Sulfur Nutrition Affects Garlic Bulb Yield and Allicin Concentration.

Improving bulb yield and allicin content of garlic is important in meeting fresh and pharmaceutical market demands. Garlic plants have a high demand for sulfur (S) since allicin contains S atoms. Two experiments were conducted to identify the effect of S application rate on garlic yield and quality. In a field trial assessing six S application rates (0-150 kg S ha-1), cultivar 'Glenlarge' produced the greatest bulb weight (~90 g) and allicin content (521 mg bulb-1) with the application of 75 kg S ha-1. In contrast, cultivar 'Southern Glen' showed no response in bulb weight or allicin. This was likely due to high soil background S concentrations masking treatment effects. Subsequently, a solution culture experiment with cv. 'Glenlarge' evaluated six S application rates (188 to 1504 mg S plant-1, nominally equivalent to 25-200 kg S ha-1). In solution culture, bulb weight and allicin concentration increased with S rate. Highest bulb weight (~53 g bulb-1) and allicin concentration (~11 mg g-1 DW) were recorded at an S application of 1504 mg S plant-1. This is the first report to conclusively demonstrate the effect of S on yield and allicin in garlic grown in solution culture.

Soil organic carbon is significantly associated with the pore geometry, microbial diversity and enzyme activity of the macro-aggregates under different land uses.

Microbial activity strongly influences the stabilization of soil organic matter (SOM), and is affected by the abiotic properties within soil aggregates, which tend to differ between land uses. Here, we assessed the effects of SOM and pore geometry on the diversity and activity of microbial communities within aggregates formed under different land uses (undisturbed, plantation, pasture, and cropping). X-ray micro-computed tomography (µCT) revealed that macro-aggregates (2-8 mm) of undisturbed soils were porous, highly-connected, and had 200% more macro-pores compared with those from pasture and cropping soils. While the macro-aggregates of undisturbed soils had greater soil organic carbon (SOC) contents and N-acetyl ß-glucosaminidase, ß-glucosidase, and phosphatase activities, those of cropped soils harboured more diverse bacterial communities. Organic carbon was positively associated with the porosity of the macro-aggregates, which was negatively associated with microbial diversity and positively associated with enzyme activity. Thus, the biophysical processes in macro-aggregates may be important for SOC stabilization within the macro-aggregates.

Seawater neutralization and gypsum amelioration of bauxite refining residue to produce a plant growth medium.

Alumina production waste (bauxite refining residue) is highly alkaline, saline, and sodic, and hence cannot support plant growth for revegetation. Gypsum (CaSO4.2H2O) amendment of bauxite residue can lower alkalinity and improve the residue Ca status, but given the large gypsum requirement, efficient gypsum use is imperative. We investigated gypsum amelioration of residue sand (RS), examining changes in RS chemistry, and growth of Rhodes grass (Chloris gayana). Furthermore, we examined whether gypsum amelioration of RS should occur before or after seawater neutralization. We found that Ca from gypsum (20 t ha-1) was retained within the surface 0.2 m of RS, regardless of whether the gypsum was applied before or after seawater neutralization. This Ca was retained both as exchangeable Ca and as a precipitate (either calcite or hydrotalcite), with ca. 50% retained as exchangeable Ca in both approaches. Gypsum at 20 t ha-1, or even lower, provided sufficient Ca for maximum growth of Rhodes grass, in the surface, but higher rates would be required to allow Ca movement down the Na-dominated profile to ameliorate a larger rooting depth - this being important in environments where there are prolonged periods of water stress. The information presented will guide the efficient use of gypsum to ameliorate bauxite refining wastes.

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.

Silver Sulfide Nanoparticles Reduce Nitrous Oxide Emissions by Inhibiting Denitrification in the Earthworm Gut.

The accumulation of Ag2S in agricultural soil via application of Ag-containing sludge potentially affects the functioning of soil microorganisms and earthworms (EWs) due to the strong antimicrobial properties of Ag. This study examined the effects of Ag2S nanoparticles (Ag2S-NPs) on the EW-mediated (Eisenia fetida and Pontoscolex corethrurus) soil N cycle. We used 16S rRNA gene-based sequencing and quantitative polymerase chain reaction to examine the bacterial community and nitrification/denitrification-related gene abundance. The presence of either EWs or Ag significantly increased denitrification and N2O emissions. However, the addition of Ag2S to EW-inhabited soil reduced N2O emissions by 14-33%. Furthermore, Ag2S caused a low-dose stimulation but a high-dose inhibition to N2O flux from the EW gut itself. Accordingly, an increase in Ag in the EW gut caused a decrease in the relative abundance of denitrifiers in both the soil and the gut, especially for the dominant genus Bacillus. Ag2S also decreased the copy numbers of nitrification gene (nxrB) and denitrification genes (napA, nirS, and nosZ) in EW gut, leading to the observed decrease in N2O emissions. Collectively, applying Ag2S-containing sludge disturbs the denitrification function of the EW gut microbiota and the cycling of N in soil-based systems.

