EFFECT OF NITRATE AND AMMONIUM NUTRITION ON THE METABOLISM OF THE ECTOMYCORRHIZAL BASIDIOMYCETE, HEBELOMA-CYLINDROSPORUM ROMAGN

Hebeloma cylindrosporum Romagn. grown on a nitrate medium exhibits a growth rate ten times greater than that on an ammonium medium lacking organic acids. Nitrate reduction is accompanied by a low rate of synthesis of organic anions and an elevated excretion of OH− ions into the external medium. The addition of a Krebs cycle organic acid, e.g., malate, citrate, succinate or pyruvate to the ammonium medium induces a strong increase in the growth of the thalli which then equals that obtained on a nitrate medium. The absorption of such precursors to 2‐oxoglutarate could permit the recharging of the Krebs cycle, otherwise inhibited because the assimilation of ammonium causes an important re‐routing of 2‐oxoglutarate. In these thalli, the soluble nitrogen content shows a clear reduction compared with the thalli cultured on ammonium medium lacking organic acids. The poor growth observed on an ammonium medium may be due in part to insufficient CO2 fixation by anaplerotic reactions which, in Hebeloma cylindrosporum Romagn., is carried out by phosphoenolpyruvate carboxykinase. Copyright © 1990, Wiley Blackwell. All rights reserved