Fast X-ray fluorescence microscopy provides high-throughput phenotyping of element distribution in seeds.

The concentration, chemical speciation, and spatial distribution of essential and toxic mineral elements in cereal seeds have important implications for human health. To identify genes responsible for element uptake, translocation, and storage, high-throughput phenotyping methods are needed to visualize element distribution and concentration in seeds. Here, we used X-ray fluorescence microscopy (µ-XRF) as a method for rapid and high-throughput phenotyping of seed libraries and developed an ImageJ-based pipeline to analyze the spatial distribution of elements. Using this method, we nondestructively scanned 4,190 ethyl methanesulfonate (EMS)-mutagenized M1 rice (Oryza sativa) seeds and 533 diverse rice accessions in a genome-wide association study (GWAS) panel to simultaneously measure concentrations and spatial distribution of elements in the embryo, endosperm, and aleurone layer. A total of 692 putative mutants and 65 loci associated with the spatial distribution of elements in rice seed were identified. This powerful method provides a basis for investigating the genetics and molecular mechanisms controlling the accumulation and spatial variations of mineral elements in plant seeds.

The role of soil in defining planetary boundaries and the safe operating space for humanity.

We use soils to provide 98.8% of our food, but we must ensure that the pressure we place on soils to provide this food in the short-term does not inadvertently push the Earth into a less hospitable state in the long-term. Using the planetary boundaries framework, we show that soils are a master variable for regulating critical Earth-system processes. Indeed, of the seven Earth-systems that have been quantified, soils play a critical and substantial role in changing the Earth-systems in at least two, either directly or indirectly, as well as smaller contributions for a further three. For the biogeochemical flows Earth-system process, soils contribute 66% of the total anthropogenic change for nitrogen and 38% for phosphorus, whilst for the land-system change Earth-system process, soils indirectly contribute 80% of global anthropogenic change. Furthermore, perturbations of soils contribute directly to 21% of climate change, 25% to ocean acidification, and 25% to stratospheric ozone depletion. We argue that urgent interventions are required to greatly improve soil management, especially for those Earth-system processes where the planetary boundary has already been exceeded and where soils make an important contribution, with this being for biogeochemical flows (both nitrogen and phosphorus), for climate change, and for land-system change. Of particular importance, it is noted that the highly inefficient use of N fertilizers results in release of excess N into the broader environment, contributes to climate change, and results in release of ozone-depleting substances. Furthermore, the use of soils for agricultural production results not only in land-system change, but also in the loss (mineralization) of organic matter with a concomitant release of CO2 contributing to both climate change and ocean acidification. Thus, there is a need to markedly improve the efficiency of fertilizer applications and to intensify usage of our most fertile soils in order to allow the restoration of degraded soils and limit further areal expansion of agriculture. Understanding, and acting upon, the role of soils is critical in ensuring that planetary boundaries are not transgressed, with no other single variable playing such a strategic role across all of the planetary boundaries.

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.

Soil and the intensification of agriculture for global food security.

Soils are the most complex and diverse ecosystem in the world. In addition to providing humanity with 98.8% of its food, soils provide a broad range of other services, from carbon storage and greenhouse gas regulation, to flood mitigation and providing support for our sprawling cities. But soil is a finite resource, and rapid human population growth coupled with increasing consumption is placing unprecedented pressure on soils through the intensification of agricultural production - the increasing of crop yield per unit area of soil. Indeed, the human population has increased from ca. 250 million in the year 1000, to 6.1 billion in the year 2000, and is projected to reach 9.8 billion by the year 2050. The current intensification of agricultural practices is already resulting in the unsustainable degradation of soils. Major forms of this degradation include the loss of organic matter and the release of greenhouse gases, the over-application of fertilizers, erosion, contamination, acidification, salinization, and loss of genetic diversity. This ongoing soil degradation is decreasing the long-term ability of soils to provide humans with services, including future food production, and is causing environmental harm. It is imperative that the global society is not shortsighted by focusing solely on the near-immediate benefits of soils, such as food supply. A failure to identify the importance of soil within increasingly intensive agricultural systems will undoubtedly have serious consequences for humanity and represents a failure to consider intergenerational equity. Of utmost importance is the need to unequivocally recognize that the degradation of soils leads to a clear economic cost through the loss of services, with such principles needing to be explicitly considered in economic frameworks and decision-making processes at all levels of governance. We contend that the concept of the Water-Food-Energy nexus must be expanded, forming the Water-Soil-Food-Energy nexus.

