Effects of zinc activity in nutrient solution on uptake, translocation, and root export of cadmium and zinc in three wheat genotypes with different zinc efficiencies

The interactions of zinc (Zn) and cadmium (Cd) in uptake and translocation are common but not consistent. We hypothesized that Cd 2+ and Zn 2+ activity in the apoplasmic solution bathing root-cells could affect Zn accumulation in plants dependent on the wheat genotype. This hypothesis was tested using seedlings of two bread wheat genotypes (Triticum aestivum L. cvs. Rushan and Cross) and one durum wheat genotype (Triticum durum L. cv. Arya) with different Zn efficiencies grown in chelate-buffered nutrient solutions with three Zn 2+ (10 −11.11 , 10 −9.11 , and 10 −8.81 µM) and two Cd 2+ (10 −11.21 and 10 −10.2 µM) activity levels. Increasing Zn 2+ activity in the nutrient solution significantly increased Zn concentration in root and shoots of all three wheat genotypes, although the magnitude of this increase was dependent on the genotype. Cadmium decreased Zn concentration in roots of "Cross" while it had no significant effect on root Zn concentration in "Rushan." At Zn 2+ = 10 −11.11 µM, Cd decreased shoot Zn concentration in "Arya" whereas it increased shoot Zn concentration at Zn 2+ = 10 −8.81 µM. Cadmium increased shoot Zn concentration of "Rushan" and "Cross" at Zn 2+ = 10 −8.81 µM but it had no significant effect on shoot Zn concentration of these genotypes at Zn 2+ = 10 −11.11 µM. The zinc-inefficient genotype "Arya" accumulated significantly more Cd in its root in comparison with "Cross" and "Rushan." Cadmium concentration in roots of "Arya" was decreased significantly with increasing Zn activity. The effect of Zn on accumulation of Cd in roots of "Cross" and "Rushan" was dependent on the dose provided, and therefore, both synergistic (at Zn 2+ = 10 −9.11 µM) and antagonistic (at Zn 2+ = 10 −8.81 µM) interactive effects were found in these genotypes. Zinc supply increased the Zn concentration of xylem sap in "Cross" and "Rushan" whereas Zn content in xylem sap of "Arya" was decreased at Zn 2+ = 10 −9.11 µM and thereafter increased at Zn 2+ = 10 −8.81 µM. Cadmium treatment reduced Zn concentration in xylem sap of "Arya," while it tended to increase Zn content in xylem sap of "Cross." At Zn-deficient conditions, greater retention of Zn in root cell walls of Zn-inefficient "Arya" resulted in lower root-to-shoot transport of Zn in this genotype. Results revealed that the effect of Cd on the root-to-shoot translocation of Zn via the xylem is dependent on wheat genotype and Zn activity in the nutrient solution.