Aseptic cultures of Paxillus involutus Fr. and clones of Betula pendula Roth, and B. puhescens Ehrh. were used in experiments, involving X-ray microanalysis and split-plate culture, to investigate the mechanism of ectomycorrhizal amelioration of zinc toxicity to Betula. Results imply that as the fungal mycelium colonizes fresh soil, zinc is adsorbed to the surface of hyphae, thereby lowering the concentration of zinc in the soil solution surrounding roots. In consequence, less zinc is taken up, and growth is better than in the non-mycorrhizal condition. The results also show that the metal may be adsorbed to electronegative sites in the hyphal cell walls and extra-hyphal, polysaccharide slime. The possible dual role of this slime in iungn^/Betula compatibility and the amelioration mechanism is discussed.
A survey was made of fungi forming ectomycorrhizas with Betula pendula Roth, and B. pubescens Ehrh. on zinc-contaminated mine tailings. Strains of Paxillus involutus Fr. from toxic and non-toxic soils were compared. There was no indication of fungal adaptation to zinc at either the inter-or intraspecific level. Most of the typical fungal associates of Betula on normal soils were also found on the zinc-rich soils. The ability of different strains of P. involutus to (a) grow in pure culture on agar, (b) form ectomycorrhizas with Betula, and (c) produce a beneficial growth and zinc uptake response in the associated plant, all in the presence of raised zinc concentrations, was not related to the zinc status of the provenance. The ameliorating influence of P. involutus on zinc toxicity to Betula seemed to be positively linked to the degree of compatibility between fungal strain and higher plant.
Clones of Betula pendula Roth, and B. pubescens Ehrh., collected as seed from zinc-contaminated mine tailings, were grown aseptically in a liquid/perlite medium. Three zinc-tolerant and four non-tolerant genotypes were compared with respect to extension growth and zinc uptake over a range of external concentrations of zinc. The effect of increasing zinc concentration on total root length and root cell length was also examined. Extension growth declined rectilinearly as the external zinc concentration increased, but the relationship between uptake and external concentration was not rectilinear. After an initial proportional increase, concentrations of zinc in tissues remained constant over much of the range of external concentrations at which some growth was possible. Fresh weight concentrations of zinc in tolerant and non-tolerant genotypes were similar at similar external concentrations. However, at a characteristic threshold external concentration, control of uptake broke down and plants were inundated with zinc. The decrease in extension growth was found to be due mainly to a progressive inhibition of cell elongation. Zinc tolerance in Betula lies in an ability to maintain proportionally more extension growth at similar internal and external zinc concentrations. Neither a tolerance mechanism involving internal detoxification, nor binding of zinc ions to electronegative sites in the cell walls, are consistent with the results. However, these findings are compatible with a tolerance mechanism involving control of uptake operating at the endodermis.
SUMMARYA survey was made of fungi forming ectomycorrhizas with Betula pendula Roth, and B. pubescens Ehrh. on zinc-contaminated mine tailings. Strains of Paxillus involutus Fr. from toxic and non-toxic soils were compared. There was no indication of fungal adaptation to zinc at either the inter-or intraspecific level. Most of the typical fungal associates of Betula on normal soils were also found on the zinc-rich soils. The ability of different strains of P. involutus to (a) grow in pure culture on agar, (b) form ectomycorrhizas with Betula, and (c) produce a beneficial growth and zinc uptake response in the associated plant, all in the presence of raised zinc concentrations, was not related to the zinc status of the provenance. The ameliorating influence of P. involutus on zinc toxicity to Betula seemed to be positively linked to the degree of compatibility between fungal strain and higher plant.
SUMMARYAseptic cultures of Paxillus involutus Fr. and clones of Betula pendula Roth, and B. puhescens Ehrh. were used in experiments, involving X-ray microanalysis and split-plate culture, to investigate the mechanism of ectomycorrhizal amelioration of zinc toxicity to Betula. Results imply that as the fungal mycelium colonizes fresh soil, zinc is adsorbed to the surface of hyphae, thereby lowering the concentration of zinc in the soil solution surrounding roots. In consequence, less zinc is taken up, and growth is better than in the non-mycorrhizal condition. The results also show that the metal may be adsorbed to electronegative sites in the hyphal cell walls and extra-hyphal, polysaccharide slime. The possible dual role of this slime in iungn^/Betula compatibility and the amelioration mechanism is discussed.
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