Minimizing experimental artefacts in synchrotron-based X-ray analyses of Fe speciation in tissues of rice plants.

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 FeIII can rapidly (within 0.5-1 min) photoreduce to FeII, 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 FeIII 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 FeIII 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 FeII, although some comparatively small differences were observed when comparing spectra of hydrated and freeze-dried FeII compounds. The results of this study have demonstrated that FeIII 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.

Manganese distribution and speciation help to explain the effects of silicate and phosphate on manganese toxicity in four crop species.

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.

Aluminum Complexation with Malate within the Root Apoplast Differs between Aluminum Resistant and Sensitive Wheat Lines.

In wheat (Triticum aestivum), it is commonly assumed that Al is detoxified by the release of organic anions into the rhizosphere, but it is also possible that detoxification occurs within the apoplast and symplast of the root itself. Using Al-resistant (ET8) and Al-sensitive (ES8) near-isogenic lines of wheat, we utilized traditional and synchrotron-based approaches to provide in situ analyses of the distribution and speciation of Al within root tissues. Some Al appeared to be complexed external to the root, in agreement with the common assumption. However, root apical tissues of ET8 accumulated four to six times more Al than ES8 when exposed to Al concentrations that reduce root elongation rate by 50% (3.5 µM Al for ES8 and 50 µM for ET8). Furthermore, in situ analyses of ET8 root tissues indicated the likely presence of Al-malate and other forms of Al, predominantly within the apoplast. To our knowledge, this is the first time that X-ray absorption near edge structure analyses have been used to examine the speciation of Al within plant tissues. The information obtained in the present study is important in developing an understanding of the underlying physiological mode of action for improved root growth in systems with elevated soluble Al.

Alleviation of Al Toxicity by Si Is Associated with the Formation of Al-Si Complexes in Root Tissues of Sorghum.

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.

Characterizing the uptake, accumulation and toxicity of silver sulfide nanoparticles in plants.

Silver nanoparticles (Ag-NPs) are used in a wide range of everyday products, leading to increasing concerns regarding their accumulation in soils and subsequent impact on plants. Using single particle inductively coupled plasma mass spectrometry (spICP-MS) and synchrotron-based techniques including X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (XFM), we characterized the uptake, speciation, and translocation of insoluble Ag2S-NPs (an environmentally-relevant form of Ag-NPs in soils) within two plant species, a monocot and a dicot. Exposure to 10 mg Ag L-1 as Ag2S-NPs for one week resulted in a substantial increase in leaf Ag concentrations (3.8 to 5.8 µg Ag g-1 dry mass). Examination using XAS revealed that most of the Ag was present as Ag2S (>91%). Furthermore, analyses using spICP-MS confirmed that these Ag2S particles within the leaves had a markedly similar size distribution to those supplied within the hydroponic solution. These observations, for the first time, provide direct evidence that plants take up Ag2S-NPs without a marked selectivity in regard to particle size and without substantial transformation (dissolution or aggregation) during translocation from roots to shoots. Furthermore, after uptake, these Ag2S-NPs reduced growth, partially due to the solubilisation of Ag+ in planta, which resulted in an upregulation of genes involved in the ethylene signalling pathway. Additionally, the upregulation of the plant defense system as a result of Ag2S-NPs exposure may have contributed to the decrease in plant growth. These results highlight the risks associated with Ag-NP accumulation in plants and subsequent trophic transfer via the food chain.

Aluminium effects on mechanical properties of cell wall analogues.

