Green Manure and Overlapped Rhizosphere Effects on Pb Chemical Forms in Soil and Plant Uptake in Maize/Canola Intercrop Systems: A Rhizobox Study

To investigate the effects of single and mixed cultures of maize and canola, green manure addition, and distance from the rhizosphere on chemical forms of soil lead (Pb) and its uptake by plants, a greenhouse experiment was conducted using rhizobox systems. After plant harvest, the soils from different parts of the rhizoboxes were sampled and analyzed for dissolved organic carbon (DOC) concentrations and pH values. A sequential extraction procedure was also carried out to differentiate the chemical forms of Pb in the soil samples. Results showed that root activity significantly affected soil DOC concentration, pH and Pb chemical forms, while the effects generally ceased from the rhizosphere through the bulk soil. The pH value increased and DOC concentration decreased with distance from the roots. Green manure addition enhanced DOC concentration and exchangeable, carbonate-associated, and organic matter-bound Pb fractions. Green manure addition reduced the Pb concentration and uptake of maize but increased those of canola. The concentration of Pb in the roots of maize and canola was increased in intercropping compared with monocropping systems. The conclusion is that chemistry and bioavailability of soil Pb are interactively influenced by overlapped rhizospheres as compared to individual rhizospheres, use of green manure, and distance from the root surfaces.     

Quantitative analysis of root and ectomycorrhizal exudates as a response to Pb, Cd and As stress

We examined exudation of low molecular weight (LMW) organic compounds of ectomycorrhizal (ECM) and non-mycorrhizal (NM) seedlings in relation to metals. Scots pine seedlings, either colonized by one of six different ECM fungi or NM, were grown in Petri dishes containing glass beads and liquid growth medium and exposed to elevated concentrations of Pb, Cd and As. Exudation of LMW organic compounds (LMW organic acids (LMWOAs), amino acids and dissolved monosaccharides) and dissolved organic carbon (DOC) was determined qualitatively and quantitatively and exudation rates were calculated. Metals had a significant impact on exudation, especially of oxalate. For Pb and Cd treatments, exudation of oxalate and total LMWOAs generally increased by 15–45% compared to nutrient controls. Production of amino acids, dissolved monosaccharides and DOC was not significantly stimulated by exposure to metals; however, there were non-significant trends towards increased exudation. Finally, exudation generally increased in the presence of mycorrhizal seedlings compared to NM seedlings. The results suggest that ECM fungi may reduce the toxicity of metals to plants through significant increases in the production of organic chelators. Axenic conditions are required to assess the full potential for production of these molecules but their overall significance in soil ecosystems needs to be determined using additional experiments under more ecologically realistic conditions.     

Spatial characteristics of aluminum uptake and translocation in roots of buckwheat (Fagopyrum esculentum)

The detoxification of aluminum (Al) in root tips of the Al accumulator buckwheat by exudation of oxalate leading to reduced Al uptake (Al resistance) is difficult to reconcile with the Al accumulation (Al tolerance). The objective of this study was to analyze resistance and tolerance mechanisms at the same time evaluating particularly possible stratification of Al uptake, Al transport and oxalate exudation along the root apex. The use of a minirhizotron made it possible to differentiate between spatial responses to Al along the root apex with regard to Al uptake and organic acid anion exudation, but also to measure at the same time Al and organic acid transport in the xylem. Al accumulates particularly in the 3‐mm root apex. The study showed that Al taken up by the 10‐mm root apex is rapidly transferred to the xylem which differentiates in the 10 to 15‐mm root zone as revealed by a microscopic study. Al induces the release of oxalate from the root apex but particularly from the subapical 6–20 mm root zone even when Al was applied only to the 5‐mm root apex suggesting a basipetal signal transduction. Citrate proved to be the most likely ligand for Al in the xylem because Al and citrate transport rates were positively correlated. In conclusion, the data presented show that the Al‐induced release of oxalate, and Al uptake as well as Al accumulation are spatially not separated in the root apex.     

