Citrate and histidine increase root zinc uptake by different mechanisms in wheat

Rationale. The objective was to describe the mechanisms behind an increase in Zn uptake by wheat roots due to the presence of citrate and histidine. Methodology. The hypothetical mechanisms of Zn uptake in the presence of metal ligands were formalized in numerical models. The simulated Zn and ligand uptakes were compared to those measured in three hydroponic experiments with two bread wheat cultivars. Results. Transpiration-driven apoplastic flux and solution-mediated dissociation of complexes resulting from limited Zn2+ diffusion could not explain the strong increase in Zn uptake by wheat roots in the presence of citrate or histidine. Transpiration-driven flux was also insufficient to cause the high histidine uptake, whereas this proved an adequate explanation for the low citrate uptake. Discussion. Wheat roots would absorb histidine and the undissociated Zn-histidine complex through membrane transporters according to a chemical potential gradient generated by plant energy conversion. Histidine and Zn-histidine could use different transport systems, or a common one with lower affinity for the complex. Zn-citrate complexes would not be absorbed directly, but rapidly dissociated after the formation of a ternary complex of Zn-citrate with a biotic ligand at the root cell membrane. The metal ion released would be internalised and the ligand recycled into the rhizosphere solution. According to the Michaelis-Menten parameters of the root uptake kinetics, the affinity of the transport systems was ranked in the order: histidine > Zn2+ > Zn-histidine > Zn-citrate. These results encourage research to identify transporters of histidine, and of complexes of Zn with histidine and citrate.