HYPHAL UPTAKE AND TRANSPORT OF NITROGEN FROM TWO ¹⁵N‐LABELLED SOURCES BY GLOMUS MOSSEAE, A VESICULAR‐ARBUSCULAR MYCORRHIZAL FUNGUS ^*

Abstract

SUMMARY Celery (Apium graveolens L.) plants were grown in pots in which the root system was separated from the soil in a side chamber by a fine mesh screen. The side chamber was treated with either an organic (ground plant tissue) or inorganic [(NH4)2SO4] source of 15N. Mycorrhizal (Glomus mosseae) and control (non‐mycorrhizal) plants were exposed to 15N over a period of 30 days (inorganic‐15N) or 88 days (organic‐16N). Mycorrhizal and control plants did not differ in shoot dry weight or shoot P content. Dry weight of root was reduced in the mycorrhiza treatments. Mycorrhizal plants derived significantly (P= 0.01) more 16N, from both N sources, than did control plants. In the inorganic‐N treatment, 15N in mycorrhizal plants was significantly (P= 0.001) and positively correlated with percent mycorrhizal fungal colonization (r= 0.58), number of hyphal crossings (±10 μ diameter) through the mesh into the area of 15N placement (r= 0.76), total length of hyphae per gram of soil (r= 0.74), and length of hyphae of 5 μ diameter in the soil (r= 0.77). No correlations were found between the 16N content of mycorrhizal plants and any parameter in the organic‐N treatment. The 16N content of control plants was not correlated with hyphal length in the outer chamber and there were no hyphal crossings of the size ( 10 μ diameter) which was counted for the mycorrhiza treatments. The presence of the organic matter (ground plant tissue) increased the total length of saprophytic hyphae per gram of soil but decreased the number of vesicular‐arbuscular mycorrhizal fungal hyphae crossing into the area of 16N placement. The mean flux of N through the hyphae of G. mosseae was 7.42 × 10−8 mol N cm−2 s−1 for the inorganic‐N treatment over a 30‐day period, and 1.74 × 10−8 mol N cm−2 s−1 for the organic‐N treatment over an 88‐day period.

Nitrogen metabolism of external hyphae of the arbuscular mycorrhizal fungus Glornus intraradices

“…However, some values were sufficiently high (7-8 % in glutamate) to suggest strongly that the external hyphae absorbed and assimilated the supplied ^"NOg^-N. These findings are supported by studies demonstrating that external hyphae of AM fungi are able to take up soil mineral N, both as NH,+-N (Ames et al, 1983;Johansen et al, 1992;Frey & Schuepp., 1993) and as NOg'-N (Johansen et al, 1993;Tobar et al, 1994), and translocate it towards the host plant. However, in these studies it was not clear whether the absorbed N was assimilated into organic compounds or translocated in inorganic form.…”

Transport of 15N from a soil compartment separated by a polytetrafluoroethylene membrane to plant roots via the hyphae of arbuscular mycorrhizal fungi

“…Using a PTFE membrane between the root compartment and hyphal compartment as a barrier to mobile ions, almost identical results were obtained in a simultaneously conducted experiment with maize (Zea mays) colonized by Glomus intraradices (Frey et al, 1998). Similar findings have been reported previously by Ames et al (1983), Johansen et al (1992Johansen et al ( , 1993Johansen et al ( , 1994Johansen et al ( , 1996, Frey & Schu$ epp (1993) and Tobar et al (1994). At a high N fertilizer concentration, the AMF-mediated transport of "&N from the hyphal compartment was reduced, as also observed by Johansen et al (1994).…”

An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient‐rich patches in soil

“…Vesicles are thought to be important storage organs (Smith & Read, 1997) but the reason they were so closely linked to N capture from the patch in this study is unclear, particularly as most of the N captured from the patch was detected in the shoots rather than in the roots. Ames et al (1983) reported that uptake of N (supplied as ("&NH % ) # SO % ) by Apium graveolens was related to the percentage of root length colonized by G. mosseae. However, in this study, the apparent relationship with percentage RLC and patch N capture was an artefact caused by an increase in both these parameters with time.…”