Dual-Isotope Tracing Reveals Substantial Postanthesis Foliar and Panicle Contributions to Grain Cadmium Accumulation in Hydroponically Grown Rice

Cadmium (Cd) accumulation in rice grains occurs through multiple exposure pathways, yet the relative contributions and associated transport mechanisms remain unclear. Here, we applied dual stable isotope labeling (112Cd/113Cd) in hydroponically grown rice under controlled conditions, with root exposure through nutrient solution as well as foliar and panicle exposure through direct solution spraying. Source apportionment was complemented by sap analysis and triple-quadrupole LA-ICP-MS imaging to examine vascular transport and allocation in the first node under the panicle (node I). Direct Cd contact with aboveground tissues inhibited growth more strongly than root exposure, inducing antioxidant responses and stomatal limitation. Isotopic mass balance revealed that under controlled hydroponic root/foliar exposure, postanthesis Cd inputs contributed 80% and 90% of grain Cd, respectively. Under triple exposure, grain Cd averaged 0.558 mg kg–1, comprising 40% foliar, 24% panicle, and 36% root contributions, demonstrating that aboveground contributions were comparable to root contributions. Transport routes diverged: root uptake elevated xylem sap Cd, whereas foliar-derived Cd showed phloem-associated localization in node I and enrichment in phloem exudates. These results highlight the role of aboveground sources in grain Cd accumulation and suggest that risk assessment in deposition-affected areas should consider both root and aboveground Cd inputs, especially during reproduction.