Localization of aluminium in tea (Camellia sinensis) leaves using low energy X-ray fluorescence spectro-microscopy

Kritzler U, Burslem DFRP. External aluminium supply regulates photosynthesis and carbon partitioning in the Al-accumulating tropical shrub Melastoma malabathricum

Aluminium (Al) is toxic to most plants, but recent research has suggested that Al addition may stimulate growth and nutrient uptake in some species capable of accumulating high tissue Al concentrations. The physiological basis of this growth response is unknown, but it may be associated with processes linked to the regulation of carbon assimilation and partitioning by Al supply. To test alternative hypotheses for the physiological mechanism explaining this response, we examined the effects of increasing Al concentrations in the growth medium on tissue nutrient concentrations and carbon assimilation in two populations of the Al-accumulator Melastoma malabathricum. Compared to seedlings grown in a control nutrient solution containing no Al, mean rates of photosynthesis and respiration increased by 46% and 27%, respectively, total non-structural carbohydrate concentrations increased by 45%, and lignin concentration in roots decreased by 26% when seedlings were grown in a nutrient solution containing 2.0 mM Al. The concentrations of P, Ca and Mg in leaves and stems increased by 31%, 22%, and 26%, respectively, in response to an increase in nutrient solution Al concentration from 0 to 2.0 mM. Elemental concentrations in roots increased for P (114%), Mg (61%) and K (5%) in response to this increase in Al concentration in the nutrient solution. Plants derived from an inherently faster-growing population had a greater relative increase in final dry mass, net photosynthetic and respiration rates and total non-structural carbohydrate concentrations in response to higher external Al supply. We conclude that growth stimulation by Al supply is associated with increases in photosynthetic and respiration rates and enhanced production of non-structural carbohydrates that are differentially allocated to roots, as well as stimulation of nutrient uptake. These responses suggest that internal carbon assimilation is up-regulated to provide the necessary resources of non-structural carbohydrates for uptake, transport and storage of Al in Melastoma malabathricum. This physiological mechanism has only been recorded previously in one other plant species, Camellia sinensis, which last shared a common ancestor with M. malabathricum more than 120 million years ago.

Subcellular compartmentalization of aluminum reduced its hazardous impact on rye photosynthesis

Aluminum (Al) toxicity limits crops growth and production in acidic soils. Compared to roots, less is known about the toxic effects of Al in leaves. Al subcellular compartmentalization is also largely unknown. Using rye (Secale cereale L.) Beira (more tolerant) and RioDeva (more sensitive to Al) genotypes, we evaluated the patterns of Al accumulation in leaf cell organelles and the photosynthetic and metabolic changes to cope with Al toxicity. The tolerant genotype accumulated less Al in all organelles, except the vacuoles. This suggests that Al compartmentalization plays a role in Al tolerance of Beira genotype. PSII efficiency, stomatal conductance, pigment biosynthesis, and photosynthesis metabolism were less affected in the tolerant genotype. In the Calvin cycle, carboxylation was compromised by Al exposure in the tolerant genotype. Other Calvin cycle-related enzymes, phoshoglycerate kinase (PGK), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), triose-phosphate isomerase (TPI), and fructose 1,6-bisphosphatase (FBPase) activities decreased in the sensitive line after 48 h of Al exposure. Consequentially, carbohydrate and organic acid metabolism were affected in a genotype-specific manner, where sugar levels increased only in the tolerant genotype. In conclusion, Al transport to the leaf and compartmentalization in the vacuoles tolerant genotype's leaf cells provide complementary mechanisms of Al tolerance, protecting the photosynthetic apparatus and thereby sustaining growth.

Multiscale imaging on Saxifraga paniculata provides new insights into yttrium uptake by plants

Yttrium (Y) has gained importance in high tech applications and, together with the other rare earth elements (REEs), is also considered to be an emerging environmental pollutant. The alpine plant Saxifraga paniculata was previously shown to display high metal tolerance and an intriguing REE accumulation potential. In this study, we analysed soil grown commercial and wild specimens of Saxifraga paniculata to assess Y accumulation and shed light on the uptake pathway. Laser ablation inductively coupled plasma mass spectrometry and synchrotron-based micro X-ray fluorescence spectroscopy was used to localise Y within the plant tissues and identify colocalized elements. Y was distributed similarly in commercial and wild specimens. Within the roots, Y was mostly located in the epidermis region. Translocation was low, but wild individuals accumulated significantly more Y than commercial ones. In plants of both origins, we observed consistent colocalization of Al, Fe, Y and Ce in all plant parts except for the hydathodes. This indicates a shared pathway during translocation and could explained by the formation of a stable organic complex with citrate, for example. Our study provides important insights into the uptake pathway of Y in S. paniculata, which can be generalised to other plants.

Imaging of Potassium and Calcium Distribution in Plant Tissues and Cells to Monitor Stress Response and Programmed Cell Death

In plants, the response to stress, such as salinity, pathogen attack, drought, high concentration of metals, hyperthermia, and hypothermia, is usually accompanied by potassium ion (K⁺) leakage from the cytosol to the cell wall, mediated by plasma membrane cation conductivity. Stress-induced electrolyte leakage co-occurs with accumulation of reactive oxygen species (ROS) and calcium ions (Ca²⁺) and often results in programmed cell death (PCD). The development of X-ray and mass spectrometry (MS) based imaging techniques has enabled insight into the spatial tissue and cell-specific redistribution of major and trace elements during the stress response. In this chapter a workflow for sample preparation, imaging, and image analysis by X-ray and MS based techniques is presented.

Soft X-ray Microscopy Techniques for Medical and Biological Imaging at TwinMic—Elettra

Progress in nanotechnology calls for material probing techniques of high sensitivity and resolution. Such techniques are also used for high-impact studies of nanoscale materials in medicine and biology. Soft X-ray microscopy has been successfully used for investigating complex biological processes occurring at micrometric and sub-micrometric length scales and is one of the most powerful tools in medicine and the life sciences. Here, we present the capabilities of the TwinMic soft X-ray microscopy end-station at the Elettra synchrotron in the context of medical and biological imaging, while we also describe novel uses and developments.