Al uptake kinetics in roots of Melastoma malabathricum L. – an Al accumulator plant

The mechanism of Al uptake in melastoma (Melastoma malabathricum L.), which accumulates Al in excess of 10 000 mg kg^–1 in its leaves and roots, was investigated. Al uptake kinetics in excised melastoma roots showed a biphasic pattern, with an initial rapid phase followed by a slow phase. It was indicated that Al uptake in the excised roots occurs mostly through passive accumulation in the apoplast. On the other hand, Al uptake rate in roots of whole melastoma plant was almost double that in excised roots. The difference of Al uptake rate between excised roots and whole plant seems to be due to transpiration-depended Al uptake. Results from a long-term experiment showed that different characteristics of Al accumulation between melastoma and barley was caused by the difference in capacity to retain Al in root symplast, rather than by the difference in uptake rate into symplast. Concentrations of oxalate in root symplastic and apoplastic fractions, and total oxalate in shoots and roots, did not change greatly with time of Al exposure compared to Al concentration, although oxalate is considered as a main Al ligand in tissue of melastoma. On the other hand, oxalate exudation to root apoplast was induced within 24 h of Al exposure; the role of such exudation was discussed.     

Characterization of cadmium uptake by the water lily Nymphaea aurora

This study characterizes cadmium (Cd) uptake by the waterlily Nymphaea aurora, (Nymphaeaceae) in two systems: a model hydroponic Cd solution and heavily polluted sludge from two sites in Israel. The uptake of Cd from hydroponic solution resulted in Cd storage in petioles and laminae of Nymphaea, as well as in the roots. The pH of the solution affected Cd solubility and availability, with pH 5.5 yielding maximum Cd content in the plant (140 mg Cd per g DW). Cd uptake was reduced by the addition of EDTA to the hydroponic growth medium, although EDTA enhanced heavy metal uptake by terrestrial plants. Nymphaea efficiently reduced the concentration of Cd in heavy metal polluted urban and industrial sludge and the amount of Cd uptake was enhanced by the addition of KCl to the sludge and by adjustment of the pH to 5.5. The inherent growth patterns of Nymphaea plants allowed Cd uptake by the shoot and root, and resulted in maximum contact between the various plant parts and the growth media. Thus, Nymphaea has potential as an optimal, highly effective phytoremediation tool for the removal of Cd from polluted waste sources.     

Quantitative evaluation of the role of organic acid exudation in the mobilization of rock phosphate by rape

Phosphorus-deficient rape plants appear to acidify part of their rhizosphere by exuding malic and citric acid. A simulation model was used to evaluate the effect of measured exudation rates on phosphate uptake from Mali rock phosphate. The model used was one on nutrient uptake, extended to include both the effect of ion uptake and exudation on rhizosphere pH and the effect of rhizosphere pH on the solubilization of rock phosphate. Only the youngest zones of the root system were assumed to exude organic acids. The transport of protons released by organic acids was described by mass flow and diffusion. An experimentally determined relation was used describing pH and phosphate concentration in the soil solution as a function of total soil acid concentration. Model parameters were determined in experiments on organic acid exudation and on the uptake of phosphate by rape from a mixture of quartz sand and rock phosphate. Results based on simulation calculations indicated that the exudation rates measured in rape plants deficient in phosphorus can provide the roots with more phosphate than is actually taken up. Presence of root hairs enhanced the effect of organic acid exudation on calculated phosphate uptake. However, increase of root hair length without exudation as an alternative strategy to increase phosphate uptake from rock phosphate appeared to be less effective than exudation of organic acids. It was concluded that organic acid exudation is a highly effective strategy to increase phosphate uptake from rock phosphate, and that it unlikely that other rhizosphere processes play an important role in rock phosphate mobilization by rape.     

