Iron–Zinc Synergy Blocks Cadmium Translocation in Rice: Minimizing Grain Contamination

Cadmium (Cd) pollution severely constrains safe rice production and threatens food security. Leveraging Fe–Zn competitive antagonism to mitigate crop Cd accumulation is a green, sustainable remediation strategy. Based on our hypothesis, we proposed that combined Fe–Zn pretreatment in seedlings and foliar spraying during the reproductive period would reduce Cd accumulation in brown rice by inhibiting root uptake, impeding translocation, and enhancing vacuolar sequestration in flag leaves. A two-year, three-season field experiment was conducted in the Cd-contaminated double-cropping rice planting area in Hunan Province. Three treatments were applied: conventional (CK), Fe–Zn pretreatment at seedling stage (FZ), and Fe–Zn pretreatment + tillering and heading spraying (FZS). This study demonstrated that FZS reduced brown rice Cd by 25%, primarily by enhancing root retention (root Cd reduced by 17–19%) and flag leaf vacuolar sequestration (flag leaf Cd 31% higher than old leaves). FZS further decreased stem–leaf Cd by 47–54% and lowered the husk-to-grain transfer coefficient from 0.22 to 0.17. Multivariate analysis identified flag leaf interception (β = −0.25) as the dominant factor regulating grain Cd, followed by panicle accumulation (β = 0.122) and Fe–Zn dosage (β = −0.061). Integrated Fe–Zn treatment blocked soil-to-grain Cd transfer via physiological barriers and flag leaf sequestration.