Transcriptomic responses to aluminum stress in tea plant leaves

Tea plant (Camellia sinensis) is a well-known Al-accumulating plant, showing a high level of aluminum (Al) tolerance. However, the molecular mechanisms of Al tolerance and accumulation are poorly understood. We carried out transcriptome analysis of tea plant leaves in response to three different Al levels (0, 1, 4 mM, for 7 days). In total, 794, 829 and 585 differentially expressed genes (DEGs) were obtained in 4 mM Al vs. 1 mM Al, 0 Al vs. 1 mM Al, and 4 mM Al vs. 0 Al comparisons, respectively. Analysis of genes related to polysaccharide and cell wall metabolism, detoxification of reactive oxygen species (ROS), cellular transport, and signal transduction were involved in the Al stress response. Furthermore, the transcription factors such as zinc finger, myeloblastosis (MYB), and WRKY played a critical role in transcriptional regulation of genes associated with Al resistance in tea plant. In addition, the genes involved in phenolics biosynthesis and decomposition were overwhelmingly upregulated in the leaves treated with either 0 Al and 4 mM Al stress, indicating they may play an important role in Al tolerance. These results will further help us to understand mechanisms of Al stress and tolerance in tea plants regulated at the transcriptional level.

Fluoride absorption, transportation and tolerance mechanism in Camellia sinensis, and its bioavailability and health risk assessment: a systematic review

Tea is the one of the most popular non-alcoholic caffeinated beverages in the world. Tea is produced from the tea plant (Camellia sinensis (L.) O. Kuntze), which is known to accumulate fluoride. This article systematically analyzes the literature concerning fluoride absorption, transportation and fluoride tolerance mechanisms in tea plants. Fluoride bioavailability and exposure levels in tea infusions are also reviewed. The circulation of fluoride within the tea plantation ecosystems is in a positive equilibrium, with greater amounts of fluoride introduced to tea orchards than removed. Water extractable fluoride and magnesium chloride (MgCl₂ ) extractable fluoride in plantation soil are the main sources of absorption by tea plant root via active trans-membrane transport and anion channels. Most fluoride is readily transported through the xylem as F^- /F-Al complexes to leaf cell walls and vacuole. The findings indicate that tea plants employ cell wall accumulation, vacuole compartmentalization, and F-Al complexes to co-detoxify fluoride and aluminum, a possible tolerance mechanism through which tea tolerates higher levels of fluoride than most plants. Furthermore, dietary and endogenous factors influence fluoride bioavailability and should be considered when exposure levels of fluoride in commercially available dried tea leaves are interpreted. The relevant current challenges and future perspectives are also discussed. © 2020 Society of Chemical Industry.

Aluminium-silicon interactions in higher plants: an update

Aluminium (Al) and silicon (Si) are abundant in soils, but their availability for plant uptake is limited by low solubility. However, Al toxicity is a major problem in naturally occurring acid soils and in soils affected by acidic precipitation. When, in 1995, we reviewed this topic for the Journal of Experimental Botany, it was clear that under certain circumstances soluble Si could ameliorate the toxic effects of Al, an effect mirrored in organisms beyond the plant kingdom. In the 25 years since our review, it has become evident that the amelioration phenomenon occurs in the root apoplast, with the formation of hydroxyaluminosilicates being part of the mechanism. A much better knowledge of the molecular basis for Si and Al uptake by plants and of Al toxicity mechanisms has been developed. However, relating this work to amelioration by Si is at an early stage. It is now clear that co-deposition of Al and Si in phytoliths is a fairly common phenomenon in the plant kingdom, and this may be important in detoxification of Al. Relatively little work on Al-Si interactions in field situations has been done in the last 25 years, and this is a key area for future development.

Effects of exogenous citric acid on the concentration and spatial distribution of Ni, Zn, Co, Cr, Mn and Fe in leaves of Noccaea caerulescens grown on a serpentine soil

The aim of this study was to show the potential of citric acid in increasing the concentration of Ni, Zn, Co, Cr, Mn and Fe in leaves of the hyperaccumulator Noccaea caerulescens. Synchrotron x-ray fluorescence (μ-XRF) images were collected to assess the distribution of metals in leaves. Applying citric acid (20 mmol kg^-1) to soil increased in 14-, 10-, 7-, 2- and 1.4- fold the concentration of Mn, Fe, Co, Ni, and Cr, respectively, compared to the control. The μ-XRF imaging revealed that Ni and Zn were not spatially correlated across the leaf. We observed a clear partitioning of Zn between veins and surrounding leaf cells while Ni was more evenly distributed between veins and leaf blade. The accumulation of metals in citric acid treated plants did not change the Ni and Zn distribution pattern in leaves but altered the Mn distribution. It seems that Mn reached toxic concentrations in leaves and we hypothesize that a mechanism driven by transpiration through the xylem was used to excrete the metal. Our results show that citric acid can enhance metal accumulation by N. caerulescens and have impact for soil remediation by either decreasing the time for clean up or increasing the access to non-labile pools of metals in soil.

Distribution of aluminium in hydrated leaves of tea (Camellia sinensis) using synchrotron- and laboratory-based X-ray fluorescence microscopy

Aluminium (Al) is highly toxic to plant growth, with soluble concentrations being elevated in the ∼40% of arable soils worldwide that are acidic. Determining the distribution of Al in plant tissues is important for understanding the mechanisms by which it is toxic and how some plants tolerate high concentrations. Synchrotron- and laboratory-based X-ray fluorescence microscopy (XFM) is a powerful technique to quantitatively analyse the distribution of elements, including in hydrated and living plants. However, analysis of light elements (z < phosphorus) is extremely challenging due to signal losses in air, and the unsuitability of vacuum environments for (fresh) hydrated plant tissues. This study uses XFM in a helium environment to avoid Al signal loss to reveal the distribution of Al in hydrated plant tissues of Tea (Camellia sinensis). The results show that Al occurs in localised areas across the foliar surface, whereas in cross-sections Al is almost exclusively concentrated in the apoplastic space above and in between adaxial epidermal cells. This distribution of Al is related to the Al tolerance of this species, and accumulation of phytotoxic elements in the apoplastic space, away from sensitive processes such as photosynthesis in the palisade mesophyll cells, is a common tolerance mechanism reported in many different plant species. This study develops an XFM method on both synchrotron and laboratory sources that overcomes the drawbacks of existing analytical techniques, permitting measurement of light elements down to Al in (fresh) hydrated plant tissues.

Visualising Silicon in Plants: Histochemistry, Silica Sculptures and Elemental Imaging

Silicon is a non-essential element for plants and is available in biota as silicic acid. Its presence has been associated with a general improvement of plant vigour and response to exogenous stresses. Plants accumulate silicon in their tissues as amorphous silica and cell walls are preferential sites. While several papers have been published on the mitigatory effects that silicon has on plants under stress, there has been less research on imaging silicon in plant tissues. Imaging offers important complementary results to molecular data, since it provides spatial information. Herein, the focus is on histochemistry coupled to optical microscopy, fluorescence and scanning electron microscopy of microwave acid extracted plant silica, techniques based on particle-induced X-ray emission, X-ray fluorescence spectrometry and mass spectrometry imaging (NanoSIMS). Sample preparation procedures will not be discussed in detail, as several reviews have already treated this subject extensively. We focus instead on the information that each technique provides by offering, for each imaging approach, examples from both silicifiers (giant horsetail and rice) and non-accumulators (Cannabis sativa L.).