Aluminium (Al) toxicity adversely impacts plant productivity in acid soils by restricting root growth and although several mechanisms are involved the physiological basis of decreased root elongation remains unclear. Understanding the primary mechanisms of Al rhizotoxicity is hindered due to the rapid effects of soluble Al on root growth and the close proximity of many cellular components within the cell wall, plasma membrane, cytosol and nucleus with which Al may react. To overcome some of these difficulties, we report on a novel method for investigating Al interactions with Komagataeibacter xylinus bacterial cellulose (BC)-pectin composites as cell wall analogues. The growth of K. xylinus in the presence of various plant cell wall polysaccharides, such as pectin, has provided a unique in vitro model system with which to investigate the interactions of Al with plant cell wall polysaccharides. The BC-pectin composites reacted in a similar way with Al as do plant cell walls, providing insights into the effects of Al on the mechanical properties of the BC-pectin composites as cell wall analogues. Our findings indicated that there were no significant effects of Al (4-160 µM) on the tensile stress, tensile strain or Young's modulus of the composites. This finding was consistent with cellulose, not pectin, being the major load bearing component in BC-pectin composites, as is also the case in plant cell walls.

Synchrotron-based X-ray absorption near-edge spectroscopy imaging for laterally resolved speciation of selenium in fresh roots and leaves of wheat and rice.

Knowledge of the distribution of selenium (Se) species within plant tissues will assist in understanding the mechanisms of Se uptake and translocation, but in situ analysis of fresh and highly hydrated plant tissues is challenging. Using synchrotron-based fluorescence X-ray absorption near-edge spectroscopy (XANES) imaging to provide laterally resolved data, the speciation of Se in fresh roots and leaves of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) supplied with 1 µM of either selenate or selenite was investigated. For plant roots exposed to selenate, the majority of the Se was efficiently converted to C-Se-C compounds (i.e. methylselenocysteine or selenomethionine) as selenate was transported radially through the root cylinder. Indeed, even in the rhizodermis which is exposed directly to the bulk solution, only 12-31% of the Se was present as uncomplexed selenate. The C-Se-C compounds were probably sequestered within the roots, whilst much of the remaining uncomplexed Se was translocated to the leaves-selenate accounting for 52-56% of the total Se in the leaves. In a similar manner, for plants exposed to selenite, the Se was efficiently converted to C-Se-C compounds within the roots, with only a small proportion of uncomplexed selenite observed within the outer root tissues. This resulted in a substantial decrease in translocation of Se from the roots to leaves of selenite-exposed plants. This study provides important information for understanding the mechanisms responsible for the uptake and subsequent transformation of Se in plants.

Identification of the primary lesion of toxic aluminum in plant roots.

Despite the rhizotoxicity of aluminum (Al) being identified over 100 years ago, there is still no consensus regarding the mechanisms whereby root elongation rate is initially reduced in the approximately 40% of arable soils worldwide that are acidic. We used high-resolution kinematic analyses, molecular biology, rheology, and advanced imaging techniques to examine soybean (Glycine max) roots exposed to Al. Using this multidisciplinary approach, we have conclusively shown that the primary lesion of Al is apoplastic. In particular, it was found that 75 µm Al reduced root growth after only 5 min (or 30 min at 30 µm Al), with Al being toxic by binding to the walls of outer cells, which directly inhibited their loosening in the elongation zone. An alteration in the biosynthesis and distribution of ethylene and auxin was a second, slower effect, causing both a transient decrease in the rate of cell elongation after 1.5 h but also a longer term gradual reduction in the length of the elongation zone. These findings show the importance of focusing on traits related to cell wall composition as well as mechanisms involved in wall loosening to overcome the deleterious effects of soluble Al.

Laterally resolved speciation of arsenic in roots of wheat and rice using fluorescence-XANES imaging.

• Accumulation of arsenic (As) within plant tissues represents a human health risk, but there remains much to learn regarding the speciation of As within plants. • We developed synchrotron-based fluorescence-X-ray absorption near-edge spectroscopy (fluorescence-XANES) imaging in hydrated and fresh plant tissues to provide laterally resolved data on the in situ speciation of As in roots of wheat (Triticum aestivum) and rice (Oryza sativa) exposed to 2 µM As(V) or As(III). • When exposed to As(V), the As was rapidly reduced to As(III) within the root, with As(V) calculated to be present only in the rhizodermis. However, no uncomplexed As(III) was detected in any root tissues, because of the efficient formation of the As(III)-thiol complex - this As species was calculated to account for all of the As in the cortex and stele. The observation that uncomplexed As(III) was below the detection limit in all root tissues explains why the transport of As to the shoots is low, given that uncomplexed As(III) is the major As species transported within the xylem and phloem. • Using fluorescence-XANES imaging, we have provided in situ data showing the accumulation and transformation of As within hydrated and fresh root tissues.

