The effect of tree-root exudates on the growth rate of ectomycorrhizal and saprotrophic fungi

Abstract

The object of this investigation was the promotion by root exudates of the growth rate of ectomycorrhizal fungi, discovered by Elias Melin in 1954. Eight ectomycorrhizal and ten non-mycorrhizal species were used as test fungi in the experiments. Different species often reacted differently: none of the eight isolated strains of Suillus luteus were promoted by pineroot exudate, whereas the growth rates of all seven strains of S. granulatus were increased. Among the other ectomycorrhizal species, S. variegatus, Laccaria bicolor, Pisolithus tinctorius and Thelephora terrestris, each represented by only one, two or three strains, usually reacted to the pine root exudate with an increased growth rate; S. bovinus and Paxillus involutus did not respond at all. Hitherto, studies of root-exudate effects on fungi have been based exclusively on the responses of ectomycorrhizal species; in the present study saprotrophic fungi were also used as test organisms. Seven out of ten saprotrophic species reacted with markedly accelerated growth when exposed to a pineroot exudate. Melin's assumption that a constituent of the root exudate, the “M-factor”, could replace the exudate growth-promoting activity was verified. By means of TLC fraction it was found that the fatty acid palmitic acid alone caused an increase in growth rate equal to that of the pine-root exudate. In line with previously published data by Gogala (1970), we also showed that certain cytokinins, especially isopentenylaminopurine, could act as substitutes for the total root exudate. Thus both palmitic acid and isopentenylaminopurine are able to function as M-factors equivalent to a root exudate.