Trace elements in Labrador Tea (Rhododendron groenlandicum): How predominant sources to the plants impact the chemical composition of hot water extracts

Labrador Tea (Rhododendron groenlandicum) has been an important food and medicinal plant to First Nations communities in North America for millenia, but little is known of its geochemical properties. Using plants from 10 sites in 4 provinces, including pristine and industrial regions, and employing the metal-free, ultraclean SWAMP laboratory facilities and procedures, we provide an estimate of the natural abundance of trace elements in the leaves, and the extent of their release during hot water extraction. Elements decrease in abundance in the order Mn > Al > Fe > Zn > Cu > Ni > V > Pb > La > Mo > Y > La > Tl > Cd > Th > Ag. The greatest concentrations of conservative, lithophile elements such as Al, La, Th and Y, are found in samples collected on lands reclaimed from open pit bitumen mines in northern Alberta, reflecting elevated inputs of atmospheric dusts. In contrast, micronutrients such as Cu and Zn are remarkably uniform which suggests that these are supplied almost exclusively by plant uptake via roots. Deionized, reverse osmosis water is more effective in removing some elements (e.g. Al, La, Y, Fe, Zn, Cd) whereas others are more readily extracted using groundwater (e.g. Cu, Ni, Pb); V behaves independently of water composition. In both types of water, the elements most readily extracted are plant micronutrients (Mn, Ni, Cu, and Zn) whereas those supplied primarily by dust exhibit much lower yields; Al shows behaviour intermediate between these two extremes. While element concentrations in the infusions increase with increasing concentrations in the leaves, the abundance of potentially toxic chalcophile elements such as Cd, Pb, Sb and Tl in the infusions are extremely low (ng/l). Plants from British Columbia, Ontario and Quebec provide evidence of atmospheric Pb contamination, yielding greater ratios of Pb/La compared to the samples from Alberta where crustal values are found. Given that this plant is common and found across the northern half of the continent, it shows great promise as a tool for biomonitoring of air quality. For consumers, Labrador Tea may represent an important dietary source of Mn.

Examining a synchrotron-based approach for in situ analyses of Al speciation in plant roots

Aluminium (Al) K- and L-edge X-ray absorption near-edge structure (XANES) has been used to examine Al speciation in minerals but it remains unclear whether it is suitable for in situ analyses of Al speciation within plants. The XANES analyses for nine standard compounds and root tissues from soybean (Glycine max), buckwheat (Fagopyrum tataricum), and Arabidopsis (Arabidopsis thaliana) were conducted in situ. It was found that K-edge XANES is suitable for differentiating between tetrahedral coordination (peak of 1566 eV) and octahedral coordination (peak of 1568 to 1571 eV) Al, but not suitable for separating Al binding to some of the common physiologically relevant compounds in plant tissues. The Al L-edge XANES, which is more sensitive to changes in the chemical environment, was then examined. However, the poorer detection limit for analyses prevented differentiation of the Al forms in the plant tissues because of their comparatively low Al concentration. Where forms of Al differ markedly, K-edge analyses are likely to be of value for the examination of Al speciation in plant tissues. However, the apparent inability of Al K-edge XANES to differentiate between some of the physiologically relevant forms of Al may potentially limit its application within plant tissues, as does the poorer sensitivity at the L-edge.

Tolerance of Eugenia dysenterica to Aluminum: Germination and Plant Growth

Native Cerrado plants are exposed to soils with low pH and high availability of Al. In this study, we measured the Al content in adult plants, and investigated the effects of various Al doses on germination and early development of Eugenia dysenterica plants. For germination tests, the seeds were soaked in Al solution and evaluated for twenty days in growth chambers. In a second experiment, young plants were cultivated in hydroponic systems with various Al concentrations to evaluate the morphological, anatomical and physiological characteristics of E. dysenterica. Anatomical changes and low germinative vigor were observed in seeds germinated in 600 and 800 μmol Al3+ L-1. In the hydroponic system, 200 μmol Al3+ L-1 stimulated root growth in young plants. The activity of antioxidant enzymes and the accumulation of phenolic compounds were greatest at the highest Al doses, preventing changes in gas exchange and chlorophyll a fluorescence. Starch grain accumulation was noted in plant cells exposed to 200 and 400 μmol Al3+ L-1. Adult E. dysenterica trees also accumulated Al in leaves, bark and seeds. These data suggest that E. dysenterica is tolerant to Al.

Localization of Aluminum in Epidermal Cells of Mature Tea Leaves

We have determined the distribution of aluminum in the epidermal cells of mature tea leaves using micro-beam particle-induced X-ray emission. The observed pattern of aluminum distribution in the epidermal cells suggests that aluminum exists in cell walls. Silicon exhibits a distribution that is nearly identical to that of aluminum, suggesting co-localization with aluminum.

Mechanisms of organic acids and boron induced tolerance of aluminum toxicity: A review

Aluminum is a major limiting abiotic factor for plant growth and productivity on acidic soils. The primary disorder of aluminum toxicity is the rapid cessation of root elongation. The root apex is the most sensitive part of this organ. Although significant literature evidence and hypothesis exist on aluminum toxicity, the explicit mechanism through which aluminum ceases root growth is still indefinable. The mechanisms of tolerance in plants have been the focus of intense research. Some plant species growing on acidic soils have developed tolerance mechanisms to overcome and mitigate aluminum toxicity, either by avoiding entry of Al³⁺ into roots (exclusion mechanism) or by being able to counterbalance toxic Al³⁺ engrossed by the root system (internal tolerance mechanism). Genes belonging to ALMT (Aluminum-activated malate transporter) and MATE (Multidrug and toxin compounds extrusion) have been identified that are involved in the aluminum-activated secretion of organic acids from roots. However, different plant species show different gene expression pattern. On the other hand, boron (B) (indispensable micronutrient) is a promising nutrient in the tolerance to aluminum toxicity. It not only hinders the adsorption of aluminum to the cell wall but also improves plant growth. This review mainly explains the critical roles of organic acid and B-induced tolerance to aluminum by summarizing the mechanisms of ALMT, MATE, internal detoxification, molecular traits and genetic engineering of crops.

