We studied the transport of 15N from a soil compartment separated from a plant root compartment by a hydrophobic polytetrafluoroethylene (PTFE) membrane to plants in the presence and absence of arbuscular mycorrhizal fungi (AMF). We have previously shown that this type of membrane efficiently inhibits mass flow and diffusion of mobile ions in the soil solution in an abiotic system, but can be penetrated by the hyphae of mycorrhizal fungi. Mycorrhizal tomatoes (Lycopersicon esculentum) colonized by Glomus mosseae were grown at two N fertilizer concentrations in a root compartment. A PTFE membrane was placed between the root compartment and an adjoining soil compartment that was inaccessible to the roots but accessible to the AMF hyphae (hyphal compartment). Additional N was applied to the hyphal compartment using uniformly 15N-labelled NH4NO3. There was a flux of 15N from the hyphal compartment to the plants even in the absence of mycorrhizal fungi. However, this flux was much higher in mycorrhizal plants, which had much higher N concentrations in their shoots and roots than did the non-mycorrhizal control plants. This was particularly apparent when the root compartment had a low N fertilizer concentration. Of the total N content of mycorrhizal plants, c. 42 and 24% at the low and high N fertilizer concentrations, respectively, were estimated to originate from the hyphal compartment by transport through AMF hyphae. In the presence of mycorrhizal fungi, the flux of 15N was about three times higher than in their absence. The results show that AMF can access a soil compartment separated by a PTFE membrane, and can contribute substantially to N uptake by plants.

Subterranean clover (Trifolium subterraneum L.) in symbiosis with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe was grown in compartmented pots. Side compartments were filled with soil where recently added (1 wk) radio-labelled cadmium (Cd) at three levels of non-labelled Cd could be accessed by either roots or arbuscular mycorrhizal hyphae. All treatments were replicated with non-mycorrhizal plants. After a growth period of 52 d roots and shoots were analysed for 109Cd, and lengths of roots and hyphae in respective labelled compartments determined. Cadmium uptake by roots was not significantly influenced by the mycorrhizal status of the plant. Uptake of Cd from hyphal compartments was higher in mycorrhizal than in non-mycorrhizal plants, corresponding to 96, 127 and 131% of that in non-mycorrhizal plants when 1, 10 and 100 mg Cd kg−1 was added, respectively. A large proportion of the increased Cd content of mycorrhizal plants was sequestered in the roots. It is concluded that extraradical hyphae of AM fungi can transport Cd from soil to plants, but that transfer from fungus to plant is restricted due to fungal immobilization. No reduction of hyphal growth into soil with up to 20 mg extractable Cd kg−1 was observed.