Zinc Isotope Fractionation in the Hyperaccumulator <i>Noccaea caerulescens</i> and the Nonaccumulating Plant <i>Thlaspi arvense</i> at Low and High Zn Supply

On the basis of our previous field survey, we postulate that the pattern and degree of zinc (Zn) isotope fractionation in the Zn hyperaccumulator <i>Noccaea caerulescens</i> (J. & C. Presl) F. K. Mey may reflect a relationship between Zn bioavailability and plant uptake strategies. Here, we investigated Zn isotope discrimination during Zn uptake and translocation in <i>N. caerulescens</i> and in a nonaccumulator <i>Thlaspi arvense</i> L. with a contrasting Zn accumulation ability in response to low (Zn-L) and high (Zn-H) Zn supplies. The average isotope fractionations of the <i>N. caerulescens</i> plant as a whole, relative to solution (Δ⁶⁶Zn_{plant‑solution}), were −0.06 and −0.12‰ at Zn-L-C and Zn-H-C, respectively, indicative of the predominance of a high-affinity (e.g., ZIP transporter proteins) transport across the root cell membrane. For <i>T. arvense</i>, plants were more enriched in light isotopes under Zn-H-A (Δ⁶⁶Zn_{plant‑solution} = −0.26‰) than under Zn-L-A and <i>N. caerulescens</i> plants, implying that a low-affinity (e.g., ion channel) transport might begin to function in the nonaccumulating plants when external Zn supply increases. Within the root tissues of both species, the apoplast fractions retained up to 30% of Zn mass under Zn-H. Moreover, the highest δ⁶⁶Zn (0.75‰–0.86‰) was found in tightly bound apoplastic Zn, pointing to the strong sequestration in roots (e.g., binding to high-affinity ligands/precipitation with phosphate) when plants suffer from high Zn stress. During translocation, the magnitude of isotope fractionation was significantly greater at Zn-H (Δ⁶⁶Zn_root–shoot = 0.79‰) than at Zn-L, indicating that fractionation mechanisms associated with root–shoot translocation might be identical to the two plant species. Hence, we clearly demonstrated that Zn isotope fractionation could provide insight into the internal sequestration mechanisms of roots when plants respond to low and high Zn supplies