Synchrotron-based X-Ray Approaches for Examining Toxic Trace Metal(loid)s in Soil-Plant Systems

Elevated levels of trace metal(loid)s reduce plant growth, both in soils contaminated by industrial activities and in acid agricultural soils. Although the adverse effects of trace metal(loid)s have long been recognized, there remains much unknown both about their behavior in soils, their toxicity to plants, and the mechanisms that plants use to tolerate elevated concentrations. Synchrotron-based approaches are being utilized increasingly in soil-plant systems to examine toxic metal(loid)s. In the present review, brief consideration is given to the theory of synchrotron radiation. Thereafter, we review the use of synchrotron-based approaches for the examination of various trace metal(loid)s in soil-plant systems, including aluminum, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, and cadmium. Within the context of this review, X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (μ-XRF) are of particular interest. These techniques can provide in situ analyses of the distribution and speciation of metal(loid)s in soil-plant systems. The information presented here serves not only to understand the behavior of trace metals in soil-plant systems, but also to provide examples of the potential applications of synchrotron radiation that can be used to advantage in other studies.

Cell wall pectin methyl-esterification and organic acids of root tips involve in aluminum tolerance in Camellia sinensis

Tea plant (Camellia sinensis (O.) Kuntze) can survive from high levels of aluminum (Al) in strongly acidic soils. However, the mechanism driving its tolerance to Al, the predominant factor limiting plant growth in acid condition, is still not fully understood. Here, two-year-old rooted cuttings of C. sinensis cultivar 'Longjingchangye' were used for Al resistance experiments. We found that the tea plants grew better in the presence of 0.4 mM Al than those grew under lower concentration of Al treatments (0 and 0.1 mM) as well as higher levels treatment (2 and 4 mM), confirming that appropriate Al increased tea plant growth. Hematoxylin staining assay showed that the apical region was the main accumulator in tea plant root. Subsequently, immunolocalization of pectins in the root tip cell wall showed a rise in low-methyl-ester pectin levels and a reduction of high-methyl-ester pectin content with the increasing Al concentration of treatments. Furthermore, we observed the increased expressions of C. sinensis pectin methylesterase (CsPME) genes along with the increasing de-esterified pectin levels during response to Al treatments. Additionally, the levels of organic acids increased steadily after treatment with 0.1, 0.4 or 2 mM Al, while they dropped after treatment with 4 mM Al. The organic acids secretion from root followed a similar trend. Similarly, a gradual increase in malate dehydrogenase (MDH), citrate synthase (CS) and glycolate oxidase (GO) enzyme activities and relevant metabolic genes expression were detected after the treatment of 0.1, 0.4 or 2 mM Al, while a sharp decrease was resulted from treatment with 4 mM Al. These results confirm that both pectin methylesterases and organic acids contribute to Al tolerance in C. sinensis.

An RNA-Seq transcriptome analysis revealing novel insights into aluminum tolerance and accumulation in tea plant

The tea plant ( Camellia sinensis L. O. Kuntze) is a high aluminum (Al) tolerant and accumulator species. Candidate genes related to Al tolerance in tea plants were assembled based on de novo transcriptome analysis. The homologs implied some common and distinct Al-tolerant mechanism between tea plants and rice, Arabidopsis and buckwheat. In addition to high Al tolerance, the tea plant exhibits good performance exposure to a proper Al level, and accumulates high Al in the leaves without any toxicity symptom. Therefore, Al was considered as a hyperaccumulator and beneficial element for tea plants. However, the whole-genome molecular mechanisms accounting for Al-tolerance and accumulation remain unknown in tea plants. In this study, transcriptome analysis by RNA-Seq following a gradient Al-level exposure was assessed to further reveal candidate genes involved. Totally more than 468 million high-quality reads were generated and 213,699 unigenes were de novo assembled, among which 8922 unigenes were all annotated in the seven databases used. A large number of transporters, transcription factors, cytochrome P450, ubiquitin ligase, organic acid biosynthesis, heat shock proteins differentially expressed in response to high Al (P ≤ 0.05) were identified, which were most likely ideal candidates involved in the Al tolerance or accumulation. Furthermore, a few of the candidate Al-responsive genes related to Al sequestration, cell wall modification and organic acid excretion have been well elucidated as was already found in Arabidopsis, rice, and buckwheat. Thus, some consistent Al-tolerance mechanisms across the species are indicated. In conclusion, the transcriptome data provided useful insights of promising candidates for further characterizing the functions of genes involved in Al tolerance and accumulation in tea plants.

Contribution of constitutive characteristics of lipids and phenolics in roots of tree species in Myrtales to aluminum tolerance

High aluminum (Al) concentration in soil solution is the most important factor restricting plant growth in acidic soils. However, various plant species naturally grow in such soils. Generally, they are highly tolerant to Al, but organic acid exudation, the most common Al tolerance mechanism, cannot explain their tolerance. Lower phospholipid and higher sterol proportions in root plasma membrane enhance Al tolerance. Other cellular components, such as cell walls and phenolics, may also be involved in Al tolerance mechanisms. In this study, the relationships between these cellular components and the Al tolerance mechanisms in Melastoma malabathricum and Melaleuca cajuputi, both highly Al-tolerant species growing in strongly acidic soils, were investigated. Both species contained lower proportions of phospholipids and higher proportions of sterols in roots, respectively. Concentrations of phenolics in roots of both species were higher than that of rice; their phenolics could form chelates with Al. In these species, phenolic concentrations and composition were the same irrespective of the presence or absence of Al in the medium, suggesting that a higher concentration of phenolics is not a physiological response to Al but a constitutive characteristic. These characteristics of cellular components in roots may be cooperatively involved in their high Al tolerance.

Aluminum, a Friend or Foe of Higher Plants in Acid Soils

Aluminum (Al) is the most abundant metal in the earth's crust, but its availability depends on soil pH. Despite this abundance, Al is not considered an essential element and so far no experimental evidence has been put forward for a biological role. In plants and other organisms, Al can have a beneficial or toxic effect, depending on factors such as, metal concentration, the chemical form of Al, growth conditions and plant species. Here we review recent advances in the study of Al in plants at physiological, biochemical and molecular levels, focusing mainly on the beneficial effect of Al in plants (stimulation of root growth, increased nutrient uptake, the increase in enzyme activity, and others). In addition, we discuss the possible mechanisms involved in improving the growth of plants cultivated in soils with acid pH, as well as mechanisms of tolerance to the toxic effect of Al.

Current status of the TwinMic beamline at Elettra: a soft X-ray transmission and emission microscopy station

The current status of the TwinMic beamline at Elettra synchrotron light source, that hosts the European twin X-ray microscopy station, is reported. The X-ray source, provided by a short hybrid undulator with source size and divergence intermediate between bending magnets and conventional undulators, is energy-tailored using a collimated plane-grating monochromator. The TwinMic spectromicroscopy experimental station combines scanning and full-field imaging in a single instrument, with contrast modes such as absorption, differential phase, interference and darkfield. The implementation of coherent diffractive imaging modalities and ptychography is ongoing. Typically, scanning transmission X-ray microscopy images are simultaneously collected in transmission and differential phase contrast and can be complemented by chemical and elemental analysis using across-absorption-edge imaging, X-ray absorption near-edge structure or low-energy X-ray fluorescence. The lateral resolutions depend on the particular imaging and contrast mode chosen. The TwinMic range of applications covers diverse research fields such as biology, biochemistry, medicine, pharmacology, environment, geochemistry, food, agriculture and materials science. They will be illustrated in the paper with representative results.

