The biology of mycorrhiza in the Ericaceae

XVIII. Chitin degradation by Hymenoscyphus ericae and transfer of chitin-nitrogen to the host plant



In acid mor-humus soils of heathland ecosystems fungi are a significant part of the soil biomass. In these organic soils chitin and hyphal wall hexosamines are major potential sources of nitrogen (N). The ability of the ericoid mycorrhizal fungus Hymenoscyphus ericae (Read) Korf & Kernan to degrade purified chitin and the hexosamines, N-acetylglucosamine, glucosamine and galactosamine when supplied as sole sources of N, was investigated in aseptic liquid culture. The fungus grew rapidly on all the organic N sources, producing the largest dry weight increase when supplied with galactosamine and similar yields on the other nitrogenous sources. Mycelial N contents of fungus grown on the hexosamines reflected the dry weight yields with the fungus grown on galactosamine haying the highest N content. After 20 d only tract quantities of N-acetylglucosamine and glucosamine were present in culture filtrate, and 15% of the galactosamine remained at the final harvest. Plants of Vaccinium macrocarpon Ait. and Calluna vulgaris (L.) Hull were grown aseptically in the mycorrhizal or non-mycorrhizal condition on agar media containing chitin or a hexosamine and compared with yields and N contents in the absence of an N source. It was revealed that significant quantities of N were transferred to the host in the mycorrhizal condition that led to enhanced growth rates. The highest yields of V. macrocarpon were observed with N-acetylglucosamine, with the other hexosamines giving yields slightly below that on chitin. The yields were reflected in the calculation of the percentage of available substrate present in the plant; up to 40% of the N-acetylglucosamine was present in the plants after 40 d.