Influence of different mineral nitrogen sources (NOn 3n −n -N vs. NHn 4n +n -N) on arbuscular mycorrhiza development and N transfer in a Glomus intraradices–cowpea symbiosis

摘要

Labeled nitrogen (15 N) was applied to a soil-based substrate in order to study the uptake of N by Glomus intraradices extraradical mycelium (ERM) from different mineral N (NO 3 − vs. NH 4 + ) sources and the subsequent transfer to cowpea plants. Fungal compartments (FCs) were placed within the plant growth substrate to simulate soil patches containing root-inaccessible, but mycorrhiza-accessible, N. The fungus was able to take up both N-forms, NO 3 − and NH 4 + . However, the amount of N transferred from the FC to the plant was higher when NO 3 − was applied to the FC. In contrast, analysis of ERM harvested from the FC showed a higher 15 N enrichment when the FC was supplied with 15NH 4 + compared with 15NO 3 − . The 15 N shoot/root ratio of plants supplied with 15NO 3 − was much higher than that of plants supplied with 15NH 4 + , indicative of a faster transfer of 15NO 3 − from the root to the shoot and a higher accumulation of 15NH 4 + in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH 4 + preferentially over NO 3 − but that export of N from the hyphae to the root and shoot may be greater following NO 3 − uptake. The need for NH 4 + to be assimilated into organically bound N prior to transport into the plant is discussed.