Foliar-Applied EDTA-Zn Reduces Grain Cd Content in Triticum aestivum L. Grown in Alkaline Fields with Cd Contamination by Regulating Vascular Bundle-Mediated Transport and Sequestration of Cd in Nodes and Rachises.

Foliar application of zinc (Zn) compounds is a promising strategy for mitigating cadmium (Cd) accumulation in wheat grains in pot experiments. However, its inconsistent field efficacy, especially in alkaline soils with long-term Cd contamination, poses a unique challenge. Herein, field experiments and demonstrations were conducted at three sites across four wheat-growing winter seasons to evaluate the efficacy of foliar Zn application in reducing grain Cd accumulation in typical Cd-contaminated alkaline soils. Foliar application of EDTA-Zn at the booting and milk stages increased grain yield by 0.65%-6.71% and significantly reduced grain Cd concentrations by 36.03% to 46.45%, ensuring that National Food Safety Standard requirements (GB 2762-2022, 0.1 mg·kg-1) are met. Three hydroponic experiments were conducted to uncover the underlying mechanisms. Foliar-applied EDTA-Zn reduced xylem Cd loading by 23.60% by downregulating the expression of genes associated with Cd transport (such as TaHMA2 and TaNramp5) in the root. It also increased Cd content in the phloem sap by 33.34%-46.12%, promoting the downward but suppressing the upward transport of Cd, thereby mitigating the reallocation of leaf Cd. Finally, it enhanced Cd retention at transport barriers (first node and rachis) by upregulating vacuolar sequestration-associated genes (TaHMA3 and TaVP1), increasing phytochelatin synthesis (by 10.65%-11.27%), and downregulating genes associated with rachis-to-grain transport of Cd (TaLCT1, TaHMA2, and TaNramp5) by 21.36%-41.19%. Our findings establish foliar-applied EDTA-Zn as a reliable, economical, and ecofriendly strategy for ensuring wheat safety in alkaline farmlands with mild-to-moderate Cd contamination.