Fien Degryse

Predicting availability of mineral elements to plants with the DGT technique: a review of experimental data and interpretation by modelling.

Environmental context. Total concentrations of mineral elements in soil bear little relation to their availability for plants. The DGT (diffusive gradients in thin-films) technique has been found to be a good predictor of trace metal uptake and P deficiency, though not consistently in all studies for all elements. This review examines the fundamental basis for the relation between DGT fluxes and plant uptake and assesses under which conditions this relation may break down. Abstract. In the DGT technique, elements are accumulated on a binding gel after their diffusive transport through a hydrogel. In this paper, we explore in more detail why – and under which conditions – DGT correlates with plant uptake. The theoretical considerations are illustrated with experimental results for metal uptake and toxicity, and for phosphorus deficiency. Strong correlations between DGT and plant uptake are predicted if the diffusive transport of the element from soil to the plant roots is rate-limiting for its uptake. If uptake is not limited by diffusive transport, DGT-fluxes and plant uptake may still correlate provided that plant uptake is not saturated. However, competitive cations may affect the plant uptake under these conditions, whereas they have no effect on the DGT flux. Moreover, labile complexes are not expected to contribute to the plant uptake if diffusion is not limiting, but they are measured with DGT. Therefore, if plant uptake is not limited by diffusion, interpretation of the observed correlation in terms of the labile species measured by DGT is inappropriate.

Metal Complexation Properties of Freshwater Dissolved Organic Matter Are Explained by Its Aromaticity and by Anthropogenic Ligands

Dissolved organic matter (DOM) in surface waters affects the fate and environmental effects of trace metals. We measured variability in the Cd, Cu, Ni, and Zn affinity of 23 DOM samples isolated by reverse osmosis from freshwaters in natural, agricultural, and urban areas. Affinities at uniform pH and ionic composition were assayed at low, environmentally relevant free Cd, Cu, Ni, and Zn activities. The C-normalized metal binding of DOM varied 4-fold (Cu) or about 10-fold (Cd, Ni, Zn) among samples. The dissolved organic carbon concentration ranged only 9-fold in the waters, illustrating that DOM quality is an equally important parameter for metal complexation as DOM quantity. The UV-absorbance of DOM explained metal affinity only for waters receiving few urban inputs, indicating that in those waters, aromatic humic substances are the dominant metal chelators. Larger metal affinities were found for DOM from waters with urban inputs. Aminopolycarboxylate ligands (mainly EDTA) were detected at concentrations up to 0.14 μM and partly explained the larger metal affinity. Nickel concentrations in these surface waters are strongly related to EDTA concentrations (R2=0.96) and this is underpinned by speciation calculations. It is concluded that metal complexation in waters with anthropogenic discharges is larger than that estimated with models that only take into account binding on humic substances.

Copper speciation and isotopic fractionation in plants: uptake and translocation mechanisms

The fractionation of stable copper (Cu) isotopes during uptake into plant roots and translocation to shoots can provide information on Cu acquisition mechanisms. Isotope fractionation ((65) Cu/(63) Cu) and intact tissue speciation techniques (X-ray absorption spectroscopy, XAS) were used to examine the uptake, translocation and speciation of Cu in strategy I (tomato-Solanum lycopersicum) and strategy II (oat-Avena sativa) plant species. Plants were grown in controlled solution cultures, under varied iron (Fe) conditions, to test whether the stimulation of Fe-acquiring mechanisms can affect Cu uptake in plants. Isotopically light Cu was preferentially incorporated into tomatoes (Δ(65) Cu(whole plant-solution ) = c. -1‰), whereas oats showed minimal isotopic fractionation, with no effect of Fe supply in either species. The heavier isotope was preferentially translocated to shoots in tomato, whereas oat plants showed no significant fractionation during translocation. The majority of Cu in the roots and leaves of both species existed as sulfur-coordinated Cu(I) species resembling glutathione/cysteine-rich proteins. The presence of isotopically light Cu in tomatoes is attributed to a reductive uptake mechanism, and the isotopic shifts within various tissues are attributed to redox cycling during translocation. The lack of isotopic discrimination in oat plants suggests that Cu uptake and translocation are not redox selective.

