Isotope Tracing Reveals Differential Allocation of Cadmium and Zinc within the Grain of Wheat ( <i>Triticum aestivum</i> L.)

Understanding how toxic cadmium (Cd) and essential zinc (Zn) are differentially allocated within wheat grains is critical for food safety and biofortification. Enriched ¹¹¹Cd and ⁶⁷Zn isotopes were introduced in wheat via rachis injection to reveal tissue-specific allocation patterns through complementary pulse-chase and long-term tracing experiments across grain development stages. The crease served as a key barrier for both elements but was incompletely developed around 10 days after anthesis, allowing both elements to readily enter the endosperm. The crease also acted as a regulatory hub, preferentially allocating Cd to the periphery and Zn to the endosperm. During grain development, Cd remobilization was restricted within maternal (crease to periphery) or filial (endosperm to embryo) tissues, whereas Zn was remobilized from maternal to filial tissues. These findings identify grain maternal tissues as irreversible Cd sinks but Zn sources for the endosperm and highlight early grain filling as the optimal timing for reducing Cd accumulation while enhancing Zn biofortification.