Stable Isotopes of Cu and Zn in Higher Plants: Evidence for Cu Reduction at the Root Surface and Two Conceptual Models for Isotopic Fractionation Processes

Abstract

Recent reports suggest that significant fractionation of stable metal isotopes occurs during biogeochemical cycling and that the uptake into higher plants is an important process. To test isotopic fractionation of copper (Cu) and zinc (Zn) during plant uptake and constrain its controls, we grew lettuce, tomato, rice and durum wheat under controlled conditions in nutrient solutions with variable metal speciation and iron (Fe) supply. The results show that the fractionation patterns of these two micronutrients are decoupled during the transport from nutrient solution to root. In roots, we found an enrichment of the heavier isotopes for Zn, in agreement with previous studies, but an enrichment of isotopically light Cu, suggesting a reduction of Cu(II) possibly at the surfaces of the root cell plasma membranes. This observation holds for both graminaceous and nongraminaceaous species and confirms that reduction is a predominant and ubiquitous mechanism for the acquisition of Cu into plants similar to the mechanism for the acquisition of iron (Fe) by the strategy I plant species. We propose two preliminary models of isotope fractionation processes of Cu and Zn in plants with different uptake strategies.

Ion Exchange Chromatography and Mass Bias Correction for Accurate and Precise Zn Isotope Ratio Measurements in Environmental Reference Materials by MC-ICP-MS

“…44 The δ 66/64 Zn JMC values of 0.79 ± 0.09‰ (2σ, n = 8) for 1573a tomato are within the large range found in the plant leaves ranging from -0.91 to 0.63‰ for herbaceous species and 0.98 ± 0.19‰, 2σ for bamboo leaves. 7,45 The several factors of fractionation such as diffusive processes in cross-cell membrane transport, Zn bioavailable speciation and rhizosphere reactions 46,47 probably are associated to this heavy isotopic compositions of 1573a tomato leaves compared with soils and sediments.…”

Daily rhythmicity of high affinity copper transport

“…10 Under deficiency, the predominant strategy for Cu acquisition consists in a mechanism of reduction of Cu 2C to Cu C and subsequent Cu C uptake. 11,12 Cu entrance is mostly dependent on the quantity of high affinity Cu transporters. Variations in external Cu concentrations in the deficiency range (below 0.5 mM 13 ) have little effect on Cu influx.…”

Cu isotopic compositions in Elsholtzia splendens: Influence of soil condition and growth period on Cu isotopic fractionation in plant tissue

“…Recent reports have suggested that Cu isotopic fractionation occurs during plant growth, and that Cu isotopic fractionation is particularly strong in strategy I plants (Jouvin et al, 2012;Ryan et al, 2013;Weinstein et al, 2011). The aim of the present study is to systematically investigate the Cu isotopic fractionation associated with plant growth in the soil-plant system using the Cu-tolerant strategy I plant Elsholtzia splendens, and to better understand the possible mechanisms and factors controlling the variations in Cu isotopic compositions in such plants.…”

“…There is now a considerable body of research on heavy metal isotopes in plants, including Fe (Arnold et al, 2015;Guelke and von Blanckenburg, 2007), Cu (Jouvin et al, 2012;Ryan et al, 2013;Weinstein et al, 2011), Zn (Couder et al, 2015;Moynier et al, 2009;Viers et al, 2015, Viers et al, 2007Weiss et al, 2005), Ca (Page et al, 2008), and Mg (Black et al, 2008;Bolou-Bi et al, 2010).…”

“…Cu isotopic fractionation associated with plant uptake and translocation of metallic nutrients was first reported by Weinstein et al (2011), who found light Cu isotope enrichment in plants relative to soil (e.g., Virginia wild rye, − 0.94‰ b Δ 65 Cu plant(stem,leaves,seeds) − soil b − 0.51‰; hairy-leaved sedge, − 0.79‰ b Δ 65 Cu plant(leaf,seeds,stem) − soil b − 0.33‰). Jouvin et al (2012) and Ryan et al (2013) carried out research on Cu isotopic fractionation in plants grown in EDTA, DEDTA + NTA, and high-and low-Fe nutrient solutions during uptake in a hydroponic plant experiment. They found light Cu isotope enrichment of Δ value of soil, and the effect of growth cycle on Cu isotopic fractionation during Cu uptake and translocation within the plant.…”