The role of proteins in the nitrogen nutrition of ectomycorrhizal plants

SUMMARY Seedings of Betula pendula L. were grown aseptically in the non‐mycorrhizal condition and in association with the mycorrhizal Fungi Hebeloma crustuliniforme (Bull, ex St. Am.) Quél Amanita muscaria (L. ex Fr.) Hooker or Paxillus incolutus (Batsch) Fr. They were supplied with the amino‐acid l ‐alanine, or with peptides of between 2 and 6 alanine units, as sole nitrogen (N) sources. The ability of the different categories of plants to use these N sources was measured in terms of dry weight yield and nitrogen content of roots and shoots. While non‐mycorrhizal plants were unable to use any of these compounds as N sources, mycorrhizal infection provided the plants with some access to all of the N sources. The fungi differed, however, in the extent to which they mobilized the nitrogen, H. crustuliniforme being generally a more effective symbiont than A. muscaria which was, in turn, more effective than P. involutus . The yields and N contents of infected plants were normally higher on the peptides than on the amino acid, and in the two most effective fungi, the highest yield and nitrogen content were obtained in the treatments containing the largest peptide units. The physiological and ecological significance of these observations is discussed with particular attention being given to the role of mycorrhizas in the nitrogen nutrition of plants in natural ecosystems.