Transpiration flow controls Zn transport in Brassica napus and Lolium multiflorum under toxic levels as evidenced from isotopic fractionation

Abstract

A B S T R A C TStable zinc (Zn) isotope fractionation between soil and plant has been used to suggest the mechanisms affecting Zn uptake under toxic conditions. Here, changes in Zn isotope composition in soil, soil solution, root and shoot were studied for ryegrass (Lolium multiflorum L.) and rape (Brassica napus L.) grown on three distinct metal-contaminated soils collected near Zn smelters (total Zn 0.7-7.5%, pH 4.8-7.3). The Zn concentrations in plants reflected a toxic Zn supply. The Zn isotopic fingerprint of total soil Zn varied from À0.05% to +0.26 AE 0.02% (d 66 Zn values relative to the JMC 3-0749L standard) among soils, but the soil solution Zn was depleted in 66 Zn, with a constant Zn isotope fractionation of about À0.1% d 66 Zn unit compared to the bulk soil. Roots were enriched with 66 Zn relative to soil solution (d 66 Zn root À d 66 Zn soil solution = D 66 Zn root-soil solution = +0.05 to +0.2 %) and shoots were strongly depleted in 66 Zn relative to roots (D 66 Zn shoot-root = À0.40 to À0.04 %). The overall d 66 Zn values in shoots reflected that of the bulk soil, but were lowered by 0.1-0.3 % units as compared to the latter. The isotope fractionation between root and shoot exhibited a markedly strong negative correlation (R 2 = 0.83) with transpiration per unit of plant weight. Thus, the enrichment with light Zn isotopes in shoot progressed with increasing water flux per unit plant biomass dry weight, showing a passive mode of Zn transport by transpiration. Besides, the light isotope enrichment in shoots compared to roots was larger for rape than for rye grass, which may be related to the higher Zn retention in rape roots. This in turn may be related to the higher cation exchange capacity of rape roots. Our finding can be of use to trace the biogeochemical cycles of Zn and evidence the tolerance strategies developed by plants in Znexcess conditions.

The influence of weathering and soil organic matter on Zn isotopes in soils

“…The Zn was then purified by a novel chromatographic separation technique on microcolumns loaded with 0.2 ml AG1-× 8 resin. This method involves successive additions of acids: 6N HCl (for column conditioning and sample loading), 1 N HCl (for matrix rinsing) and 1N HNO 3 /HBr (for zinc fraction elution) (Couder et al, 2015). Upon separation the eluate was dried down and digested with 100 μl concentrated HNO 3 to dissolve potential co-eluted organics.…”

“…Crushed soil samples (~2 mg) were dry-ashed for 24 h at 450°C to remove organic matter (Couder et al, 2015). The dry-ashed samples were dissolved by applying the tri-acid digestion technique (with concentrated 14 M HNO 3 24 M HF and re-dissolution in 6 M HCl) in a Teflon Savillex® beaker placed on a hot plate (120°C) for evaporation until dryness (Couder et al, 2015). The Zn was then purified by a novel chromatographic separation technique on microcolumns loaded with 0.2 ml AG1-× 8 resin.…”

“…Zinc mobilization by plant roots or root exudates preferentially releases heavy Zn isotopes (Smolders et al, 2013;Houben et al, 2014). Within the plant, heavy Zn isotopes sorb onto the root surface, and light Zn isotopes are preferentially transported into aerial plant parts (Arnold et al, 2010;Aucour et al, 2011;Jouvin et al, 2012;Moynier et al, 2009;Weiss et al, 2005;Viers et al, 2007Viers et al, , 2015Couder et al, 2015;Tang et al, 2016;Caldelas and Weiss, 2017). Plant litter at the soil surface is therefore generally isotopically lighter, and organic matter decomposition is considered to lead to the retention of heavier Zn isotopes by humification (Viers et al, 2015).…”

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

“…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).…”

“…As mineral elements are taken up by the plant root and translocated throughout the plant, Cu in the upper stems and branches is translocated upward from lower stems and branches. It is generally assumed that two main mechanisms are involved in the transport of Cu into stems and branches: diffusion of Cu over relatively short distances according to a concentration gradient (Moynier et al, 2009;Weinstein et al, 2011), and convective mass flow in the plant, controlled by transpiration (Barberon and Geldner, 2014;Couder et al, 2015;Lorenz et al, 1994).…”