Miaomiao Cheng

Silver sulfide nanoparticles (Ag2S-NPs) are taken up by plants and are phytotoxic

Abstract Silver nanoparticles (NPs) are used in more consumer products than any other nanomaterial and their release into the environment is unavoidable. Of primary concern is the wastewater stream in which most silver NPs are transformed to silver sulfide NPs (Ag2S-NPs) before being applied to agricultural soils within biosolids. While Ag2S-NPs are assumed to be biologically inert, nothing is known of their effects on terrestrial plants. The phytotoxicity of Ag and its accumulation was examined in short-term (24 h) and longer-term (2-week) solution culture experiments with cowpea (Vigna unguiculata L. Walp.) and wheat (Triticum aestivum L.) exposed to Ag2S-NPs (0–20 mg Ag L−1), metallic Ag-NPs (0–1.6 mg Ag L−1), or ionic Ag (AgNO3; 0–0.086 mg Ag L−1). Although not inducing any effects during 24-h exposure, Ag2S-NPs reduced growth by up to 52% over a 2-week period. This toxicity did not result from their dissolution and release of toxic Ag+ in the rooting medium, with soluble Ag concentrations remaining below 0.001 mg Ag L−1. Rather, Ag accumulated as Ag2S in the root and shoot tissues when plants were exposed to Ag2S-NPs, consistent with their direct uptake. Importantly, this differed from the form of Ag present in tissues of plants exposed to AgNO3. For the first time, our findings have shown that Ag2S-NPs exert toxic effects through their direct accumulation in terrestrial plant tissues. These findings need to be considered to ensure high yield of food crops, and to avoid increasing Ag in the food chain.

Synchrotron-based techniques shed light on mechanisms of plant sensitivity and tolerance to high manganese in the root environment

Mn toxicity results from apoplastic accumulation in soybean leaves, while tolerance occurs through vacuolar Mn(II) sequestration in two lupin species and oxidation to Mn(III) in trichomes of sunflower. Plant species differ in response to high available manganese (Mn), but the mechanisms of sensitivity and tolerance are poorly understood. In solution culture, greater than or equal to 30 µm Mn decreased the growth of soybean (Glycine max), but white lupin (Lupinus albus), narrow-leafed lupin (Lupin angustifolius), and sunflower (Helianthus annuus) grew well at 100 µm Mn. Differences in species’ tolerance to high Mn could not be explained simply by differences in root, stem, or leaf Mn status, being 8.6, 17.1, 6.8, and 9.5 mmol kg–1 leaf fresh mass at 100 µm Mn. Furthermore, x-ray absorption near edge structure analyses identified the predominance of Mn(II), bound mostly to malate or citrate, in roots and stems of all four species. Rather, differences in tolerance were due to variations in Mn distribution and speciation within leaves. In Mn-sensitive soybean, in situ analysis of fresh leaves using x-ray fluorescence microscopy combined with x-ray absorption near edge structure showed high Mn in the veins, and manganite [Mn(III)] accumulated in necrotic lesions apparently through low Mn sequestration in vacuoles or other vesicles. In the two lupin species, most Mn accumulated in vacuoles as either soluble Mn(II) malate or citrate. In sunflower, Mn was sequestered as manganite at the base of nonglandular trichomes. Hence, tolerance to high Mn was ascribed to effective sinks for Mn in leaves, as Mn(II) within vacuoles or through oxidation of Mn(II) to Mn(III) in trichomes. These two mechanisms prevented Mn accumulation in the cytoplasm and apoplast, thereby ensuring tolerance to high Mn in the root environment.

Cadmium accumulation is enhanced by ammonium compared to nitrate in two hyperaccumulators, without affecting speciation

Highlight Supply of NH4 + compared with NO3 − increased plant Cd uptake, translocation and accumulation in Carpobrotus rossii and Solanum nigrum. Cd-S was the dominant Cd species, with Cd being transported as free ions in xylem.

Contrasting effects of organic amendments on phytoextraction of heavy metals in a contaminated sediment

Background and aimsSoil amendments are often added to polluted soils to increase phytoremediation efficiency. Here we investigated the potential of a range of organic amendments for phytoextraction of heavy metals in a contaminated sediment.MethodsTwo experiments compared adsorption and phytoextraction of heavy metals by a Cd-hyperaccumulator Carpobrotus rossii grown in the contaminated sediment amended with six organic amendments.ResultsThe adsorption capacity as measured by Langmuir adsorption maximum followed the order of Cr > Zn > Cu > Cd, and the effect of organic amendments followed the order of chicken manure > cow manure > brown coal > golden wattle biochar > blue gum biochar > radiata pine biochar. The addition of amendments increased the adsorption of heavy metals, with brown coal resulting in the lowest concentrations of water-extractable Cd, Cu and Zn. Two manures resulted in the highest concentrations of these water-extractable heavy metals in the rhizosphere soil of C. rossii. Furthermore, brown coal resulted in higher shoot accumulation of these heavy metals than three wood-derived biochars, whilst the manures generally had the lowest accumulation of Cd and Cu although they increased shoot biomass.ConclusionsThe addition of brown coal decreased whereas manure addition increased the mobility (water-extractable fraction) of heavy metals in rhizosphere soil. Phytoextraction of Cd and Cu was greater with brown coal than with biochars or manures. Brown coal is suitable for enhancing phytoextraction of these heavy metals because it could increase their accumulation in shoots of C. rossii and decrease the risk of leaching of these heavy metals into groundwater.

