Fractionation of Stable Zinc Isotopes in the Field-Grown Zinc Hyperaccumulator Noccaea caerulescens and the Zinc-Tolerant Plant Silene vulgaris

Stable Zn isotope signatures offer a potential tool for tracing Zn uptake and transfer mechanisms within plant-soil systems. Zinc isotopic compositions were determined in the Zn hyperaccumulator Noccaea caerulescens collected at a Zn-contaminated site (Viviez), a serpentine site (Vosges), and a noncontaminated site (Sainte Eulalie) in France. Meanwhile, a Zn-tolerant plant (Silene vulgaris) was also collected at Viviez for comparison. While delta Zn-66 was substantially differentiated among N. caerulescens from the three localities, they all exhibited an enrichment in heavy Zn isotopes of 0.40-0.72 parts per thousand from soil to root, followed by a depletion in heavy Zn from root to shoot (-0.10 to 0.50 parts per thousand). The enrichment of heavy Zn in roots is ascribed to the transport systems responsible for Zn absorption into root symplast and root-to-shoot translocation, while the depletion in heavy Zn in shoots is likely to be mediated by a diffusive process and an efficient translocation driven by energy-required transporters (e.g., NcHMA4). The mass balance yielded a bulk Zn isotopic composition between plant and soil (triangle(66) Znplant-soil) of -0.01 parts per thousand to 0.63 parts per thousand in N. caerulescens, indicative of high-and/or low-affinity transport systems operating in the three ecotypes. In S. vulgaris, however, there was no significant isotope fractionation between whole plant and rhizosphere soil and between root and shoot, suggesting that this species appears to have a particular Zn homeostasis. We confirm that quantifying stable Zn isotopes is useful for understanding Zn accumulation mechanisms in plants.