The biology of mycorrhiza in the Ericaceae: XIX. 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 typical 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 the fermentation (F) horizon. Much of this chitin-nitrogen is thought to be contained in the mycelial walls of soil fungi. Experiments were therefore designed to test the hypothesis that such sources of N could be rendered accessible to the ericaceous plants by their fungal endophytes. Mycelium of the ericoid endophyte Hymenoscyphus ericae (Read) Korf & Kernan and of the ectomycorrhizal fungus Suillus bovinus (Fr.) O. Krantze were grown in liquid culture before being killed and added either 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 Ait. and Calluna vulgaris L. The abilities of the test organisms to utilize the nitrogen contained in the intact mycelial necromass, or in its fractions, were assessed by determining their yields and nitrogen concentrations of their tissues. It was revealed that H. ericae was able to produce significantly higher yield when grown on intact fungal necromass than when provided with equivalent concentrations of N in the form 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 ericaceous plants grown with these nitrogenous substrates in the mycorrhizal condition. Without H. ericae they had no access to the substrates. The possible ecological implications of these results are discussed.