Kinetics and nature of aluminium rhizotoxic effects: a review

Acid soils with elevated levels of soluble aluminium (Al) comprise ~40% of the world's arable land, but there remains much uncertainty regarding the mechanisms by which Al is rhizotoxic. This review examines the kinetics of the toxic effects of Al on the root elongation rate (RER), its effects on root tissues, and its location at a subcellular level. Depending upon the concentration and plant species, soluble Al decreases the RER in a median time of 73min, but in as little as 5min in soybean. This is initially due to a decreased rate at which cells expand anisotropically in the elongation zone. Thereafter, rhizodermal and outer cortical cells rupture through decreased cell wall relaxation. It is in this region where most Al accumulates in the apoplast. Subsequently, Al impacts root growth at a subcellular level through adverse effects on the plasma membrane (PM), cytoplasm, and nucleus. At the PM, Al alters permeability, fluidity, and integrity in as little as 0.5h, whilst it also depolarizes the PM and reduces H(+)-ATPase activity. The Al potentially crosses the PM within 0.5h where it is able to bind to the nucleus and inhibit cell division; sequestration within the vacuole is required to reduce the toxic effects of Al within the cytoplasm. This review demonstrates the increasing evidence of the importance of the initial Al-induced inhibition of wall loosening, but there is evidence also of the deleterious effects of Al on other cellular processes which are important for long-term root growth and function.

Distribution and Speciation of Cu in the Root Border Cells of Rice by STXM Combined with NEXAFS

Root border cells (RBCs) serve plants in their initial line of defense against stress from the presence of heavy metals in the soil. In this research, light microscopy and synchrotron-based scanning transmission X-ray microscopy (STXM) combined with near edge X-ray absorption fine structure spectroscopy (NEXAFS) with a nanoscale spatial resolution were used to investigate the effects of copper (Cu) upon the RBCs, as well as its distribution and speciation within the RBCs of rice (Oryza sativa L.) under aeroponic culture. The results indicated that with increasing exposure time and concentration, the attached RBCs were surrounded by a thick mucilage layer which changed in form from an ellipse into a strip in response to Cu ion stress. Copper was present as Cu(II), which accumulated not only in the cell wall but also in the cytoplasm. To our knowledge, this is the first time that STXM has been used in combination with NEXAFS to provide new insight into the distribution and speciation of metal elements in isolated plant cells.

Aluminium Accumulation and Intra-Tree Distribution Patterns in Three Arbor aluminosa (Symplocos) Species from Central Sulawesi

Accumulation of Aluminium (Al) at concentrations far above 1,000 mg kg-1 in aboveground plant tissues of Arbor aluminosa (Symplocos) species is the main reason why traditional Indonesian weavers rely on their leaves and bark as a mordant for dyeing textile. Recently, Symplocos species have become a flagship species for the conservation efforts of weaving communities due to their traditionally non-sustainable sampling and increasing demand for Symplocos plant material. Here we investigated Symplocos odoratissima, S. ophirensis and S. ambangensis at three montane rainforest sites in Central Sulawesi to measure Al levels in different tissues and organs. The highest Al concentrations were found in old leaves (24,180 ± 7,236 mg·kg-1 dry weight, mean ± SD), while young leaves had significantly lower Al levels (20,708 ± 7,025 mg·kg-1). Al accumulation was also lower in bark and wood tissue of the trunk (17,231 ± 8,356 mg·kg-1 and 5,181 ± 2,032 mg·kg-1, respectively). Two Al excluding species (Syzigium sp. and Lithocarpus sp.) contained only high Al levels in their roots. Moreover, no difference was found in soil pH (4.7 ± 0.61) and nutrient (K, Ca, Fe, Mg) availability at different soil levels and within or outside the crown of Symplocos trees, except for the upper soil layer. Furthermore, a positive and significant correlation between Al and Ca concentrations was found at the whole plant level for Symplocos, and at the leaf level for S. ophirensis and S. ambangensis, suggesting a potential role of Ca in Al uptake and/or detoxification within the plant. Our results provide evidence for strong Al accumulation in Symplocos species and illustrate that both Al accumulation and exclusion represent two co-occurring strategies of montane rainforest plants for dealing with Al toxicity. Indonesian weavers should be encouraged to harvest old leaves, which have the most efficient mordant capacity due to high Al concentrations.

The effects of aluminium on plant growth in a temperate and deciduous aluminium accumulating species

Aluminium (Al) is a phytotoxic element affecting the growth and yield of many crop plants, especially in the tropics. Yet, some plants are able to accumulate high levels of Al. The monogeneric family Symplocaceae represents an Al accumulating family including many tropical and evergreen species with high Al levels in their above ground plant tissues. It is unclear, however, whether Al accumulation also characterises temperate species of Symplocos, and whether or not the uptake has a beneficial growth effect. Here, we investigate if the temperate, deciduous species Symplocos paniculata is able to accumulate Al by growing seedlings and saplings in a hydroponic setup at pH 4 with and without Al. Pyrocatechol-violet (PCV) and aluminon staining was performed to visualize Al accumulation in various plant tissues. Both seedlings and saplings accumulate Al in their tissues if available. Mean Al levels in leaves were 4107 (±1474 mg kg^-1) and 4290 (±4025 mg kg^-1) for the seedlings and saplings, respectively. The saplings treated without Al showed a high mortality rate unlike the Al accumulating ones. The seedlings, however, showed no difference in growth and vitality between the two treatments. The saplings treated with Al showed new twig, leaf and root development, resulting in a considerable biomass increase. PCV and aluminon staining indicated the presence of Al in leaf, wood and bark tissue of the plants. S. paniculata shares the capacity to accumulate Al with its tropical sister species and is suggested to be a facultative accumulator. Whether or not Al has a beneficial effect remains unclear, due to developmental differences between seedlings and saplings. Al is suggested to be transported via the xylem transport system into the leaves, which show the highest Al levels. Radial transport via ray parenchyma to bark tissue is also likely given the high Al concentrations in the bark tissue.

