Effects and mechanisms of foliar application of different medium and trace elements on cadmium accumulation in brown rice

Objectives Foliar application of physiological barrier agents containing secondary and micronutrients is a common practice for the safe rice production in cadmium (Cd)-contaminated farmland. However, their effectiveness varies considerably depending on the nutrient type and cultivation conditions. This study aimed to comparatively evaluate the effects and underlying mechanisms of foliar application of common secondary and micronutrient fertilizers, including CaCl₂, MgSO₄, Na₂SiO₃, Na₂SeO₃, MnCl₂, FeSO₄, ZnSO₄, and (NH₄)₂MoO₄ on Cd accumulation in brown rice, thereby providing a scientific basis for the selection and formulation optimization of foliar barrier agents.

Methods Two-year pot and field experiments were conducted with rice. The above nutrient solutions were applied as foliar sprays at the heading and grain-filling stages, with water spraying as the control. Treatments significantly affecting brown rice Cd content were screened by measuring Cd concentration. Further analysis included determining Cd content, accumulation, and translocation factors in different plant parts. Combined with the analysis of subcellular Cd distribution and the expression levels of Cd transporter-related genes in samples collected at the grain-filling stage, the effects and mechanisms by which foliar application of different nutrients regulates Cd content in brown rice were investigated.

Results Foliar application of CaCl₂, MgSO₄, Na₂SeO₃, MnCl₂, and (NH₄)₂MoO₄ had no significant effect on brown rice Cd content. In contrast, application of Na₂SiO₃, ZnSO₄, and FeSO₄ significantly reduced brown rice Cd content in both consecutive years of pot and field experiments. Under the Na₂SiO₃, ZnSO₄, and FeSO₄ treatments, there was no significant change in total Cd accumulation in the aboveground parts. However, Cd content and accumulation in the flag leaf were significantly higher than in the control. The proportion of Cd allocated to flag leaf organelles decreased significantly, while the proportion in the cell wall (Na₂SiO₃, FeSO₄ treatment) and the soluble fraction (ZnSO₄ treatment) increased significantly. Concurrently, the translocation factor of Cd from the flag leaf to internode I (internodes below the ear) decreased significantly by 63.3%, 62.0%, and 71.6%, respectively. Compared to other functional leaves, the flag leaf had a more significant impact on Cd translocation. Under the Na₂SiO₃, ZnSO₄, and FeSO₄ treatments, the expression of OsZIP7 (a gene associated with Cd vascular loading) and OsHMA2 (a gene related to long-distance transport) was significantly downregulated, while the expression of OsCAL1 (a Cd efflux-related gene) was significantly upregulated. This enhanced Cd retention in the flag leaf, inhibited Cd translocation to ear organs, and consequently reduced Cd content in brown rice.

Conclusions Foliar application of Na₂SiO₃, ZnSO₄, or FeSO₄ effectively reduced grain Cd accumulation by enhancing Cd retention in the flag leaves (the last fully expanded leaf) and restricting its translocation efficiency from these leaves to the first internode. These treatments altered the subcellular distribution of Cd within the flag leaves. Specifically, Na₂SiO₃ and FeSO₄ enhanced Cd immobilization in the cell wall and soluble fractions, whereas ZnSO₄ primarily promoted Cd chelation and fixation within the soluble fraction. Furthermore, all three treatments significantly downregulated the expression of Cd transporter genes OsZIP7and OsHMA2, while upregulating OsCAL1. In contrast, foliar spraying of CaCl₂, MgSO₄, Na₂SeO₃, MnCl₂, or (NH₄)₂MoO₄ had no significant effect on rice grain Cd content.