The ability of ryegrass (Lolium perenne L.) to take up and utilize aspartic acid (Asp) and serine (Ser), and the effect of colonization of the roots by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum (Thax. sensu Gerd.) were studied. The seedlings were grown under controlled conditions in a series of micro-lysimeters. All plants were fed with a nutrient solution containing either nitrate, Asp or Ser as the sole N source. After 49 d, they were supplied with 15N labelled nitrate, Asp or Ser for 1 h and harvested.

AM colonization increased the growth and total N content of the plants in all cases. Similarly, the amount of Asp or Ser taken up was higher in AM than in control plants. There were no differences in biomass production between the nitrate and Ser-fed plants. However uptake rates were lower for Ser than for nitrate. Growth of the Asp-fed plants was significantly less than the other two treatments, and uptake of 15N-Asp was lower than uptake of 15N-Ser. Analysis of 15N incorporation into the amino acids extracted from the roots suggests the hydrolysis of Ser followed by re-assimilation of the resulting ammonia via the GS-GOGAT cycle. There were no differences in the patterns of accumulation of amino acids in the root-zone of control and AM-ryegrass. The implication of these results for the pathway of nitrogen transfer between plants is discussed.