate uptake and xylem loading of sulfate in beech roots differ in th e regulation by organic sulfur compounds from herbaceous plants.