New Phytologist, October 1998, Vol.140(2), pp.319-329
Beech nuts ( L.) were germinated and grown in soil inoculated with the ectomycorrhizal fungus or for 18–20 wk. The success of mycorrhizal infection was monitored by measuring the ergosterol contents of the mycorrhizas. Ergosterol levels ranged from 122±23 μg g d. wt ( mycorrhizas) to 94±36 μg g d. wt ( mycorrhizas), indicating that ectomycorrhizal symbiosis was established. In root incubation chambers, rates of sulphate uptake and the xylem loading of sulphate of excised mycorrhizas were investigated. Both types of mycorrhizas showed saturation kinetics in external sulphate concentrations from 2·5–1000 μmol l. Linearization of these kinetics revealed two phases with low apparent ( mycorrhizas: 15±3 μmol l; mycorrhizas: 13±3 μmol l) and ( mycorrhizas: 19±3 nmol h g f. wt; mycorrhizas: 25±4 nmol h g f. wt) at low external sulphate concentrations and significantly higher kinetic constants at higher sulphate supplies. Relative xylem loading, i.e. the portion of sulphate loaded into the xylem that was taken up, remained constant over the entire concentration range investigated (. 4–7% of the sulphate taken up). If trees were supplied for 72 h with different N and sulphur concentrations, both uptake of sulphate and relative xylem loading were unaffected by sulphur availability, but modulated by N supply. Nitrogen depletion diminished the rates of sulphate uptake in and mycorrhizas. In response to higher N availability combined with sulphur depletion, sulphate uptake of mycorrhizas, but not of mycorrhizas, increased. Organic compounds considered to be possible signals for the regulation of sulphate uptake were fed to excised mycorrhizas. ‐Cysteine but not ‐methionine and glutathione (γ‐Glu‐Cys‐Gly) inhibited sulphate uptake of the two mycorrhizas and xylem loading of sulphate was stimulated rather than inhibited by ‐Cys in both types. In mycorrhizas glutathione had a similar effect. ‐Acetyl‐‐serine (OAS), a precursor of ‐cysteine, stimulated sulphate uptake, but did not affect xylem loading. Apparently, OAS, generated in N metabolism, and ‐cysteine, a product of assimilatory sulphate reduction, act as antagonists, together mediating regulation of sulphate uptake.
Sulphate Uptake ; Xylem Loading Of Sulphate ; Ectomycorrhizas ; L. Beech