The rhizotoxicity of metal cations is related to their strength of binding to hard ligands.

Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognized as rhizotoxic for approximately 100 yr, but there is no consensus on its mode of action. The authors contend that the primary mechanism of rhizotoxicity of many metal cations is nonspecific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands, especially carboxylate ligands of the cell-wall pectic matrix. Specifically, the authors propose that metal cations have a common toxic mechanism through inhibiting the controlled relaxation of the cell wall as required for elongation. Metal cations such as Al(3+) and Hg(2+), which bind strongly to hard ligands, are toxic at relatively low concentrations because they bind strongly to the walls of cells in the rhizodermis and outer cortex of the root elongation zone with little movement into the inner tissues. In contrast, metal cations such as Ca(2+), Na(+), Mn(2+), and Zn(2+) , which bind weakly to hard ligands, bind only weakly to the cell wall and move farther into the root cylinder. Only at high concentrations is their weak binding sufficient to inhibit the relaxation of the cell wall. Finally, different mechanisms would explain why certain metal cations (for example, Tl(+), Ag(+), Cs(+), and Cu(2+)) are sometimes more toxic than expected through binding to hard ligands. The data presented in the present study demonstrate the importance of strength of binding to hard ligands in influencing a range of important physiological processes within roots through nonspecific mechanisms.

Fate of ZnO nanoparticles in soils and cowpea (Vigna unguiculata).

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 Zn(2+) 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 confirmed 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 of Zn from ZnO-NPs and soluble Zn in soils indicates that the ZnO-NPs used in this study did not constitute nanospecific risks.

In situ speciation and distribution of toxic selenium in hydrated roots of cowpea.

The speciation and spatial distribution of selenium (Se) in hydrated plant tissues is not well understood. Using synchrotron-based x-ray absorption spectroscopy and x-ray fluorescence microscopy (two-dimensional scanning [and associated mathematical model] and computed tomography), the speciation and distribution of toxic Se were examined within hydrated roots of cowpea (Vigna unguiculata) exposed to either 20 µM selenite or selenate. Based upon bulk solution concentrations, selenate was 9-fold more toxic to the roots than selenite, most likely due to increased accumulation of organoselenium (e.g. selenomethionine) in selenate-treated roots. Specifically, uptake of selenate (probably by sulfate transporters) occurred at a much higher rate than for selenite (apparently by both passive diffusion and phosphate transporters), with bulk root tissue Se concentrations approximately 18-fold higher in the selenate treatment. Although the proportion of Se converted to organic forms was higher for selenite (100%) than for selenate (26%), the absolute concentration of organoselenium was actually approximately 5-fold higher for selenate-treated roots. In addition, the longitudinal and radial distribution of Se in roots differed markedly: the highest tissue concentrations were in the endodermis and cortex approximately 4 mm or more behind the apex when exposed to selenate but in the meristem (approximately 1 mm from the apex) when exposed to selenite. The examination of the distribution and speciation of Se in hydrated roots provides valuable data in understanding Se uptake, transport, and toxicity.

Quantitative determination of metal and metalloid spatial distribution in hydrated and fresh roots of cowpea using synchrotron-based X-ray fluorescence microscopy.

Many metals and metalloids, jointly termed metal(loid)s, are toxic to plants even at low levels. This has limited the study of their uptake, distribution, and modes of action in plant roots grown at physiologically relevant concentrations. Synchrotron-based X-ray fluorescence microscopy was used to examine metal(loid)s in hydrated cowpea (Vigna unguiculata L.) roots exposed to Zn(II), Ni(II), Mn(II), Cu(II), Hg(II), Se(IV), Se(VI), As(III), or As(V). Development of a mathematical model enabled in situ quantitative determination of their distribution in root tissues. The binding strength of metals influenced the extent of their movement through the root cylinder, which influenced the toxic effects exerted-metals (e.g. Cu, Hg) that bind more strongly to hard ligands had high concentrations in the rhizodermis and caused this tissue to rupture, while other metals (e.g. Ni, Zn) moved further into the root cylinder and did not cause ruptures. When longitudinal distributions were examined, the highest Se concentration in roots exposed to Se(VI) was in the more proximal root tissues, suggesting that Se(VI) is readily loaded into the stele. This contrasted with other metal(loid)s (e.g. Mn, As), which accumulated in the apex. These differences in metal(loid) spatial distribution provide valuable quantitative data on metal(loid) physiology, including uptake, transport, and toxicity in plant roots.