Application of sewage sludge containing environmentally-relevant silver sulfide nanoparticles increases emissions of nitrous oxide in saline soils.

Silver (Ag) is released from a range of products and accumulates in agricultural soils as silver sulfide (Ag2S) through the application of Ag-containing biosolids as a soil amendment. Although Ag2S is comparatively stable, its solubility increases with salinity, potentially altering its impacts on microbial communities due to the anti-microbial properties of Ag. In this study, we investigated the impacts of Ag on the microbially mediated N cycle in saline soils by examining the relationship between the (bio)availability of Ag2S and microbial functioning following the application of Ag2S-containing sludge. Synchrotron-based X-ray absorption spectroscopy (XAS) revealed that the Ag2S was stable within the soil, although extractable Ag concentrations increased up to 18-fold in soils with higher salinity. However, the extractable Ag accounted for <0.05% of the total Ag in all soils and had no impact on plant biomass or soil bacterial biomass. Interestingly, at high soil salinity, Ag2S significantly increased cumulative N2O emissions from 80.9 to 229.2 mg kg-1 dry soil (by 180%) compared to the corresponding control sludge treatment, which was ascribed to the increased abundance of nitrification and denitrification-related genes (amoA, nxrB, narG, napA, nirS, and nosZ) and increased relative abundance of denitrifiers (Rhodanobacter, Salinimicrobium, and Zunongwangia). Together, our findings show that the application of Ag2S-containing sludge to a saline soil can disrupt the N cycle and increase N2O emissions from agroecosystems.

Understanding the delayed expression of Al resistance in signal grass (Urochloa decumbens).

BACKGROUND AND AIMS: Signal grass (Urochloa decumbens) is a widely used pasture grass in tropical and sub-tropical areas due to its high aluminiun (Al) resistance. However, the underlying mechanisms conferring this resistance are not clearly understood. METHODS: The Al concentrations of bulk root tissues and the intracellular compartment were examined, including the impact of a metabolic inhibitor, carbonyl cyanide m-chlorophenyl hydrazone (CCCP). Next, we examined changes in the properties of signal grass root tissues following exposure to toxic levels of Al, including the cell wall cation exchange capacity (CEC), degree of methylation and concentrations of cell wall fractions. KEY RESULTS: Although signal grass was highly resistant to Al, there was a delay of 24-48 h before the expression of this resistance. We found that this delay in the expression of Al resistance was not related to the total Al concentration in the bulk apical root tissues, nor was it related to changes in the Al bound to the cell wall. We also examined changes in other properties of the cell wall, including the CEC, degree of methylation and changes in the concentration of pectin, hemicellulose and cellulose. We noted that concentrations of intracellular Al decreased by approx. 50 % at the same time that the root elongation rate improved after 24-48 h. Using CCCP as a metabolic inhibitor, we found that the intracellular Al concentration increased approx. 14-fold and that the CCCP prevented the subsequent decrease in intracellular Al. CONCLUSIONS: Our results indicate that the delayed expression of Al resistance was not associated with the Al concentration in the bulk apical root tissues or bound to the cell wall, nor was it associated with changes in other properties of the cell wall. Rather, signal grass has an energy-dependent Al exclusion mechanism, and this mechanism requires 24-48 h to exclude Al from the intracellular compartment.

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.

Evaluating effects of iron on manganese toxicity in soybean and sunflower using synchrotron-based X-ray fluorescence microscopy and X-ray absorption spectroscopy.