Absorption of foliar applied Zn fertilizers by trichomes in soybean and tomato

Leaf trichomes are not part of the primary pathway through which foliar-applied Zn fertilizer moves across the leaf surface in soybean and tomato.

Sodium chloride decreases cadmium accumulation and changes the response of metabolites to cadmium stress in the halophyte Carpobrotus rossii

Background and Aims Salinity affects the bioavailability of cadmium (Cd) in soils and Cd accumulation in plants, but the associated mechanisms remain unclear. This study aimed to assess the metabolic response to NaCl and Cd and the relationship between metabolites and Cd accumulation in the halophyte Carpobrotus rossii, which has potential for Cd phytoextraction. Methods Plants were grown in nutrient solution with 0-400 mm NaCl in the presence of 5 or 15 µm Cd, with varied or constant solution Cd2+ activity. Plant growth and Cd uptake were measured, and the accumulation of peptides, and organic and amino acids in plant tissues were assessed. Key Results The addition of NaCl to Cd-containing solutions improved plant growth along with 70-87 % less shoot Cd accumulation, resulting from decreases in Cd root uptake and root-to-shoot translocation irrespective of Cd2+ activity in solutions. Moreover, Cd exposure increased the concentration of phytochelatins, which correlated positively with Cd concentrations in plants regardless of NaCl addition. In comparison, Cd inhibited the synthesis of organic acids in shoots and roots in the absence of NaCl, but increased it in shoots in the presence of NaCl. While Cd increased the concentrations of amino acids in plant shoots, the effect of NaCl on the synthesis of amino acids was inconsistent. Conclusions Our data provide the first evidence that NaCl decreased Cd shoot accumulation in C. rossii by decreasing Cd root uptake and root-to-shoot translocation even under constant Cd2+ activity. The present study also supports the important role of peptides and organic acids, particular of phytochelatins, in Cd tolerance and accumulation although the changes of those metabolites was not the main reason for the decreased Cd accumulation.

Salinity decreases Cd translocation by altering Cd speciation in the halophytic Cd-accumulator Carpobrotus rossii

Background and Aims Salt has been shown to affect Cd translocation and accumulation in plants but the associated mechanisms are unclear. This study examined the effects of salt type and concentration on Cd uptake, translocation and accumulation in Carpobrotus rossii. Methods Plants were grown in nutrient solution with the same Cd concentration or Cd2+ activity in the presence of 25 mm NaNO3, 12.5 mm Na2SO4 or 25 mm NaCl for ≤10 d. Plant growth and Cd uptake were measured and the accumulation of peptides and organic acids, and Cd speciation in plant tissues were analysed. Key Results Salt addition decreased shoot Cd accumulation by >50 % due to decreased root-to-shoot translocation, irrespective of salt type. Synchrotron-based X-ray absorption spectroscopy revealed that, after 10 d, 61-94 % Cd was bound to S-containing ligands (Cd-S) in both roots and shoots, but its speciation was not affected by salt. In contrast, Cd in the xylem sap was present either as free Cd2+ or complexes with carboxyl groups (Cd-OH). When plants were exposed to Cd for ≤24 h, 70 % of the Cd in the roots was present as Cd-OH rather than Cd-S. However, NaCl addition decreased the proportion of Cd-OH in the roots within 24 h by forming Cd-Cl complexes and increasing the proportion of Cd-S. This increase in Cd-S complexes by salt was not due to changes in glutathione and phytochelatin synthesis. Conclusions Salt addition decreased shoot Cd accumulation by decreasing Cd root-to-shoot translocation due to the rapid formation of Cd-S complexes (low mobility) within the root, without changing the concentrations of glutathione and phytochelatins.

Time-resolved analyses of elemental distribution and concentration in living plants: An example using manganese toxicity in cowpea leaves

Knowledge of elemental distribution and concentration within plant tissues is crucial in the understanding of almost every process that occurs within plants. However, analytical limitations have hindered the microscopic determination of changes over time in the location and concentration of nutrients and contaminants in living plant tissues. We developed a novel method using synchrotron-based micro X-ray fluorescence (μ-XRF) that allows for laterally-resolved, multi-element, kinetic analyses of plant leaf tissues in vivo. To test the utility of this approach, we examined changes in the accumulation of Mn in unifoliate leaves of 7-d-old cowpea (Vigna unguiculata) plants grown for 48 h at 0.2 and 30 μM Mn in solution. Repeated μ-XRF scanning did not damage leaf tissues demonstrating the validity of the method. Exposure to 30 μM Mn for 48 h increased the initial number of small spots of localized high Mn and their concentration rose from 40 to 670 mg Mn kg-1 fresh mass. Extension of the two-dimensional μ-XRF scans to a three-dimensional geometry provided further assessment of Mn localization and concentration. This method shows the value of synchrotron-based μ-XRF analyses for time-resolved in vivo analysis of elemental dynamics in plant sciences.