Leaf-applied sodium chloride promotes cadmium accumulation in durum wheat grain

Cadmium (Cd) accumulation in durum wheat grain is a growing concern. Among the factors affecting Cd accumulation in plants, soil chloride (Cl) concentration plays a critical role. The effect of leaf NaCl application on grain Cd was studied in greenhouse-grown durum wheat (Triticum turgidum L. durum, cv. Balcali-2000) by immersing (10 s) intact flag leaves into Cd and/or NaCl-containing solutions for 14 times during heading and dough stages. Immersing flag leaves in solutions containing increasing amount of Cd resulted in substantial increases in grain Cd concentration. Adding NaCl alone or in combination with the Cd-containing immersion solution promoted accumulation of Cd in the grains, by up to 41%. In contrast, Zn concentrations of grains were not affected or even decreased by the NaCl treatments. This is likely due to the effect of Cl complexing Cd and reducing positive charge on the metal ion, an effect that is much smaller for Zn. Charge reduction or removal (CdCl2 0 species) would increase the diffusivity/lipophilicity of Cd and enhance its capability to penetrate the leaf epidermis and across membranes. Of even more significance to human health was the ability of Cl alone to penetrate leaf tissue and mobilize and enhance shoot Cd transfer to grains, yet reducing or not affecting Zn transfer

Uptake and retranslocation of leaf-applied cadmium (Cd-109) in diploid, tetraploid and hexaploid wheats

WOS: 000085380200008PubMed ID: 10938828Uptake and retranslocation of leaf-applied radiolabelled cadmium (Cd-109) was studied in three diploid (Triticum monococcum, AA), four tetraploid (Triticum turgidum, BBAA) and two hexaploid (Triticum aestivum, BBAADD) wheat genotypes grown for 9 d under controlled environmental conditions in nutrient solution. Among the tetraploid wheats, two genotypes were primitive (ssp. dicoccum) and two genotypes modern wheats (ssp. durum), Radiolabelled Cd was applied by immersing the tips (3 cm) of mature leaf into a Cd-109 radiolabelled solution. There was a substantial variation in the uptake and export of Cd-109 among and within wheat species. On average, diploid wheats (AA) absorbed and translocated more Cd-109 than other wheats. The largest variation in Cd-109 uptake was found within tetraploid wheats (BBAA), Primitive tetraploid wheats (ssp, dicoccum) had a greater uptake capacity for Cd-109 than modern tetraploid wheats (ssp, durum), In all wheats studied, the amount of the Cd-109 exported from the treated leaf into the roots and the remainder of the shoots was poorly related to the total absorption, For example, bread wheat cultivars were more or less similar in total absorption, but differed greatly in the amount of Cd-109 retranslocated. The diploid wheat genotype 'FAL-43' absorbed the lowest amount of Cd-109, but retranslocated the greatest amount of Cd-109 in roots and remainder of shoots, The results indicate the existence of substantial genotypic variation in the uptake and retranslocation of leaf-applied Cd-109. This variation is discussed in terms of potential genotypic differences in binding of Cd to cell walls and the composition of phloem sap ligands possibly affecting Cd transport into sink organs

Influence of varied zinc supply on re-translocation of cadmium (Cd-109) and rubidium (Rb-86) applied on mature leaf of durum wheat seedlings

WOS: 000086529500029Effect of varied zinc (Zn) supply (0, 0.1, 1, 5 mu M) on re-translocation of radio-labeled cadmium (Cd-109) and rubidium (Rb-86) from mature leaf to root and other parts of shoot was studied in 11-day-old durum wheat (Triticum durum cv. C-1252) plants grown in nutrient solution under controlled environmental conditions. Application of Cd-109 and Rb-86 was carried out by immersing the tips (3 cm) of mature leaf in radio-labeled solutions for 10 s at three different times over a 42 h period. Differences in Zn supply for 11 days did not affect plant growth nor did it cause visual leaf symptoms, such as necrosis and chlorosis, at either the lowest or the highest Zn supply. Only at the nil Zn supply (0 mu M), shoot and root dry weights tended to decrease and increase, respectively, causing a lower shoot/root dry weight ratio. Partitioning of more dry matter to roots rather than shoots, a typical phenomena for Zn-deficient plants in nutrient solution experiments, indicated existence of a mild Zn deficiency stress at the nil-Zn treatment. Irrespective of Zn supply, plants could, on average, retranslocate 3.8% and 38% of the total absorbed Cd-109 and Rb-86 from the treated leaf to roots and other parts of shoots within 42 h, respectively. At nil-Zn treatment, 2.8% of the total absorbed Cd-109 was re-translocated from the treated leaf, particularly into roots. The highest re-translocation of Cd-109 (6.5%) was found in plants supplied with 0.1 mu M Zn. Increases in Zn supply from 0.1 mu M reduced Cd-109 re-translocation from 6.5% to 4.3% at 1 mu M Zn and 1.3% at 5 mu M Zn. With the exception of the nil-Zn treatment, the proportion of re-translocated Cd-109 was greater in the remainder of the shoot than in the roots. Contrary to the Cd-109 results, re-translocation of Rb-86 was not (at 0, 0.1 and 1 mu M Zn), or only slightly (at 5 mu M), affected by changing Zn supply. The results indicate an inhibitory action of increased concentrations of Zn in shoot tissues on phloem-mediated Cd transport. This effect is discussed in relation to competitive inhibition of Cd loading into phloem sap by Zn