Roots regulate ion transport in the rhizosphere to counteract reduced mobility in dry soil

Diffusion of ions in the soil depends on soil moisture content. In a dry soil, transport of nutrients towards the root and the concomitant uptake could be reduced. However, pot and field experiments showed that this is not always the case. The objective of this paper was to investigate possible mechanisms of plants to counteract reduced nutrient supply due to water shortage. A split root system was used to investigate P and K inflow of oat and sugar beet at different soil moisture contents (Θ) without water shortage for the plant. The measured average P and K inflows were compared to model calculations considering diffusion, mass-flow, sorption and uptake processes. In the calculations, soil dryness impeded diffusion and decreased nutrient inflow as expected. Measured K inflow was decreased in a similar way indicating that Θ influences K diffusion. In contrast to this, measured P inflow was not influenced by Θ and under-estimated by the model. Low and high molecular exudates were collected at different water supply levels showing that exudation rate of both compounds was increased at water shortage. Especially the high molecular exudates (i.e. mainly mucilage) from water-stressed plants increased P concentration in soil solution under dry conditions in an incubation experiment. Calculated inflow considering this increased P concentration agreed well with measured P inflow indicating that exudation of mucilage could be a mechanism to overcome nutrient transport problems due to soil dryness.     

Experimental Assessment of Using Soluble Humic Substances for Remediation of Heavy Metal Polluted Soils

Since heavy metals are nondegradable and strongly bonded in soils, remediation of heavy metal polluted soils by extraction is difficult and current extraction techniques require harsh chemicals such as ethylenediaminetetraacetic acid (EDTA). However, use of EDTA is environmentally problematic because of costs, persistence, toxicity and deterioration of soil structure. Therefore, the potential of soluble natural humic substances (HS) to extract heavy metals from contaminated soils is tested as an environmentally friendly substitute for EDTA. A strongly polluted, calcareous urban soil (CRC soil) and a moderately polluted agricultural soil (CUP soil) were extracted at neutral pH in batch mode by three HS solutions from beech and Norway spruce litter (Beech-HS and Spruce-HS) and processed cow slurry (Bio-HS), all containing 25 mM dissolved organic carbon (DOC). After 10 extractions with a solution to soil ratio of 5:1 (L/kg), 8% to 39% of the total Cd, Cu, Ni and Pb soil contents, lowest for Ni and highest for Cu/Pb, were extracted. Natural and processed HS samples had comparable capacities to extract the heavy metals. A comparison of 100 mM DOC of Bio-HS and EDTA as extractants for Cu from the CRC soil showed extraction of 67% by EDTA and 41% by Bio-HS, indicating somewhat higher efficiency of EDTA than of HS. Sequential extraction of the CRC soil after Bio-HS and EDTA extraction showed removal of exchangeable, carbonate- and metal oxide-bound Cu but also of some residual Cu. It is therefore concluded that HS appears to be an attractive and promising alternative to EDTA as remediation agent for heavy metal polluted soils provided cheap HS of good quality is easily available.     

A field lysimeter study of heavy metal movement down the profile of soils with multiple metal pollution during chelate-enhanced phytoremediation

The agricultural soils near a copper smelter in southeast China were found to be highly contaminated with Cu, Pb, Zn, and Cd. Metal migration from the soil to groundwater presents an environmental risk that depends on the physicochemical properties of the contaminated soils. Soil solution samples were obtained using lysimeters from a loam soil with multiple metal pollutions over a period of about 1 yr. A field lysimeter study was also conducted to examine the potential use of (S, S')-ethylenediamine-N, N'-disuccinic acid trisodium salt (EDDSNa3) in chelate-enhanced phytoremedation and to evaluate the leaching of heavy metals. The average heavy metal concentrations in the soil solution (without the addition of EDDS) were high (e.g., 0.15 mg Pb L(-1) at a 50-cm depth) compared to the upper limit for protection of groundwater in China, but varied during the sampling period. Cu concentrations were not correlated with pH or dissolved organic carbon (DOC), but Zn and Cd concentrations were related to soil solution pH. EDDS enhanced metal solubility in the soil, but plant metal uptake by Elsholtzia splendens Nakai did not increase accordingly. There may be an increasing risk of groundwater pollution by Cu and the EDDS enhanced phytoremediation technique needs to be carefully applied to minimize this side effect.     

