How important is the mycorrhizal pathway for plant Zn uptake?

Formation of arbuscular mycorrhizas can enhance plant uptake of immobile nutrients such as zinc (Zn) and phosphorus (P). Enhancement of Zn uptake by arbuscular mycorrhizal (AM) fungi on Zn-deficient soils has been studied previously, however, the quantity of Zn that is contributed by the AM pathway of uptake to the plant has not previously been reported for soil of any Zn status. We grew a mycorrhiza-defective mutant tomato (Solanum lycopersicum L.) genotype (rmc) and its mycorrhizal wild-type progenitor (76R) in pots containing a hyphal compartment (HC) accessible only by the external hyphae of AM fungi, and containing the radioisotope Zn-65. This was repeated at three soil Zn concentrations, ranging from low to high. We estimated the amount of Zn delivered via both the AM and direct (root) pathways. Up to 24 % of Zn in the shoots of the AM plants was delivered via the AM pathway at the lowest soil Zn treatment. This decreased significantly, to 8 %, as soil Zn concentration increased. No Zn-65 was detected in the tissues of the non-mycorrhizal genotype. The relative contribution to shoot Zn by the AM pathway of uptake was highest when soil Zn was low, and decreased with increasing soil Zn concentration.