DGT-measured fluxes explain the chloride-enhanced cadmium uptake by plants at low but not at high Cd supply

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal uptake by plants in a wide range of soils. With the DGT technique, diffusion fluxes of trace metals through a diffusion layer towards a resin layer are measured. The DGT technique therefore mimics the metal uptake by plants if uptake is limited by diffusion of the free ion to the plant roots, which may not be the case at high metal supply. This study addresses the capability of DGT to predict cadmium (Cd) uptake by plants at varying Cd supply. To test the performance of DGT in such conditions, we used the chloride (Cl-) enhancement effect, i.e. the increase in Cd solution concentrations-due to chloride complexation of Cd-and Cd uptake with increasing Cl- concentrations, as previously characterized in pot, field and solution culture experiments. The uptake of Cd by spinach was assessed in soil amended with Cd (0.4-10.5 mg Cd kg-1) and NaCl (up to 120 mM) in a factorial design. Treatments with NaNO3 were included as a reference to correct for ionic strengths effects. The effect of Cl- on the shoot Cd concentrations was significant at background Cd but diminished with increasing soil Cd. Increasing Cl- concentrations increased the root area based Cd uptake fluxes by more than a factor of 5 at low soil Cd, but had no significant effect at high soil Cd. Short-term uptake of Cd in spinach from nutrient solutions confirmed these trends. In contrast, increasing Cl - concentrations increased the DGT measured fluxes by a factor of 5 at all Cd levels. As a result, DGT fluxes were able to explain soil Cl - effects on plant Cd concentrations at low but not at high Cd supply. This example illustrates under which conditions DGT mimics trace metal bioavailability. If biouptake is controlled by diffusive limitations, DGT should be a successful tool for predicting ion uptake across different conditions. © 2008 Springer Science+Business Media B.V.Carla Oporto, Erik Smolders, Fien Degryse, Liesbeth Verheyen, Carlo Vandecasteel

Metal complexes increase uptake of Zn and Cu by plants: implications for uptake and deficiency studies in chelator-buffered solutions

The uptake of trace metals by plants is commonly assumed to depend on the free metal-ion activity, rather than on the total concentration of dissolved metal. Although this free-ion hypothesis has proved to be useful for the interpretation and prediction of metal uptake, several exceptions have been reported where metal complexes also affected metal uptake by plants. In this study, we measured uptake of Zn and Cu by spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) in chelator-buffered or resin (Chelex)-buffered solutions, under Zn-deficient and non-deficient conditions. Several ligands, with differing dissociation rates, were used in the chelator-buffered solutions. At the same free-ion activity, Cu and Zn uptake was less in Chelex-buffered than in chelator-buffered solutions. In the chelator-buffered solution, uptake of Cu and Zn at same free-ion activity and same total concentration followed the order: NTA > HEDTA > EDTA > CDTA, i.e., the same order as the dissociation rate. These differences in metal uptake were also reflected in the deficiency symptoms and plant yield in the experiments where Zn deficiency was imposed. The critical Zn2+ activity for Zn deficiency varied by one order of magnitude depending on the buffer, and followed the order HEDTA < CDTA < resin-buffered (no soluble ligand). These results suggest that, when present, aqueous complexes can increase metal uptake by plants because uptake is rate-limited by diffusion of the free ion to the root or cell surface. Thus, the critical free-ion activity in chelator-buffered solutions depends on the type and concentration of the ligand employed. © 2006 Springer Science+Business Media B.V.F. Degryse, E. Smolders, D. R. Parke