Improved nitrogen uptake and transport from ¹⁵N‐labelled nitrate by external hyphae of arbuscular mycorrhiza under water‐stressed conditions

Abstract

SUMMARY The significance of the external mycelium of arbuscular mycorrhiza for uptake and transport of N from 15 N‐labelled nitrate in benefiting plant nutrition, was evaluated under either well‐irrigated or water‐stressed conditions. Plants of lettuce ( Lactuca sativa L.) were grown under controlled conditions in a neutral agricultural soil/sand mix either non‐mycorrhizal or in association with the arbuscular‐mycorrhizal fungus, Glomus fasciculatum (Taxter sensu Gerd.) Gerd. and Trappe. The pots comprised a two‐compartment system, where a fine nylon mesh screen prevented the development of roots in a hyphal compartment. 15 NO 3 − was applied to this hyphal compartment where access of the root was not possible. At harvest, the 15 N enrichment in plant tissues was the same for both mycorrhizal and non‐mycorrhizal plants under optimal water supply conditions. However, under water‐stressed conditions, where the mass flow and diffusion of NO 3 − ions to the roots can be affected, the 15 N enrichment was four times higher in mycorrhizal than in non‐mycorrhizal plants. This provides evidence of hyphal transport of N from a nitrate source, supporting the view that arbuscular mycorrhiza can be important for the N‐nutrition of plants in relatively dry agricultural soil where nitrate is actually the predominant nitrogen form.