Significant lead isotope ‘fractionation’ in maize records plant lead uptake, transfer, and detoxification mechanisms

Lead isotope analysis is the main method to trace the sources and cycling of Pb in the biosphere system. The linchpin of such application hinges on the assumption that there is negligible or no biologically mediated isotopic fractionation of Pb occurs in the environment. However, recent measurements by high-precision multi-collector mass spectrometry revealed that biological isotope fractionation of heavy mass elements is a prevalent phenomenon. This study shows that compared with the Pb sources, the maize plant (Zea mays L.) organs exhibit a wider range of Pb isotope compositions and a depletion of radioactive Pb isotopes (²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb). Moreover, three independent studies consistently indicate that the ²⁰⁶Pb/²⁰⁷Pb ratio of maize organs varies as root/leaf > stem/grain, reflecting a continuous loss of light Pb isotopes during transportation. The conventional wisdom fails to account for these phenomena, suggesting that maize may undergo Pb isotope fractionation during the absorption and transportation of Pb. However, compared with other non-traditional metal isotopes, Pb isotope exhibits a more significant fractionation magnitude. We tentatively attribute this fractionation to the Pb tolerance mechanism of maize and its selective absorption of various forms of Pb, which requires further research to validate. Findings from this study mandate caution in future Pb source tracing in plants using Pb isotope methods and open up applications in using Pb isotopic fractionation to track Pb uptake and transfer pathways and decipher the associated detoxification mechanisms in plants.