SULFATE UPTAKE AND XYLEM LOADING OF MYCORRHIZAL BEECH ROOTS

Beech nuts (Fagus sylvatica L.) were germinated and grown in soil inoc ulated with the ectomycorrhizal fungus Laccaria laccata or Paxillus in volutus for 18-20 wk. The success of mycorrhizal infection was monitor ed by measuring the ergosterol contents of the mycorrhizas. Ergosterol levels ranged from 122 +/- 23 mu g g(-1) d. wt (Laccaria mycorrhizas) to 94 +/- 36 mu g g(-1) d. wt (Paxillus mycorrhizas), indicating that ectomycorrhizal symbiosis was established. In root incubation chamber s, rates of sulphate uptake and the xylem loading of sulphate of excis ed mycorrhizas were investigated. Both types of mycorrhizas showed sat uration kinetics in external sulphate concentrations from 2.5-1000 mu mol l(-1). Linearization of these kinetics revealed two phases with lo w apparent K-m (Laccaria mycorrhizas: 15 +/- 3 mu mol l(-1); Paxillus mycorrhizas: 13 +/- 3 mu mol l(-1)) and V-max (Laccaria mycorrhizas: 1 9 +/- 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 significa ntly higher kinetic constants at higher sulphate supplies. Relative xy lem loading, i.e. the portion of sulphate loaded, into the xylem that was taken up, remained constant over the entire concentration range in vestigated (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 availab ility, but modulated by N supply. Nitrogen depletion diminished the ra tes of sulphate uptake in Laccaria and Paxillus mycorrhizas. In respon se to higher N availability combined with sulphur depletion, sulphate uptake of Laccaria mycorrhizas, but not of Paxillus mycorrhizas, incre ased. Organic compounds considered to be possible signals for the regu lation of sulphate uptake were fed to excised mycorrhizas. L-Cysteine but not L-methionine and glutathione (gamma-Glu-Cys-Gly) inhibited sul phate uptake of the two mycorrhizas and xylem loading of sulphate was stimulated rather than inhibited by L-Cys in both types. In Paxillus m ycorrhizas glutathione had a similar effect. O-Acetyl-L-serine (OAS), a precursor of L-cysteine, stimulated sulphate uptake, but did not aff ect xylem loading. Apparently, GAS, generated in N metabolism, and L-c ysteine, a product of assimilatory sulphate reduction, act as antagoni sts, together mediating regulation of sulphate uptake.