Phytochelatins as biomarkers for heavy metal stress in maize (Zea mays L.) and wheat (Triticum aestivum L.): combined effects of copper and cadmium

Heavy metal contaminated soils often show increased levels of more than one metal, e.g. copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb) or nickel (Ni). In case such soils are used for crop production, prediction of yield reduction or quality decline due to heavy metals in the soil is inadequate when based only on chemical soil analysis. The use of biomarkers such as phytochelatins (PC), non-protein thiols specifically induced in plants upon exposure to heavy metals, may be an additional tool or diagnostic criterion in heavy metal research and in practice. In the present work, Cu and Cd uptake and induction of PC synthesis are studied with hydroponically grown maize and wheat plants exposed to mixtures of the two metals. We observed a close positive relationship between the concentrations of Cd and PC in the plant shoot material. A decreased shoot concentration of Cd after addition of Cu, due to metal competition at common root absorption sites, coincided with lower shoot PC levels. Also differences in metal uptake and xylary metal transport among the two plant species were reflected in corresponding differences in PC concentration. The observed direct relationship between shoot PC concentration and the degree of metal-induced growth inhibition makes the use of PC promising for the purpose tested for.     

Enhanced phytoextraction: I. Effect of EDTA and citric acid on heavy metal mobility in a calcareous soil

Phytoextraction, the use of plants to extract heavy metals from contaminated soils, could be an interesting alternative to conventional remediation technologies. However, calcareous soils with relatively high total metal contents are difficult to phytoremediate due to low soluble metal concentrations. Soil amendments such as ethylene diaminetetraacetate (EDTA) have been suggested to increase heavy metal bioavailability and uptake in aboveground plant parts. Strong persistence of EDTA and risks of leaching of potentially toxic metals and essential nutrients have led to research on easily biodegradable soil amendments such as citric acid. In our research, EDTA is regarded as a scientific benchmark with which degradable alternatives are compared for enhanced phytoextraction purposes. The effects of increasing doses of EDTA (0.1,1,10 mmol kg(-1) dry soil) and citric acid (0.01, 0.05, 0.25, 0.442, 0.5 mol kg(-1) dry soil) on bioavailable fractions of Cu, Zn, Cd, and Pb were assessed in one part of our study and results are presented in this article. The evolution of labile soil fractions of heavy metals over time was evaluated using water paste saturation extraction (approximately soluble fraction), extraction with 1 M NH4OAc at pH 7 (approximately exchangeable fraction), and extraction with 0.5 M NH4OAc + 05 M HOAc + 0.02 M EDTA at pH 4.65 (approximately potentially bioavailable fraction). Both citric acid and EDTA produced a rapid initial increase in labile heavy metal fractions. Metal mobilization remained constant in time for soils treated with EDTA, but a strong exponential decrease of labile metal fractions was noted for soils treated with citric acid. The half life of heavy metal mobilization by citric acid varied between 1.5 and 5.7 d. In the following article, the effect of heavy metal mobilization on uptake by Helianthus annuus will be presented.     

铜污染旱地红壤的络合诱导植物修复作用

研究EDTA和低分子量有机酸对污染旱地红壤Cu形态变化及金属忍耐－富集型植物印度芥菜生长和Cu吸收的影响,以探讨Cu污染土壤的有机络合诱导植物吸取修复作用,结果表明,在芥菜营养生长旺盛期施用EDTA可显著提高土壤Cu的活性,土壤水浸提态Cu和交换态Cu均显著上升;这种活化效应随外加Cu浓度的上升和EDTA用量的增加而增大;柠檬酸和酸对土壤Cu的活化效应小,只对水浸提态Cu略有影响;EDTA可显著增加芥菜茎叶、根的Cu浓度和吸收量,等摩尔量柠檬酸和苹果酸对土壤铜形态变化和芥菜Cu吸收没有显著作用。     