Aluminum uptake and migration from the soil compartment into Betula pendula for two different environments: a polluted and environmentally protected area of Poland

This paper presents the impact of soil contamination on aluminum (Al) concentrations in plant parts of Betula pendula and a possible way of migration and transformation of Al in the soil-root-stem-twig-leaf system. A new procedure of Al fractionation based on extraction in water phase was applied to obtain and measure the most available forms of Al in soils and B. pendula samples. In addition, total Al content was determined in biological samples and pseudo total Al content in soil samples collected under plant saplings, using atomic absorption spectrometry with flame atomization. A number of relations concerning the occurrence of Al and Ca in soils and plant parts of B. pendula (tap roots, lateral roots, stem, twigs, and leaves) were observed. Based on the research findings, the mechanism of Al migration from soil to the leaves of B. pendula can be presented. It was found that aluminum uptake may be limited in roots by high calcium concentration. The application of a new procedure based on the simple sequential extraction of water-soluble fractions (the most available and exchangeable fractions of Al) can be used as an effective tool for the estimation of aluminum toxicity in soils and plants.

Aluminium uptake and translocation in Al hyperaccumulator Rumex obtusifolius is affected by low-molecular-weight organic acids content and soil pH

BACKGROUND AND AIMS

High Al resistance of Rumex obtusifolius together with its ability to accumulate Al has never been studied in weakly acidic conditions (pH > 5.8) and is not sufficiently described in real soil conditions. The potential elucidation of the role of organic acids in plant can explain the Al tolerance mechanism.

METHODS

We established a pot experiment with R. obtusifolius planted in slightly acidic and alkaline soils. For the manipulation of Al availability, both soils were untreated and treated by lime and superphosphate. We determined mobile Al concentrations in soils and concentrations of Al and organic acids in organs.

RESULTS

Al availability correlated positively to the extraction of organic acids (citric acid < oxalic acid) in soils. Monovalent Al cations were the most abundant mobile Al forms with positive charge in soils. Liming and superphosphate application were ambiguous measures for changing Al mobility in soils. Elevated transport of total Al from belowground organs into leaves was recorded in both lime-treated soils and in superphosphate-treated alkaline soil as a result of sufficient amount of Ca available from soil solution as well as from superphosphate that can probably modify distribution of total Al in R. obtusifolius as a representative of "oxalate plants." The highest concentrations of Al and organic acids were recorded in the leaves, followed by the stem and belowground organ infusions.

CONCLUSIONS

In alkaline soil, R. obtusifolius is an Al-hyperaccumulator with the highest concentrations of oxalate in leaves, of malate in stems, and of citrate in belowground organs. These organic acids form strong complexes with Al that can play a key role in internal Al tolerance but the used methods did not allow us to distinguish the proportion of total Al-organic complexes to the free organic acids.

Selective and sensitive turn-on chemosensor for Al(iii) ions applicable in living organisms: nanomolar detection in aqueous medium

A new crystallographically characterized non-cytotoxic CHEF based highly sensitive Al(iii) ion selective chemosensor (L) is useful to detect the Al(iii) ions' distribution in A549 cell lines and also able to sense Al(iii) ions in tea extract.

Leaf optical properties are affected by the location and type of deposited biominerals

This study aimed to relate the properties of incrusted plant tissues and structures as well as biomineral concentrations and localization with leaf reflectance and transmittance spectra from 280nm to 880nm in the grasses Phragmites australis, Phalaris arundinacea, Molinia caerulea and Deschampsia cespitosa, and the sedge Carex elata. Redundancy analysis revealed that prickle-hair length on adaxial surface and thickness of lower epidermis exerted significant effects in P. australis; prickle-hair density at abaxial leaf surface and thickness of epidermis on adaxial leaf surface in P. arundinacea; thickness of epidermis on adaxial leaf in D. cespitosa; prickle-hair density on adaxial leaf surface and thickness of cuticle in M. caerulea; and prickle-hair density on adaxial leaf surface and cuticle thickness of the lower side in C. elata. Micro-PIXE and LEXRF elemental localization analysis show that all of these structures and tissues are encrusted by Si and/or by Ca. Reflectance spectra were significantly affected by the Ca concentrations, while Si and Mg concentrations and the Ca concentrations significantly affected transmittance spectra. High concentrations of Mg were detected in epidermal vacuoles of P. arundinacea, M. caerulea and D. cespitosa. Al co-localises with Si in the cuticle, epidermis and/or prickle hairs.

Spatial X-ray fluorescence micro-imaging of minerals in grain tissues of wheat and related genotypes

Wheat and its related genotypes show distinct distribution patterns for mineral nutrients in maternal and filial tissues in grains. X-ray-based imaging techniques are very informative to identify genotypes with contrasting tissue-specific localization of different elements. This can help in the selection of suitable genotypes for nutritional improvement of food grain crops. Understanding mineral localization in cereal grains is important for their nutritional improvement. Spatial distribution of mineral nutrients (Mg, P, S, K, Ca, Fe, Zn, Mn and Cu) was investigated between and within the maternal and filial tissues in grains of two wheat cultivars (Triticum aestivum Cv. WH291 and WL711), a landrace (T. aestivum L. IITR26) and a related wild species Aegilops kotschyi, using micro-proton-induced X-ray emission (µ-PIXE) and micro-X-ray fluorescence (µ-XRF). Aleurone and scutellum were major storage tissues for macro (P, K, Ca and Mg) as well as micro (Fe, Zn, Cu and Mn) nutrients. Distinct elemental distribution patterns were observed in each of the four genotypes. A. kotschyi, the wild relative of wheat and the landrace, T. aestivum L. IITR26, accumulated more Zn and Fe in scutellum and aleurone than the cultivated wheat varieties, WH291 and WL711. The landrace IITR26, accumulated far more S in grains, Mn in scutellum, aleurone and embryo region, Ca and Cu in aleurone and scutellum, and Mg, K and P in scutellum than the other genotypes. Unlike wheat, lower Mn and higher Fe, Cu and Zn concentrations were noticed in the pigment strand of A. kotschyi. Multivariate statistical analysis, performed on mineral distribution in major grain tissues (aleurone, scutellum, endosperm and embryo region) resolved the four genotypes into distinct clusters.

Superior aluminium (Al) tolerance of Stylosanthes is achieved mainly by malate synthesis through an Al-enhanced malic enzyme, SgME1

Stylosanthes (stylo) is a dominant leguminous forage in the tropics. Previous studies suggest that stylo has great potential for aluminium (Al) tolerance, but little is known about the underlying mechanism. A novel malic enzyme, SgME1, was identified from the Al-tolerant genotype TPRC2001-1 after 72 h Al exposure by two-dimensional electrophoresis, and the encoding gene was cloned and characterized via heterologous expression in yeast, Arabidopsis thaliana and bean (Phaseolus vulgaris) hairy roots. Internal Al detoxification might be mainly responsible for the 72 h Al tolerance of TPRC2001-1, as indicated by 5.8-fold higher root malate concentrations and approximately two-fold higher Al concentrations in roots and root symplasts of TPRC2001-1 than those of the Al-sensitive genotype Fine-stem. An accompanying increase in malate secretion might also reduce a fraction of Al uptake in TPRC2001-1. Gene and protein expression of SgME1 was only enhanced in TPRC2001-1 after 72 h Al exposure. Overexpressing SgME1 enhanced malate synthesis and rescued yeast, A. thaliana and bean hairy roots from Al toxicity via increasing intracellular malate concentrations and/or accompanied malate exudation. These results provide strong evidence that superior Al tolerance of stylo is mainly conferred by Al-enhanced malate synthesis, functionally controlled by SgME1.

