NITROGEN ASSIMILATION IN LOLIUM-PERENNE COLONIZED BY THE ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS-FASCICULATUM

To investigate nitrogen assimilation in Lolium perenne L. colonized by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum (Thax. sen su Gerd.), nitrate uptake, key enzyme activities, and N-15 incorporati on into free amino acids were measured. After a 4-h labelling period w ith [N-15]nitrate, N-15 content was higher in roots and shoots of AM-p lants than in those of control plants. Glutamine synthetase (GS) and n itrate reductase (NR) activities were increased in shoots of AM-plants , but not in roots. More label was incorporated into amino acids in sh oots of AM plants. Glutamine, glutamate, alanine and gamma-aminobutyri c acid were the major sinks for N-15 in roots and shoots of control an d AM plants. Interactions between mycorrhizal colonization, phosphate and nitrate nutrition and NR activity were investigated in plants whic h received different amounts of phosphate or nitrate. In shoots of con trol plants, NR activity was not stimulated by high levels of phosphat e nutrition but was stimulated by high levels of nitrate. At 4 mM nitr ate in the nutrient solution, NR activity was similar in control and A M plants. We concluded that mycorrhizal effects on nitrate assimilatio n are not mediated via improved phosphate nutrition, but could be due to improved nitrogen uptake and translocation.