Mobilization of Cu and Zn by root exudates of dicotyledonous plants in resin-buffered solutions and in soil

It has been frequently suggested that root exudates play a role in trace metal mobilization and uptake by plants, but there is little in vivo evidence. We studied root exudation of dicotyledonous plants in relation to mobilization and uptake of Cu and Zn in nutrient solutions and in a calcareous soil at varying Cu and Zn supply. Spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) were grown on resin-buffered nutrient solutions at varying free ion activities of Cu (pCu 13.0–10.4) and Zn (pZn 10.1–6.6). The Cu and Zn concentrations in the nutrient solution increased with time, except in plant-free controls, indicating that the plant roots released organic ligands that mobilized Cu and Zn from the resin. At same pCu, soluble Cu increased more at low Zn supply, as long as Zn deficiency effects on growth were small. Zinc deficiency was observed in most treatment solutions with pZn ≥ 9.3, but not in nutrient solutions of a smaller volume/plant ratio in which higher Zn concentrations were observed at same pZn. Root exudates of Zn-deficient plants showed higher specific UV absorbance (SUVA, an indicator of aromaticity and metal affinity) than those of non-deficient plants. Measurement of the metal diffusion flux with the DGT technique showed that the Cu and Zn complexes in the nutrient solutions were highly labile. Diffusive transport (through the unstirred layer surrounding the roots) of the free ion only could not explain the observed plant uptake of Cu and of Zn at low Zn2+ activity. The Cu and Zn uptake by the plants was well explained if it was assumed that the complexes with root exudates contributed 0.4% (Cu) or 20% (Zn) relative to the free ion. In the soil experiment, metal concentrations and organic C concentrations were larger in the solution of planted soils than in unplanted controls. The SUVA of the soil solution after plant growth was higher for unamended soils, on which the plants were Zn-deficient, than for Zn-amended soils. In conclusion, root exudates of dicotyledonous plants are able to mobilize Cu and Zn, and plants appear to respond to Zn deficiency by exuding root exudates with higher metal affinity.

Diffusion limitations in root uptake of cadmium and zinc, but not nickel, and resulting bias in the Michaelis constant

It has long been recognized that diffusive boundary layers affect the determination of active transport parameters, but this has been largely overlooked in plant physiological research. We studied the short-term uptake of cadmium (Cd), zinc (Zn), and nickel (Ni) by spinach (Spinacia oleracea) and tomato (Lycopersicon esculentum) in solutions with or without metal complexes. At same free ion concentration, the presence of complexes, which enhance the diffusion flux, increased the uptake of Cd and Zn, whereas Ni uptake was unaffected. Competition effects of protons on Cd and Zn uptake were observed only at a very large degree of buffering, while competition of magnesium ions on Ni uptake was observed even in unbuffered solutions. These results strongly suggest that uptake of Cd and Zn is limited by diffusion of the free ion to the roots, except at very high degree of solution buffering, whereas Ni uptake is generally internalization limited. All results could be well described by a model that combined a diffusion equation with a competitive Michaelis-Menten equation. Direct uptake of the complex was estimated to be a major contribution only at millimolar concentrations of the complex or at very large ratios of complex to free ion concentration. The true Km for uptake of Cd2+ and Zn2+ was estimated at <5 nm, three orders of magnitude smaller than the Km measured in unbuffered solutions. Published Michaelis constants for plant uptake of Cd and Zn likely strongly overestimate physiological ones and should not be interpreted as an indicator of transporter affinity.

Effect of Organic P Forms and P Present in Inorganic Colloids on the Determination of Dissolved P in Environmental Samples by the Diffusive Gradient in Thin Films Technique, Ion Chromatography, and Colorimetry

The speciation of P in environmental samples is operationally defined, since it depends on the analytical method used. In this study, we compared four methods to measure P in solution: ion chromatography (IC), the malachite green colorimetric method (CM), the diffusive gradient in thin films technique (DGT) and, for total dissolved P, optical inductively coupled plasma (ICP). These methods were compared on three sets of solutions (filtered over <0.45 μm): solutions with model organic P compounds, suspensions of synthesized inorganic Fe and Al colloids loaded with P, and environmental samples. The environmentally relevant organic P compounds were only marginally detected by CM and IC. Substantial fractions of certain organic P compounds contributed to the DGT measurement. Colorimetric analysis of DGT eluates detected in general less P than ICP analysis, indicating that these organic P compounds sorbed on the zero sink layer. Phosphorus associated with inorganic colloids was completely recovered by CM, but not by IC and least by DGT. Measurements on a wide set of 271 environmental samples (soil pore waters, groundwaters, and surface waters) suggest that surface water P is largely present as orthophosphate and phosphate sorbed onto inorganic colloids, whereas organic P contributes more in groundwaters.

Uptake of Metals from Soil into Vegetables

The consumption of locally-produced vegetables by humans may be an important exposure pathway for soil contaminants in many urban settings and for agricultural land use. Hence, prediction of metal and metalloid uptake by vegetables from contaminated soils is an important part of the Human Health Risk Assessment procedure. The behaviour of metals (cadmium, chromium, cobalt, copper, mercury, molybdenum, nickel, lead and zinc) and metalloids (arsenic, boron and selenium) in contaminated soils depends to a large extent on the intrinsic charge, valence and speciation of the contaminant ion, and soil properties such as pH, redox status and contents of clay and/or organic matter. However, chemistry and behaviour of the contaminant in soil alone cannot predict soil-to-plant transfer. Root uptake, root selectivity, ion interactions, rhizosphere processes, leaf uptake from the atmosphere, and plant partitioning are important processes that ultimately govern the accumulation of metals and metalloids in edible vegetable tissues. Mechanistic models to accurately describe all these processes have not yet been developed, let alone validated under field conditions. Hence, to estimate risks by vegetable consumption, empirical models have been used to correlate concentrations of metals and metalloids in contaminated soils, soil physico-chemical characteristics, and concentrations of elements in vegetable tissues. These models should only be used within the bounds of their calibration, and often need to be re-calibrated or validated using local soil and environmental conditions on a regional or site-specific basis.

