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Iron (Fe) plays an important role within environmental systems. Synchrotron-based X-ray approaches, including X-ray absorption spectroscopy (XAS), provide powerful tools for in situ analyses of Fe speciation, but beam damage during analysis may alter Fe speciation during its measurement. XAS was used to examine whether experimental conditions affect the analysis of Fe speciation in plant tissues. Even when analyzed in a cryostat at 12 K, it was found that Fe III can rapidly (within 0.5-1 min) photoreduce to Fe II , although the magnitude of photoreduction varied depending upon the hydration of the sample, the coordination chemistry of the Fe, as well as other properties. For example, photoreduction of Fe III was considerably higher for aqueous standard compounds than for hydrated plant-root tissues. The use of freeze-dried samples in the cryostat (12 K) markedly reduced the magnitude of this Fe III photoreduction, and there was no evidence that the freeze-drying process itself resulted in experimental artefacts under the current experimental conditions, such as through the oxidation of Fe II , although some comparatively small differences were observed when comparing spectra of hydrated and freeze-dried Fe II compounds. The results of this study have demonstrated that Fe III photoreduction can occur during X-ray analysis, and provides suitable conditions to preserve Fe speciation to minimize the extent of beam damage when analyzing environmental samples. All studies utilizing XAS are encouraged to include a preliminary experiment to determine if beam damage is occurring, and, where appropriate, to take the necessary steps (such as freeze drying) to overcome these issues.

Summary: Soil acidity and waterlogging increase manganese (Mn) in leaf tissues to potentially toxic concentrations, an effect reportedly alleviated by increased silicon (Si) and phosphorus (P) supply. Effects of Si and P on Mn toxicity were studied in four plant species using synchrotron‐based micro X‐ray fluorescence (μ‐XRF) and nanoscale secondary ion mass spectrometry (NanoSIMS) to determine Mn distribution in leaf tissues and using synchrotron‐based X‐ray absorption spectroscopy (XAS) to measure Mn speciation in leaves, stems and roots. A concentration of 30 μM Mn in solution was toxic to cowpea and soybean, with 400 μM Mn toxic to sunflower but not white lupin. Unexpectedly, μ‐XRF analysis revealed that 1.4 mM Si in solution decreased Mn toxicity symptoms through increased Mn localization in leaf tissues. NanoSIMS showed Mn and Si co‐localized in the apoplast of soybean epidermal cells and basal cells of sunflower trichomes. Concomitantly, added Si decreased oxidation of Mn(II) to Mn(III) and Mn(IV). An increase from 5 to 50 μM P in solution changed some Mn toxicity symptoms but had little effect on Mn distribution or speciation. We conclude that Si increases localized apoplastic sorption of Mn in cowpea, soybean and sunflower leaves thereby decreasing free Mn2+ accumulation in the apoplast or cytoplasm. [ABSTRACT FROM AUTHOR]

Silicon is reported to reduce the toxic effects of Al on root elongation but the in planta mechanism by which this occurs remains unclear. Using seedlings of soybean (Glycine max) and sorghum (Sorghum bicolor), we examined the effect of up to 2 mM Si on root elongation rate (RER) in Al-toxic nutrient solutions. Synchrotron-based low energy X-ray fluorescence (LEXRF) was then used for the in situ examination of the distribution of Al and Si within cross-sections cut from the apical tissues of sorghum roots. The addition of Si potentially increased RER in Al-toxic solutions, with RER being up to ca. 0.3 mm h-1 (14%) higher for soybean and ca. 0.2 mm h-1 (17%) higher for sorghum relative to solutions without added Si. This improvement in RER could not be attributed to a change in Al-chemistry of the bulk nutrient solution, nor was it due to a change in the concentration of Al within the apical (0–10 mm) root tissues. Using LEXRF to examine sorghum, it was demonstrated that in roots exposed to both Al and Si, much of the Al was co-located with Si in the mucigel and outer apoplast. These observations suggest that Si reduces the toxicity of Al in planta through formation of Al–Si complexes in mucigel and outer cellular tissues, thereby decreasing the binding of Al to the cell wall where it is known to inhibit wall loosening as required for cell elongation. [ABSTRACT FROM AUTHOR]

Highlights • A novel method using synchrotron-based micro X-ray fluorescence. • Provides laterally-resolved, multi-element, kinetic, in vivo analyses of plants. • Allows a wide range of studies for the examination of plant responses. Abstract The distribution and concentration of nutrients and contaminants affect almost every metabolic process in plants but analytical limitations have hindered the determination of microscopic changes over time within living plant tissues. We developed a novel method using synchrotron-based micro X-ray fluorescence (μ-XRF) that, for the first time, allows quantification of the spatial and temporal changes of multiple elements in the same area of living leaves. The utility of this approach was tested by examining changes over 48 h in unifoliate leaves of 7-d-old cowpea (Vigna unguiculata) plants simultaneously at 0.2 and 30 μM Mn in nutrient solution, with 30 μM Mn known to be toxic to cowpea and cause the formation of Mn-dense lesions. The fast X-ray fluorescence detector system reduced dwell on living leaf samples. This produced no evidence of tissue damage through repeated μ-XRF scanning, thereby overcoming previously noted experimental artifacts. This permitted, for the first time, visual and quantitative assessments of spatial and temporal changes in nutrient concentrations. By focusing on changes in Mn status, this study illustrated extension of two-dimensional μ-XRF scans to a three-dimensional geometry of Mn kinetics in the same area of leaves. The multi-element potential of this method was exemplified through the measurement of distributions and concentrations of K, Ca, Fe, Cu, and Zn within living plant leaves. This novel method and accompanying information on changes in Mn distribution showed the potential for microscopic, time-resolved, in vivo examination of changes in elemental distribution. We consider that this method will be of benefit for a wide range of studies, including functional characterization of molecular biology, examining changes in the distribution of nutrients, and understanding the movement and toxicity of contaminants. [ABSTRACT FROM AUTHOR]

The increasing use of zinc oxide nanoparticles (ZnO-NPs) in various commercial products is prompting detailed investigation regarding the fate of these materials in the environment. There is, however, a lack of information comparing the transformation of ZnO-NPs with soluble Zn2+ in both soils and plants. Synchrotron-based techniques were used to examine the uptake and transformation of Zn in various tissues of cowpea (Vigna unguiculata (L.) Walp.) exposed to ZnO-NPs or ZnCL2 following growth in either solution or soil culture. In solution culture, soluble Zn (ZnCL2) was more toxic than the ZnO-NPs, although there was substantial accumulation of ZnO-NPs on the root surface. When grown in soil, however, there was no significant difference in plant growth and accumulation or speciation of Zn between soluble Zn and ZnO-NP treatments, indicating that the added ZnO-NPs underwent rapid dissolution following their entry into the soil. This was conhrmed by an incubation experiment with two soils, in which ZnO-NPs could not be detected after incubation for 1 h. The speciation of Zn was similar in shoot tissues for both soluble Zn and ZnO-NPs treatments and no upward translocation of ZnO-NPs from roots to shoots was observed in either solution or soil culture. Under the current experimental conditions, the similarity in uptake and toxicity ot Zn from ZnO-NPs and soluble Zn in soils indicates that the ZnO-NPs used in this study did not constitute nanospecific risks. [ABSTRACT FROM AUTHOR]