Effect of nitrate and ammonium concentration on nitrate reductase activity in five species of mycorrhizal fungi

The nitrate reducing capacity of pure cultures of Cenococcum geophilum (Sow.) Ferd. & Winge, Paxillus involutes (Batsch: Fr.) Fr. (strains 1 and 2), Piloderma croceum Erikss. & Hjortst., Suillus variegatus (Fr.) O. Kuntze (strains 1 and 2) and an ectendomycorrhizal (E-strain) fungus was measured using an in vivo nitrate reductase (EC 1.6.6.3) assay. Differences between species and strains were established. The nitrate concentration of the culture medium influenced the nitrate reductase activities of the E-strain fungus and one strain of S. variegatus. The nitrate reductase activity of certain species and strains was a function of nitrate concentration. Addition of ammonium to the growth medium did not have any significant effect on the in vivo or in vitro nitrate reductase activity. The in vivo nitrate reductase activity in the mycelia of C. geophilum and the E-strain fungus decreased during 28 day growth in modified Melin-Norkrans medium. For mycelia of Paxillus involutus, Piloderma croceum and S. variegatus grown on agar the in vitro assays showed higher nitrate reductase activity than the in vivo assays.