Changes in trace metal species and other components of the rhizosphere during growth of radish

Changes in the properties of soil solution in the rhizosphere of developing radish plants were investigated. Variations in these properties were expected to affect the distribution and speciation of metals in the soil and soil solution. Applications of essential nutrients were linked to plant transpiration rates and prevented excess addition of nutrient ions, so that subtle changes in soil solution composition would not be obscured. Soil solution pH, the concentration of dissolved organic carbon (DOC) and the concentrations of major and trace elements in solution were found to vary over time. Strict control of fertilizer additions led to the maintenance of a relatively low ionic strength in the soil solution, and under such conditions trace metal solubility appeared to be highy influenced by the concentration of DOC. A chemical speciation analysis was performed which showed that, while dissolved Cd and Zn were largely uncomplexed in unplanted soil, Cd and Zn in the rhizosphere existed mainly as complexed forms. It is hypothesized that this is partly a result of Ca-metal-ligand equilibrium in solution, with higher Ca concentrations in unplanted soil leading to more of the Cd and Zn in solution existing in the uncomplexed state. Changes in the concentrations of uncomplexed Cd and Zn with time gave the best correlations with changes in plant uptake of these metals over time, supporting the hypothesis that plants mainly absorb the free metal ion from soil solution.     

A Laboratory Assessment of Potentials and Limitations of Using EDTA,Rhamnolipids, and Compost-derived Humic Substances (HS) in Enhanced Phytoextraction of Copper and Zinc Polluted Calcareous Soils

Phytoextraction is an economically and environmentally attractive in-situ method for cleaning heavy metal polluted soil. Phytoextraction is a rather slow process, but it can be enhanced by the application of chelating agents such as the synthetic ethylenediaminetetraacetic acid (EDTA). However, EDTA is persistent, toxic, and can promote heavy metal leaching. Replacement of EDTA by natural, non-toxic compounds such as humic substances (HS) or rhamnolipids (bacterial-produced biosurfactants) might be environmentally attractive but before recommending such alternatives, their suitability must be assessed. Therefore, compost-derived HS and rhamnolipids were compared with EDTA as natural non-toxic alternatives in a multi-step batch extraction test. The test included 10 steps carried out on two Cu and Zn polluted calcareous soils using a solution:soil ratio of 10 (L/kg). In each step, soil was extracted with an extractant containing EDTA, HS, or rhamnolipids corresponding to 250 mmol DOC/kg of soil (3 g C/kg). By HS extraction, each step resulted in the release of ～0.29 mg Cu/L and ～0.19 mg Zn/L, which is considered to enhance plant uptake without leading to unacceptable leaching and toxification of the plants (and the environment), suggesting HS can enhance phytoextraction. In contrast, the EDTA and the rhamnolipid treatments were found to be unsuitable because the EDTA released Cu and Zn in concentrations that may be toxic to plants and can lead to leaching, whereas the rhamnolipids showed insufficient capacity to mobilize Cu (and Zn). However, future investigations in the field are needed to confirm these laboratory results.     

A comparative study on the aluminium- and copper-induced organic acid exudation from wheat roots

It is well established that aluminium (Al) and some heavy metals can elicit organic acid exudation from a range of species. In the present research we found that copper (Cu) can also induce organic acid exudation from the roots of wheat, rye, triticale, maize and soybean. Using intact wheat plants, we made a comparative study of Al- and Cu- induced organic acid exudation. In 5-day-old wheat seedlings, severe Cu stress (40 µ M CuCl₂) mainly induced the exudation of malate and citrate, and Al-tolerant genotypes could release significantly greater amounts of malate than Al-sensitive genotypes. The time course of the exudation of malate and citrate from the roots of 5-day-old seedlings of wheat (cv. Atlas) in 200 µ M AlCl₃ was similar to that in 40 µ M CuCl₂. In older wheat plants (15-day-old), moderate Cu stress (12 µ M CuCl₂) induced the exudation of large amounts of citrate and addition of Al or La sharply reduced Cu-induced citrate exudation, while Cu or La did not affect Al-induced malate efflux. When half of the root system of Atlas wheat was immersed in Al- or Cu-containing solution and the remaining half in Al- or Cu-free solution, organic acids were only exuded into the solution containing Al or Cu. This suggests that no long distance signal transport is involved in organic acid exudation induced by Al or Cu, and that direct contact of Al or Cu with plant roots is a prerequisite for the induction of organic acid exudation. The anion-channel inhibitor niflumic acid (NIF) significantly stimulated the exudation of both citrate and malate from 5-day-old wheat seedlings under severe Al or Cu stress. Our results suggest that Cu-induced organic acid efflux may be a common response, which may play a role in alleviating Cu toxicity in plants.     