Localization of fluoride and aluminum in subcellular fractions of tea leaves and roots

The tea plant is a fluoride (F) and aluminum (Al) hyperaccumulator. High concentrations of F and Al have always been found in tea leaves without symptoms of toxicity, which may be related to the special localization of F and Al in tea leaves. In this study, we for the first time determined the subcellular localization of F and Al in tea roots and leaves and provided evidence of the detoxification mechanisms of high concentrations of F and Al in tea plants. Results revealed that 52.3 and 71.8% of the total F accumulated in the soluble fraction of tea roots and leaves, and vacuoles contained 98.1% of the total F measured in the protoplasts of tea leaves. Cell walls contained 69.8 and 75.2% of the total Al detected in the tea roots and leaves, respectively, and 73.2% of Al sequestered in cell walls was immobilized by pectin and hemicellulose components. Meanwhile, 88.3% of the Al measured in protoplasts was stored in the vacuoles of tea leaves. Our results suggested that the subcellular distributions of F and Al in tea plants play two important roles in the detoxification of F and Al toxicities. First, most of the F and Al was sequestered in the vacuole fractions in tea leaves, which could reduce their toxicities to organelles. Second, Al can be immobilized in the pectin and hemicellulose components of cell walls, which could suppress the uptake of Al by tea roots.

Evaluation of micro-energy dispersive X-ray fluorescence and histochemical tests for aluminium detection in plants from High Altitude Rocky Complexes, Southeast Brazil

The soils developed under High Altitude Rocky Complexes in Brazil are generally of very low chemical fertility, with low base saturation and high exchangeable aluminium concentration. This stressful condition imposes evolutionary pressures that lead to ecological success of plant species that are able to tolerate or accumulate high amounts of aluminium. Several analytical methods are currently available for elemental mapping of biological structures, such as micro-X-ray fluorescence (μ-EDX) and histochemical tests. The aim of this study was to combine μ-EDX analysis and histochemical tests to quantify aluminium in plants from High Altitude Rocky Complexes, identifying the main sites for Al-accumulation. Among the studied species, five showed total Al concentration higher than 1000 mg kg−1. The main Al-hyperaccumulator plants, Lavoisiera pectinata, Lycopodium clavatum and Trembleya parviflora presented positive reactions in the histochemical tests using Chrome Azurol and Aluminon. Strong positive correlations were observed between the total Al concentrations and data obtained by μ-EDX analysis. The μ-EDX analysis is a potential tool to map and quantify Al in hyperaccumulator species, and a valuable technique due to its non-destructive capacity. Histochemical tests can be helpful to indicate the accumulation pattern of samples before they are submitted for further μ-EDX scrutiny.

Evaluation of chitooligosaccharide application on mineral accumulation and plant growth in Phaseolus vulgaris

Chitooligosaccharides (COS) - water soluble derivatives from chitin, are an interesting group of molecules for several biological applications, for they can enter plant cells and bind negatively charged molecules. Several studies reported an enhanced plant growth and higher crop yield due to chitosan application in soil grown plants, but no studies have looked on the effect of COS application on plant mineral nutrient dynamics in hydroponically grown plants. In this study, Phaseolus vulgaris was grown in hydroponic culture and the effect of three different concentrations of COS on plant growth and mineral accumulation was assessed. There were significant changes in mineral allocations for Mo, B, Zn, P, Pb, Cd, Mn, Fe, Mg, Ca, Cu, Na, Al and K among treatments. Plant morphology was severely affected in high doses of COS, as well as lignin concentration in the stem and the leaves, but not in the roots. Chlorophyll A, B and carotenoid concentrations did not change significantly among treatments, suggesting that even at higher concentrations, COS application did not affect photosynthetic pigment accumulation. Plants grown at high COS levels had shorter shoots and roots, suggesting that COS can be phytotoxic to the plant. The present study is the first detailed report on the effect of COS application on mineral nutrition in plants, and opens the door for future studies that aim at utilizing COS in biofortification or phytoremediation programs.

From soil to leaves--aluminum fractionation by single step extraction procedures in polluted and protected areas

The paper presents the fractionation of aluminum in the samples of soil and plants of different species using a selective single-step extraction method. The study was conducted in the area located near a chemical plant, which for many years served as a post-crystallization leachate disposal site storing chemical waste (sector I), and in the area around the site: in Wielkopolski National Park, Rogalin Landscape Park and toward the infiltration ponds at the "Dębina" groundwater well-field for the city of Poznań (Poland) (sector II). The results of aluminum fractionation in samples of soil, leaves and plants showed heavy pollution with aluminum, especially in the water soluble aluminum fraction - Alsw (maximum concentration of aluminum in soil extract was 234.8 ± 4.8 mg kg(-1), in the leaves of Betula pendula it was 107.4 ± 1.8 mg kg(-1) and in the plants of Artemisia vulgaris (root) and Medicago sativa (leaves) it amounted to 464.7 ± 10.7 mg kg(-1)and 146.8 ± 1.2 mg kg(-1) respectively). In addition, the paper presents the problem of organic aluminum fractionation in biological samples and it shows the relationship between aluminum concentration in soil and the analysed woody and herbaceous species.

Pattern of iron distribution in maternal and filial tissues in wheat grains with contrasting levels of iron

Iron insufficiency is a worldwide problem in human diets. In cereals like wheat, the bran layer of the grains is an important source of iron. However, the dietary availability of iron in wheat flour is limited due to the loss of the iron-rich bran during milling and processing and the presence of anti-nutrients like phytic acid that keep iron strongly chelated in the grain. The present study investigated the localization of iron and phosphorus in grain tissues of wheat genotypes with contrasting grain iron content using synchrotron-based micro-X-ray fluorescence (micro-XRF) and micro-proton-induced X-ray emission (micro-PIXE). X-ray absorption near-edge spectroscopy (XANES) was employed to determine the proportion of divalent and trivalent forms of Fe in the grains. It revealed the abundance of oxygen, phosphorus, and sulphur in the local chemical environment of Fe in grains, as Fe-O-P-R and Fe-O-S-R coordination. Contrasting differences were noticed in tissue-specific relative localization of Fe, P, and S among the different genotypes, suggesting a possible effect of localization pattern on iron bioavailability. The current study reports the shift in iron distribution from maternal to filial tissues of grains during the evolution of wheat from its wild relatives to the present-day cultivated varieties, and thus suggests the value of detailed physical localization studies in varietal improvement programmes for food crops.

