The Role of Proteins in the Nitrogen Nutrition of Ectomycorrhizal Plants

S ummary The ability of mycorrhizal (M) and non‐mycorrhizal (NM) plants of Pinus contorta (Dougl. ex Loud) to utilize protein as a nitrogen source was examined. Mycorrhizas were synthesized with the fungal symbionts Paxillus involutus, Rhizopogon roseolas, Suillus bovines and Pisolithus tinctorius . The plants were grown under aseptic conditions and provided with nitrogen in the form of either protein or ammonium. Dry weight yields of plants infected with P. involutus, R. roseolus and S. bovinus were significantly higher than those of NM plants when grown on protein as the N source. In these associations yields of M plants on protein were similar to those obtained on ammonium‐N. Non‐mycorrhizal plants had little ability to use protein N and the same was true of plants infected with P. tinctorius . Plants in those associations which provided a yield increase also contained larger quantities of N. Calculations show that in all these cases some of the increases of N content arise through utilization of protein. The implications of the results are discussed in relation to the nitrogen nutrition of the plant and to the pattern of N mobilization in soil. It is proposed that the ability of mycorrhizal associations to utilize protein N will lead not only to an increased supply of N to the plant but also to more effective competition with the decomposer population and to an overall tightening of the nitrogen cycle.