EDTA-Enhanced Phytoremediation of Heavy Metals: A Review

The increase in heavy metal terrestrial ecosystems’ contamination through anthropogenic activities is a widespread and serious global problem due to their various environmental and human implications. For these reasons, several techniques, including phytoremediation of heavy metals, have been extensively studied. In spite of significant recent advancement, ethylene diamine tetraacetic acid (EDTA)-enhanced heavy metal phytoextraction as well as related ecological risks are still topical and remain an important area of research. In fact, EDTA favors the solubilization of metals and metalloids in soils, and was therefore extensively studied during the last two decades in order to improve phytoextraction efficiency and reduce treatment duration. This review highlights the recent findings (2010–2012) and mechanisms behind EDTA-enhanced (1) solubilization of heavy metals in soil, (2) mobilization/transport of soluble metals towards plant root zone, and (3) metal absorption by plant roots and translocation towards aerial parts. The review also presents potential risks associated with EDTA-enhanced phytoextraction: (1) environmental persistence of EDTA and/or metal-EDTA complex; (2) potential toxicity of EDTA and/or metal-EDTA complex to plants; and (3) leaching and contamination of groundwater. Moreover, field-scale cost of EDTA-enhanced remediation and the role of EDTA in time required for heavy metal remediation is discussed.     

Fine Root Decomposition and Cycling of Cu,Ni, Pb,and Zn at Forest Sites Near Smelters in Sudbury,ON, and Rouyn-Noranda,QU, Canada

This study determined rates of in situ fine root decomposition and changes in trace metals concentration during decomposition at sites in Sudbury, ON, and Rouyn-Noranda, QU, with elevated or background concentrations of Cu, Ni, Pb, and/or Zn in the soil, and correlated the depth gradients of Cu, Ni, Pb, and Zn for soils and roots at the same sites. Fine roots were extracted from soil cores within root traps several times over 12 months; biomass and metal concentrations were measured. Live roots were collected from 30-cm soil cores, separated into three depths. Elevated soil metal concentrations did not necessarily reduce fine root decomposition, and effects on decomposition were similar to those previously reported for surface foliar litter at the same sites. Decomposing roots at only the high metal sites demonstrated increased metal concentrations with time. Root tissue concentrations of Cu, Ni, and Zn, but not Pb, at lower soil depths were generally higher than expected from soil metal concentrations. This could be explained by reallocation of essential metals, although these metals were likely also more available for uptake at depth due to lower DOC concentrations. This study means that for risk assessment, separate determinations of altered decomposition for roots and leaf litter are likely not necessary for predicting ecosystem effects, a pragmatically useful conclusion given the labor intensity of the fine root studies. This study also suggests that for risk assessment of plant community exposure to metals, prediction of exposure to metals should probably consider soil layers that do not have substantially elevated metal concentrations, as their soil characteristics, or plant processes, may result in unexpected exposure.     

