Effect of plant nitrogen status on the contribution of arbuscular mycorrhizal hyphae to plant nitrogen uptake

The contribution of hyphae of Glomus mosseae (Nicol. and Gerd,) Gerd. and T rappe (BEG 107) to the acquisition of mineral nitrogen by Triticum aestivum L. cv, Hano (wheat) was tested under conditions of low P and high N (+ N - P) or low N (-N-P), Mycorrhizal colonisation increased the shoot dry weigh t and plant tissue concentrations of P and cations, However, N tissue conce ntrations of mycorrhizal plants were not increased, although nitrate reduct ase activities were significantly higher (in vivo activity) in + N-P mycorr hizal compared to non-mycorrhizal roots. Ser ere plant N deficiency reduced the percentage root length colonised (but not the percentage viable coloni sation), hyphal length, total N-15 uptake bg hyphae and dry weight of mycor rhizal plants. Although mycorrhizal colonisation did not affect the overall plant N status, hyphae transported 1% (-N-P) and 7% (+N-P) of the N-15-lab elled NH4NO3 to mycorrhizal plants ol er 48 h, The higher rate of hyphal N uptake was apparently related to the more extensive hyphal growth at the hi gher level of plant N supply. However, the hyphal N supply was not sufficie ntly high to sustain adequate N nutrition of the plants supplied with very low amounts of N to the roots. Conversely, a sufficient N supply to the roo ts was important for the development of an extensive mycelium.