THE BIOLOGY OF MYCORRHIZA IN THE ERICACEAE .19. FUNGAL MYCELIUM AS A NITROGEN-SOURCE FOR THE ERICOID MYCORRHIZAL FUNGUS HYMENOSCYPHUS-ERICAE AND ITS HOST PLANTS

Measurements of the chitin content of the rooting horizons of a typica l mor-humus heathland soil, indicate that chitin can contain in excess of 20 % of the total nitrogen in the litter (L) horizon and 30 % in t he fermentation (F) horizon. Much of this chitin-nitrogen is thought t o be contained in the mycelial walls of soil fungi. Experiments were t herefore designed to test the hypothesis that such sources of N could be rendered accessible to the ericaceous plants by their fungal endoph ytes. Mycelium of the ericoid endophyte Hymenoscyphus ericae (Read) Ko rf & Kernan and of the ectomycorrhizal fungus Suillus bovinus (Fr.) O. Krantze were grown in liquid culture before being killed and added ei ther in the intact condition, or after fractionation, as sole sources of N to sterile media upon which were grown H. ericae in pure culture, or mycorrhizal and non-mycorrhizal plants of Vaccinium macrocarpon Ai t. and Calluna vulgaris L. The abilities of the test organisms to util ize the nitrogen contained in the intact mycelial necromass, or in its fractions, were assessed by determining their yields and nitrogen con centrations of their tissues. It was revealed that H. ericae was able to produce significantly higher yield when grown on intact fungal necr omass than when provided with equivalent concentrations of N in the fo rm of ammonium. Its yields on mycelial fractions were lower, but still significantly greater than those obtained in the controls lacking N. Significantly greater yields and N contents were also found in the eri caceous plants grown with these nitrogenous substrates in the mycorrhi zal condition. Without H. ericae they had no access to the substrates. The possible ecological implications of these results are discussed.