Enhanced Phytoremediation Capacity of Chenopodium album L. Grown on Pb-Contaminated Soils Using EDTA and Reduction of Leaching Risk

Leaching of metals due to enhanced mobility during ethylenediaminetetraacetic acid (EDTA)-assisted phytoextraction has been demonstrated as one of the potential hazards associated with this technology. This study was conducted to determine phytoextraction efficiency of Chenopodium album L. for Pb and EDTA-assisted (1.5, 3, and 9 mmol kg^?1) phytoextraction and potential for leaching of Pb. The results demonstrated that BCF_shoot (bioconcentration factor) was relatively higher than the BCF_root. Translocation factor in the shoot was higher than the roots. Thus, plant species would be applicable for Pb phytoextraction. EDTA enhanced translocation of Pb from roots to shoots. Lead content in the plant parts was maximum in the shoot and root of 9EDTA and 3EDTA, respectively. However, there was no significant difference between 3EDTA and 9EDTA. Lead concentration in the plant parts increased significantly from vegetative stage into flowering stage. Lead content taken up by the plant was lowest when EDTA was applied in a single dose. Therefore, application of EDTA in several increments rather than a single split reduced the leaching risk. Totally, optimum phytoextraction was observed when 3 mmol kg^?1 EDTA was added in triple dosage 60 days after the plant cultivation under triple application mode. The results indicated the plant has the potential for Pb phytoextraction, but it should not be used unless the biomass containing such accumulated metal is removed for disposal. Significant improvement over current ETDA-assisted phytoextraction of Pb may be possible but should be implemented cautiously because of environmental risk.     

Effects of EDTA and EDDS on Heavy Metal Activation and Accumulation of Metals by Soybean in Alkaline Soil

The application of chelating agents for phytoextraction has demonstrated that it is an efficient method to activate heavy metals in polluted soil. We conducted pot experiments using soybean, which has been considered an indicator plant, to study the effects of EDTA and EDDS on heavy metals’ activation, and on the soybean. The study results indicated that EDDS decreased the chlorophyll content of the leaves and increased the malondialdehyde (MDA) content of the soybean. EDTA also decreased the chlorophyll content of the leaves. EDDS had a strong influence on activating Cu (2583-8900-fold) and Zn. The addition of 5 mmol kg^?1 of EDDS markedly increased the uptake of metals. Compared with the control, EDDS increased the Cu uptake (100-205-fold). EDTA greatly increased the activation of heavy metals; it also increased Cu uptake in a concentration-dependent manner. EDTA also increased the biological concentration factor (BCF) and the transfer factor (TF) in a concentration-dependent manner. The BCF and the TF reached maximum levels when 5 mmol kg^?1 EDDS was applied to the pots.     

Protonophore- and pH-insensitive glucose and sucrose accumulation detected by FRET nanosensors in Arabidopsis root tips

Although soil contains only traces of soluble carbohydrates, plant roots take up glucose and sucrose efficiently when supplied in artificial media. Soluble carbohydrates and other small metabolites found in soil are in part products from exudation from plant roots. The molecular nature of the transporters for uptake and exudation is unknown. Here, fluorescence resonance energy transfer (FRET) glucose and sucrose sensors were used to characterize accumulation and elimination of glucose and sucrose in Arabidopsis roots tips. Using an improved image acquisition set-up, FRET responses to perfusion with carbohydrates were detectable in roots within less than 10 sec and over a wide concentration range. Accumulation was fully reversible within 10-180 sec after glucose or sucrose had been withdrawn; elimination may be caused by metabolism and/or efflux. The rate of elimination was unaffected by pre-incubation with high concentrations of glucose, suggesting that elimination is not due to accumulation in a short-term buffer such as the vacuole. Glucose and sucrose accumulation was insensitive to protonophores, was comparable in media differing in potassium levels, and was similar at pH 5.8, 6.8 and 7.8, suggesting that both influx and efflux may be mediated by proton-independent transport systems. High-resolution expression mapping in root tips showed that only a few proton-dependent transport of the STP (Sugar Transport Protein) and SUT/SUC (Sucrose Transporter/Carrier) families are expressed in the external cell layers of root tips. The root expression maps may help to pinpoint candidate genes for uptake and release of carbohydrates from roots.