Sulphate uptake and xylem loading of mycorrhizal beech roots

Beech nuts (Fagus sylvatica L.) were germinated and grown in soil inoculated with the ectomycorrhizal fungus Laccaria laccata or Paxillus involutus 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^-1 d. wt (Laccaria mycorrhizas) to 94±36 μg g^-1 d. wt (Paxillus 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^-1 . Linearization of these kinetics revealed two phases with low apparent K_m (Laccaria mycorrhizas: 15±3 μmol l^-1 ; Paxillus mycorrhizas: 13±3 μmol l^-1 ) and V_max (Laccaria mycorrhizas: 19±3 nmol h^-1 g^-1 f. wt; Paxillus mycorrhizas: 25±4 nmol h^-1 g^-1 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 (c. 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 Laccaria and Paxillus mycorrhizas. In response to higher N availability combined with sulphur depletion, sulphate uptake of Laccaria mycorrhizas, but not of Paxillus mycorrhizas, increased. Organic compounds considered to be possible signals for the regulation of sulphate uptake were fed to excised mycorrhizas. l-Cysteine but not l-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 l-Cys in both types. In Paxillus mycorrhizas glutathione had a similar effect. O-Acetyl-l-serine (OAS), a precursor of l-cysteine, stimulated sulphate uptake, but did not affect xylem loading. Apparently, OAS, generated in N metabolism, and l-cysteine, a product of assimilatory sulphate reduction, act as antagonists, together mediating regulation of sulphate uptake.