Nickel and Zinc Isotope Fractionation in Hyperaccumulating and Nonaccumulating Plants

Abstract

Until now, there has been little data on the isotope fractionation of nickel (Ni) in higher plants and how this can be affected by plant Ni and zinc (Zn) homeostasis. A hydroponic cultivation was conducted to investigate the isotope fractionation of Ni and Zn during plant uptake and translocation processes. The nonaccumulator Thlaspi arvense, the Ni hyperaccumulator Alyssum murale and the Ni and Zn hyperaccumulator Noccaea caerulescens were grown in low (2 μM) and high (50 μM) Ni and Zn solutions. Results showed that plants were inclined to absorb light Ni isotopes, presumably due to the functioning of low-affinity transport systems across root cell membrane. The Ni isotope fractionation between plant and solution was greater in the hyperaccumulators grown in low Zn treatments (Δ(60)Ni(plant-solution) = -0.90 to -0.63‰) than that in the nonaccumulator T. arvense (Δ(60)Ni(plant-solution) = -0.21‰), thus indicating a greater permeability of the low-affinity transport system in hyperaccumulators. Light isotope enrichment of Zn was observed in most of the plants (Δ(66)Zn(plant-solution) = -0.23 to -0.10‰), but to a lesser extent than for Ni. The rapid uptake of Zn on the root surfaces caused concentration gradients, which induced ion diffusion in the rhizosphere and could result in light Zn isotope enrichment in the hyperaccumulator N. caerulescens. In high Zn treatment, Zn could compete with Ni during the uptake process, which reduced Ni concentration in plants and decreased the extent of Ni isotope fractionation (Δ(60)Ni(plant-solution) = -0.11 to -0.07‰), indicating that plants might take up Ni through a low-affinity transport system of Zn. We propose that isotope composition analysis for transition elements could become an empirical tool to study plant physiological processes.

The Isotope Geochemistry of Ni

“…Isotopically lighter Zn in shoots relative to roots was first reported by Weiss et al, (2005) and similar features are observed in the Ni isotopic studies of plants (Deng et al, 2014;Estrade et 795 al., 2015). In a comparison of shoot-root isotopic differences of Zn and Ni in the same species, Deng et al (2014) noted a more modest fractionations for Ni.…”

“…It should be re-emphasised that most of the species analysed for their Ni isotopic compositions are hyperaccumulators, in which Ni is anomalously enriched shoots relative to roots. The two non-accumulators studied so far show contrasting behaviour, 800 with both positive, Thlapsi arvensa (Deng et al, 2014) and negative, Euphorbia spinosa values of Δ 60/58 Ni shoots-roots , see Figure 9. Some variability in Δ 60/58 Ni between roots, stems and leaves may be related to the growth stage of the plants .…”

“…In a comparison of shoot-root isotopic differences of Zn and Ni in the same species, Deng et al (2014) noted a more modest fractionations for Ni. They attributed this to higher Ni mobility in plants relative to Zn.…”

“…Laboratory experiments (Deng et al, 2014) and a field study reported isotopic analyses for several Ni hyperaccumulators, together with a non-accumulator species for comparison. In all but one case, the plants have net isotopically light compositions relative to dissolved Ni in growth media or inferred for soil water (Δ 60/58 Ni bulk plant-solution = -0.06 to -0.8), see 780 Figure 9.…”

Current status and challenges in developing nickel phytomining: an agronomic perspective

“…High nickel phytoavailability is essential for successful Ni phytomining (Massoura et al 2004), as Ni hyperaccumulator plants take up Ni from the same soil labile Ni pools as 'normal' plants Shallari et al 2001). Nickel hyperaccumulator plants have efficient root absorption mechanisms that deplete the phytoavailable Ni pools to the extent that the soil Ni chemical equilibrium is changed (Centofanti et al 2012;Deng et al 2014). As a result, Ni from non-labile pools replenishes the labile pool over time to maintain equilibration (Centofanti et al 2012), but this is a slow process and depends on the local buffering system (Massoura et al 2004).…”