Arbuscular mycorrhizal symbiosis and phosphorus nutrition: Effects on amino acid production and turnover in leek

This study examines leek (Allium porrum L.) plants colonized by the arbuscu lar mycorrhizal fungus Glomus etunicatum Becker Sr Gerdemann under conditio ns where the symbiosis is actively affecting host plant growth and carbon m etabolism. Isotopic labeling with N-15 nitrate together with amino acid ana lysis and mass spectrometry was used to assess the free amino acid pools of root and shoot tissues in colonized plants (M) compared to uncolonized pla nts grown either with (NM+P) or without (NM-P) supplemental phosphate. Tota l amino acid levels in roots were lower in M than in NM-P plants and this e ffect also was induced partially in NM+P plants by supplemental phosphate. The relative proportions of different amino acids were not affected by the AM symbiosis nor were amino acid levels in leaves affected by either coloni zation or increased phosphate supply. Fractional labeling was substantial i n all free amino acids after a six-week labeling period and enrichments wer e higher on average in M than in NM-P. There were no indications of effects of mycorrhizas upon relative enrichments in different amino acids. However , a higher incorporation of label was found in Asp, Glu, and Gin in short t erm N-15 labeling experiments (72 h) with M than NM-P plants. The average l abeling in all amino acids was higher in the roots of M plants. h lore rapi d turnover ef amino acids was noted in the M plant rests, however this was probably due to the lower concentration of amino acids found in the plant. These results suggest that there are indeed effects of AM symbioses on over all nitrogen handling by the host hut that these are secondary effects of t he improved phosphate supply to the host and do not cause significant shift s in levels or fluxes through different amino acids.