Isotopic fractionation of Zn in tomato plants suggests the role of root exudates on Zn uptake

Abstract

AIMS: Phytosiderophore-chelated Zn can be absorbed in grasses. Root exudates of dicotyledonous plants can mobilize soil Zn but it is unclear how this affects Zn bioavailability. Stable Zn isotope shifts can indicate exudate-facilitated Zn uptake, since complexation of Zn2+ by organic ligands in solution yields a small, but detectable, enrichment of the heavy Zn isotope due to thermodynamic fractionation. METHODS: Tomato seedlings were grown in resin-buffered nutrient solution in which free Zn2+ concentrations are buffered, in a factorial design of two Zn levels and two solution volumes. The latter factor allowed altering the exudate concentrations in the solution. Dissolved Cu concentrations in the resin buffered system were used as a sensitive index of metal mobilization resulting from root activity. In addition, seedlings were grown in Zn deficient soil with and without Zn addition. RESULTS: The dissolved Cu concentration increased with Zn deficiency and was highest at the lowest solution volume, suggesting metal mobilization by root exudates. At low Zn supply, Zn in the plant was enriched in heavy Zn (66Zn) and this was most pronounced at small solution volume. Similarly, Zn deficiency in soil enriched tomato shoot Zn with heavy isotope in this plant. INTERPRETATION: Zinc deficiency increases the contribution of Zn-exudate complexes, which are enriched in the heavy isotope compared to the free ion, to Zn uptake by transporting Zn from the bulk solution or soil to the roots where they likely dissociate and release Zn2+.