Aluminum exclusion and aluminum tolerance in woody plants

The aluminum (Al) cation Al(3) (+) is highly rhizotoxic and is a major stress factor to plants on acid soils, which cover large areas of tropical and boreal regions. Many woody plant species are native to acid soils and are well adapted to high Al(3) (+) conditions. In tropical regions, both woody Al accumulator and non-Al accumulator plants occur, whereas in boreal regions woody plants are non-Al accumulators. The mechanisms of these adaptations can be divided into those that facilitate the exclusion of Al(3) (+) from root cells (exclusion mechanisms) and those that enable plants to tolerate Al(3) (+) once it has entered the root and shoot symplast (internal tolerance mechanisms). The biochemical and molecular basis of these mechanisms have been intensively studied in several crop plants and the model plant Arabidopsis. In this review, we examine the current understanding of Al(3) (+) exclusion and tolerance mechanisms from woody plants. In addition, we discuss the ecology of woody non-Al accumulator and Al accumulator plants, and present examples of Al(3) (+) adaptations in woody plant populations. This paper complements previous reviews focusing on crop plants and provides insights into evolutionary processes operating in plant communities that are widespread on acid soils.

Changes of growth, photosynthesis and alteration of leaf antioxidative defence system of tea [Camellia sinensis (L.) O. Kuntze] seedlings under aluminum stress

Tea [Camellia sinensis (L.) O. Kuntze] is an aluminum (Al) hyperaccumulator plant and is commercially important due to its high content of antioxidants. Although Al induced growth is well-known for the plants growing in acid soil, yet the cause underlying the stimulatory effect of Al has not been fully understood. To investigate the possible role of Al in growth induction, we studied morphological, physiological as well as biochemical changes of tea plant under different Al concentrations (0-4,000 μM). In hydroponics, Al (15 μM), enhanced shoot and root growth, but at higher concentrations, it caused oxidative damage which culminated in a cascade of biochemical changes, Al content increased concurrently with the maturity of the leaf as well as stem tissues than their younger counterparts. Hematoxylin staining indicated that Al accumulation started after 6 h of exposure in the tips of young roots and accumulation was dose dependent. The physiological parameters such as pigments, photosynthetic rate, transpiration and stomatal conductance were declined due to Al toxicity. Alteration in activated oxygen metabolism was also evidenced by increasing lipid peroxidation, membrane injury, evolution of superoxide anions and accumulation of H(2)O(2). Contents of phenols initially exhibited an acceleration which gradually plummeted at higher levels whereas total sugar and starch contents decimated beyond 15 μM of Al concentration. Activities of antioxidant defense enzymes were increased with the elevated concentration of Al. Expression of citrate synthase gene was up-regulated in the mature leaves, young as well as old roots simultaneously with increased concentration of Al in those parts; indicating the formation of Al-citrate complex. These results cooperatively specified that Al concentration at lower level promoted growth but turned out to be a stressor at elevated stages indicating the sensitivity of the cultivar (T-78) to Al.

Use of micro-PIXE to determine spatial distributions of copper in Brassica carinata plants exposed to CuSO4 or CuEDDS

A better understanding of the mechanisms that govern copper (Cu) uptake, distribution and tolerance in Brassica carinata plants in the presence of chelators is needed before significant progress in chelate-assisted Cu phytoextraction can be made. The aims of this study were therefore to characterise (S,S)-N,N'-ethylenediamine disuccinic acid (EDDS)-assisted Cu uptake, and to compare the spatial distribution patterns of Cu in the roots and leaves of B. carinata plants. The plants were treated with 30 μM or 150 μM CuSO(4) or CuEDDS in hydroponic solution. Quantitative Cu distribution maps and concentration profiles across root and leaf cross-sections of the desorbed plants were obtained by micro-proton induced X-ray emission. In roots, the 30 μM treatments with both CuSO(4) and CuEDDS resulted in higher Cu concentrations in epidermal/cortical regions. At 150 μM CuSO(4), Cu was mainly accumulated in root vascular bundles, whereas with 150 μM CuEDDS, Cu was detected in endodermis and the adjacent inner cortical cell layer. Under all treatments, except with a H(+)-ATP-ase inhibitor, the Cu in leaves was localised mainly in vascular tissues. The incubation of plants with 150 μM CuEDDS enhanced metal translocation to shoots, in comparison to the corresponding CuSO(4) treatment. Inhibition of H(+)-ATPase activity resulted in reduced Cu accumulation in 30 μM CuEDDS-treated roots and 150 μM CuEDDS-treated leaves, and induced changes in Cu distribution in the leaves. This indicates that active mechanisms are involved in retaining Cu in the leaf vascular tissues, which prevent its transport to photosynthetically active tissues. The physiological significance of EDDS-assisted Cu uptake is discussed.

Conostegia xalapensis (Melastomataceae): an aluminum accumulator plant

In acidic soils, an excess of Al³⁺ is toxic to most plants. The Melastomataceae family includes Al-accumulator genera that tolerate high Al³⁺ by accumulating it in their tissues. Conostegia xalapensis is a common shrub in Mexico and Central America colonizing mainly disturbed areas. Here, we determined whether C. xalapensis is an Al accumulator, and whether it has internal tolerance mechanisms to Al. Soil samples collected from two pastures in the state of Veracruz, Mexico, had low pH and high Al³⁺ concentrations along with low Ca²⁺ levels. Leaves of C. xalapensis from pastures showed up to 19,000 mg Al kg⁻¹ DW (dry weight). In laboratory experiments, 8-month-old seedlings treated with 0.5 and 1.0 mM AlCl₃ for 24 days showed higher number of lateral roots and biomass. Pyrocatechol violet and hematoxylin staining evidenced that Al localized in epidermis and mesophyll cells in leaves and in epidermis and vascular pith in roots. Scanning electron microscope-energy dispersive X-ray microanalysis of Al-treated leaves corroborated that Al is in abaxial and adaxial epidermis and in mesophyll cells (31.2%) in 1.0 mM Al-treatment. Roots of Al-treated plants had glutathione reductase (EC 1.6.4.2) and superoxide dismutase (EC 1.15.1.1) activity higher, and low levels of O₂*⁻ and H₂O₂. C. xalapensis is an Al-accumulator plant that can grow in acidic soils with higher Al³⁺ concentrations, and can be considered as an indicator species for soils with potential Al toxicity.