Optimized foliar 24-epibrassinolide reduces grain cadmium and increases wheat yield by modulating root-to-shoot Cd dynamics, expanding temporary pools, and redirecting Cd to tillers

Cadmium (Cd) is a highly toxic heavy metal that suppresses crop growth and poses risks to human health. 2,4-Epibrassinolide (EBL) is known to promote plant growth, alleviate abiotic stress, and improve yield and quality. Here, we investigated whether the dose and timing of foliar EBL application could simultaneously reduce grain Cd accumulation and increase wheat yield. In the absence of EBL, wheat adopted a protective strategy by preferentially allocating Cd to tillers and strengthening Cd retention in roots and rachises. The source of grain Cd shifted from post-anthesis uptake under low to moderate stress to remobilization from root reserves under high stress, under which post-anthesis Cd efflux was induced. Exogenous EBL further enhanced these protective strategies. Post-anthesis application of 100 μmol·L⁻¹ EBL proved most effective, reducing grain Cd by 12%-56% and increasing yield by 31%-35% (P<0.05). The mechanisms differed sharply between pre- and post-anthesis application, depending on the Cd stress level. Under low Cd, pre-anthesis application only increased biomass without decreasing grain Cd, whereas post-anthesis application reduced grain Cd by inhibiting post-anthesis uptake and increasing temporary Cd storage in stems and leaves. Under moderate and high Cd, pre-anthesis application alleviated Cd-induced damage to vascular bundle cells in nodes, promoted post-anthesis root Cd efflux and its remobilization to aboveground tissues. In contrast, post-anthesis application primarily increased temporary Cd pools in stems and leaves, ultimately reducing grain Cd concentration. Collectively, post-anthesis spraying reduced grain Cd and increased yield by redirecting more Cd to tillers and away from the main stem, thereby protecting grain development. These findings demonstrate that optimizing EBL application timing can simultaneously reduce grain Cd accumulation and safeguard wheat yield under Cd-contaminated conditions.