Ozkutlu et al. 2026 — Zinc application reduces cadmium in wheat grain
A greenhouse trial using durum wheat grown in calcareous Turkish soils contaminated with increasing cadmium doses demonstrates that combined soil plus foliar zinc fertilization substantially reduces grain cadmium concentration, with foliar ZnSO4 spray alone reducing grain Cd by up to 70 percent in uncontaminated soils, though at the highest soil Cd dose (5.0 mg Cd kg-1 soil) the reduction reaches only 9 to 11 percent and grain Cd values still exceed the Codex Alimentarius and Chinese GB 2762-2022 maximum level of 200 µg kg-1 (ppb). The study establishes the practical ceiling of Zn-based mitigation and shows that Zn management alone is insufficient for food safety compliance when soil Cd is substantially elevated.
Key numbers
Grain Cd concentrations by soil Cd dose (best treatment = high soil Zn + foliar Zn):
| Soil Cd dose (mg kg-1) | Lowest grain Cd (high Zn + foliar Zn), µg kg-1 | Highest grain Cd (low Zn), µg kg-1 | Codex limit, µg kg-1 |
|---|---|---|---|
| Cd0 (background ~0.005) | 49.97 ± 4.26 | 54.5 (approx.) | 200 |
| Cd0.2 | 118.31 ± 4.76 | 238.89 ± 24.84 | 200 |
| Cd1.0 | 631.48 ± 81.39 | 1224.32 ± 47.55 | 200 |
| Cd5.0 | 2460.93 ± 554.41 | 3153.72 ± 207.55 | 200 |
Reduction magnitude by foliar Zn in Cd-free soil: 70.49% reduction versus inadequate Zn fertilization. In highest Cd soil (Cd5): 9.31% reduction. Combined soil + foliar Zn increased grain yield by 49.75% in Cd-free soil and by 32.79% at the Cd5 dose versus inadequate Zn. Grain Zn concentration increased by 77.67% (Cd0) and 173.06% (Cd5) with foliar Zn versus inadequate Zn. Correlation of foliar Zn treatment vs grain yield: R = 0.720 (p < 0.001); vs grain Cd: R = -0.048 (NS). Soil Zn application vs grain yield: R = 0.374 (p < 0.001); vs grain Cd: R = -0.086 (p < 0.05). Method: ICP-OES (Varian Vista-PRO); certified reference material SRM 1567B wheat flour, recovery 97-103%. 72 experimental units (3 soil Zn levels × 2 foliar Zn treatments × 4 Cd doses × 4 replicates per treatment).
Methods (brief)
Completely randomized block factorial greenhouse trial. Durum wheat cv. Diyarbakir 81, Osmaniye Turkey (calcareous soil pH 8.08, 14.2% CaCO3, very low baseline Cd 0.005 mg kg-1 DTPA-extractable). Spiked with CdCl2 at 0, 0.2, 1.0, and 5.0 mg Cd kg-1 soil; Zn applied at 0.5, 5.0, and 10.0 mg Zn kg-1 soil, with or without 0.3% ZnSO4·7H2O foliar spray at three applications during flowering. Grain harvested at maturity (149 days), acid digestion with HNO3/H2O2, ICP-OES. Limitation: greenhouse only; calcareous soil conditions specific to arid/semiarid regions; does not address field conditions or biological Cd from natural soil contamination.
Implications
Certification: Supports the principle that Zn biofortification can partially mitigate Cd uptake in wheat, but demonstrates a hard ceiling: at ecologically relevant elevated Cd soil levels (Cd1 and Cd5 doses), Zn intervention alone cannot bring grain Cd within Codex limits. HMT&C wheat or wheat-containing product standards should not rely on Zn fertilization as a sufficient mitigation pathway when sourcing from high-Cd soils. Supply-chain screening of soil Cd at origin is the primary control.
Courses: Useful illustration of the Zn-Cd competition mechanism (shared divalent cation transporter) and the phytochelatin synthesis trade-off. Provides quantitative estimates of mitigation ceiling that are more useful than qualitative claims.
App: Mitigation credit for zinc-biofortified wheat products should be applied conservatively; the magnitude of benefit is soil-Cd-level dependent.