Distribution and speciation of Mn in hydrated roots of cowpea at levels inhibiting root growth.

The phytotoxicity of Mn is important globally due to its increased solubility in acid or waterlogged soils. Short-term (≤24 h) solution culture studies with 150 µM Mn were conducted to investigate the in situ distribution and speciation of Mn in apical tissues of hydrated roots of cowpea [Vigna unguiculata (L.) Walp. cv. Red Caloona] using synchrotron-based techniques. Accumulation of Mn was rapid; exposure to 150 µM Mn for only 5 min resulting in substantial Mn accumulation in the root cap and associated mucigel. The highest tissue concentrations of Mn were in the root cap, with linear combination fitting of the data suggesting that ≥80% of this Mn(II) was associated with citrate. Interestingly, although the primary site of Mn toxicity is typically the shoots, concentrations of Mn in the stele of the root were not noticeably higher than in the surrounding cortical tissues in the short-term (≤24 h). The data provided here from the in situ analyses of hydrated roots exposed to excess Mn are, to our knowledge, the first of this type to be reported for Mn and provide important information regarding plant responses to high Mn in the rooting environment.

Examination of the distribution of arsenic in hydrated and fresh cowpea roots using two- and three-dimensional techniques.

Arsenic (As) is considered to be the environmental contaminant of greatest concern due to its potential accumulation in the food chain and in humans. Using novel synchrotron-based x-ray fluorescence techniques (including sequential computed tomography), short-term solution culture studies were used to examine the spatial distribution of As in hydrated and fresh roots of cowpea (Vigna unguiculata 'Red Caloona') seedlings exposed to 4 or 20 µm arsenate [As(V)] or 4 or 20 µm arsenite. For plants exposed to As(V), the highest concentrations were observed internally at the root apex (meristem), with As also accumulating in the root border cells and at the endodermis. When exposed to arsenite, the endodermis was again a site of accumulation, although no As was observed in border cells. For As(V), subsequent transfer of seedlings to an As-free solution resulted in a decrease in tissue As concentrations, but growth did not improve. These data suggest that, under our experimental conditions, the accumulation of As causes permanent damage to the meristem. In addition, we suggest that root border cells possibly contribute to the plant's ability to tolerate excess As(V) by accumulating high levels of As and limiting its movement into the root.

Toxicity of metals to roots of cowpea in relation to their binding strength.

Metal phytotoxicity is important in both environmental and agricultural systems. A solution culture study examined the toxicity of 26 metals to roots of cowpea (Vigna unguiculata (L.) Walp.); new data were collected for 15 metals and published data for 11 metals. Metal toxicity, calculated as causing a 50% reduction in root elongation rate, was determined based on either the measured concentration in the bulk solution (EC50(b)) or the calculated activity at the outer surface of the plasma membrane (EA50(0)°). The EC50(b) values ranged from 0.007 µM for Tl to 98,000 µM for K, with the order of rhizotoxicity to cowpea, from most to least toxic, being Tl = Ag > Cu > Hg = Ni = Ga = Ru = In > Sc = Cd = Gd = La = Co = Cs = Pb > Zn = Al = H > Mn > Ba = Sr > Li > Mg > Ca = Na > K. The EA50(0)° values suggest that the binding of metals to hard ligands is an important, general, nonspecific mechanism of toxicity, a hypothesis supported by the similar toxicity symptoms to roots of cowpea by many metals. However, additional mechanisms, such as strong binding to soft ligands, substantially increase rhizotoxicity of some metals, especially Tl, Ag, and Cs. Besides direct toxic effects, osmotic effects or reduced activity of Ca(2+) at the outer surface of the root plasma membrane (and resultant Ca deficiency) may decrease short-term root growth.