The performance of DGT versus conventional soil phosphorus tests in tropical soils - an isotope dilution study

Background and aimsA soil test that samples nutrients only from fractions that are accessible to plants will predict availability and uptake more robustly than empirical tests. This can be tested by comparison of the isotope ratios (specific activity, SA) of the nutrient between plant and the soil extract. This study was set up to assess this requirement for the diffusive gradients in thin films technique (DGT), recently proposed as a soil P test, in comparison with conventional soil P tests viz. Olsen, Colwell, Bray-1, Mehlich-3, ammonium oxalate, anion exchange membranes (AEM) and 0.01 M CaCl2 solution.MethodsMaize (Zea mays L.) was grown in two P-deficient soils from western Kenya with contrasting P sorption characteristics, amended with a low and a high P rate and labelled with 33P.ResultsThe SA in the plant shoot corresponded with that of the extracts of the different soil tests, except for CaCl2 and ammonium oxalate extracts, at the low P rate in the soil with low P sorption capacity, Teso soil. For the high P rate on this soil, differences in SA between maize shoot and soil test were small for all established soil tests, but significant for the Colwell, Bray-1, Mehlich-3 and AEM tests. The SA in the soil extracts was significantly smaller than that in the maize shoot for Sega the strongly P-sorbing soil at both P rates for all conventional tests, including AEM. This indicates that these tests extracted P from a pool that is not accessible to the plant. For the DGT test, however, there was no difference in SA between the maize shoot and the soil test, for any of the treatments.ConclusionsMost conventional soil tests can extract a fraction of P which is not available to maize. The DGT technique, however, only samples P from the plant-accessible pool.

Copper isotope fractionation during equilibration with natural and synthetic ligands

As copper (Cu) stable isotopes emerge as a tool for tracing Cu biogeochemical cycling, an understanding of how Cu isotopes fractionate during complexation with soluble organic ligands in natural waters and soil solutions is required. A Donnan dialysis technique was employed to assess the isotopic fractionation of Cu during complexation with the soluble synthetic ligands ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA) and desferrioxamine B (DFOB), as well as with Suwannee River fulvic acid (SRFA). The results indicated enrichment of the heavy isotope ((65)Cu) in the complexes, with Δ(65)Cu complex-free values ranging from +0.14 to +0.84‰. A strong linear correlation was found between the logarithms of the stability constants of the Cu complexes and the magnitudes of isotopic fractionation. These results show that complexation of Cu by organic ligands can affect the isotopic signature of the free Cu ion. This free Cu is considered the most bioavailable species, and hence, our results highlight the importance of understanding fractionation processes in the uptake medium when using Cu isotopes to study the uptake mechanisms of organisms. These data contribute a vital piece to the emerging picture of Cu isotope cycling in the natural environment, as organic complexation plays a key role in the Cu cycle.

Isotopic fractionation of Zn in tomato plants suggests the role of root exudates on Zn uptake

AimsPhytosiderophore-chelated Zn can be absorbed in grasses. Root exudates of dicotyledonous plants can mobilize soil Zn but it is unclear how this affects Zn bioavailability. Stable Zn isotope shifts can indicate exudate-facilitated Zn uptake, since complexation of Zn2+ by organic ligands in solution yields a small, but detectable, enrichment of the heavy Zn isotope due to thermodynamic fractionation.MethodsTomato seedlings were grown in resin-buffered nutrient solution in which free Zn2+ concentrations are buffered, in a factorial design of two Zn levels and two solution volumes. The latter factor allowed altering the exudate concentrations in the solution. Dissolved Cu concentrations in the resin buffered system were used as a sensitive index of metal mobilization resulting from root activity. In addition, seedlings were grown in Zn deficient soil with and without Zn addition.ResultsThe dissolved Cu concentration increased with Zn deficiency and was highest at the lowest solution volume, suggesting metal mobilization by root exudates. At low Zn supply, Zn in the plant was enriched in heavy Zn (66Zn) and this was most pronounced at small solution volume. Similarly, Zn deficiency in soil enriched tomato shoot Zn with heavy isotope in this plant.InterpretationZinc deficiency increases the contribution of Zn-exudate complexes, which are enriched in the heavy isotope compared to the free ion, to Zn uptake by transporting Zn from the bulk solution or soil to the roots where they likely dissociate and release Zn2+.