With similar chemistry, Mn and Fe interact in their many essential roles in plants but the magnitude and mechanisms involved of these interactions are poorly understood. Leaves of soybean (a Mn-sensitive species) developed a mild chlorosis and small dark spots and distorted trifoliate leaves with 30 µM Mn and 0.6 µM Fe in nutrient solution (pH 5.6; 3 mM ionic strength). At 0.6 µM Fe, lower alternate leaves of sunflower (a Mn-tolerant species) were chlorotic at 30 µM Mn and had a pale chlorosis and necrosis at 400 µM Mn. A concentration of 30 and 300 µM Fe in solution alleviated these typical symptoms of Mn toxicity and decreased the concentration of Mn from >3000 to ca. 800 mg kg-1 dry mass (DM) in all leaf tissues. As expected, increased Fe supply increased Fe in leaves from <100 up to 1350 mg Fe kg-1 DM. In situ synchrotron-based X-ray fluorescence microscopy showed that increased Fe supply caused an overall decrease in Mn in the leaf tissue but had little effect on the pattern of its distribution. Similarly, X-ray absorption spectroscopy identified only slight effects of Fe supply on Mn speciation in leaf tissues. Thus, the results of this study indicate that increased Fe supply ameliorated Mn toxicity in soybean and sunflower largely through decreased Mn uptake and translocation to leaf tissues rather than through changes in Mn distribution or speciation within the leaves.

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.

Chemical and physical influence of sodic soils on the coleoptile length and root growth angle of wheat genotypes.

BACKGROUND AND AIMS: High exchangeable sodium percentage (ESP) and bulk density of sodic soils can reduce seedling emergence. This study examined variation in seedling coleoptile length and seminal root angle of wheat (Triticum aestivum. L) genotypes to determine whether these traits vary between genotypes that differ in their tolerance to sodic soils. METHODS: Wheat genotypes were grown in three different experiments. First, four wheat genotypes were grown using soils of three ESPs (4, 10 and 17 %) and secondly in soils of three different bulk densities (1.2, 1.4 and 1.5 g cm-3) and ESP 10 %. Thirdly, seedling coleoptile length and seminal root angle were determined for 16 genotypes grown in a soil of ESP 10 % and bulk density 1.2 g cm-2. Seminal root angle and coleoptile length measurements from the current study were compared with seedling emergence rate and force measured previously. KEY RESULTS: The seedling coleoptile length of all genotypes decreased with increasing soil ESP and bulk density, but with no significant differences between genotypes. In contrast, seminal root angles differed significantly between genotypes, but were not significantly affected by ESP or bulk density. There was an inverse relationship between the seminal root angle of the 16 genotypes and seedling emergence rate (R2 = 0.89) and also between seminal root angle and seedling emergence force (R2 = 0.61). CONCLUSIONS: Lack of significant variation in coleoptile length between genotypes suggests that this may not be a suitable characteristic to identify wheat tolerance to sodic conditions. However, a narrower seminal root angle was correlated with rate and force of seedling emergence, traits likely to improve establishment. The mechanism underlying this correlation is not yet clear. Genotypes with a narrow root angle had greater root depth. One possible mechanism might be that genotypes with narrow root angles were able to take up more soil moisture at depth, leading to a higher proportion of seedling emergence.

Absorption of foliar-applied Zn in sunflower (Helianthus annuus): importance of the cuticle, stomata and trichomes.

Background and Aims: The pathways whereby foliar-applied nutrients move across the leaf surface remain unclear. The aim of the present study was to examine the pathways by which foliar-applied Zn moves across the sunflower (Helianthus annuus) leaf surface, considering the potential importance of the cuticle, stomata and trichomes. Methods: Using synchrotron-based X-ray florescence microscopy and nanoscale secondary ion mass spectrometry (NanoSIMS), the absorption of foliar-applied ZnSO4 and nano-ZnO were studied in sunflower. The speciation of Zn was also examined using synchrotron-based X-ray absorption spectroscopy. Key Results: Non-glandular trichomes (NGTs) were particularly important for foliar Zn absorption, with Zn preferentially accumulating within trichomes in ≤15 min. The cuticle was also found to have a role, with Zn appearing to move across the cuticle before accumulating in the walls of the epidermal cells. After 6 h, the total Zn that accumulated in the NGTs was approx. 1.9 times higher than in the cuticular tissues. No marked accumulation of Zn was found within the stomatal cavity, probably indicating a limited contribution of the stomatal pathway. Once absorbed, the Zn accumulated in the walls of the epidermal and the vascular cells, and trichome bases of both leaf sides, with the bundle sheath extensions that connected to the trichomes seemingly facilitating this translocation. Finally, the absorption of nano-ZnO was substantially lower than for ZnSO4, with Zn probably moving across the leaf surface as soluble Zn rather than nanoparticles. Conclusions: In sunflower, both the trichomes and cuticle appear to be important for foliar Zn absorption.

Reforestation of agricultural land in the tropics: The relative contribution of soil, living biomass and debris pools to carbon sequestration.

Tropical regions of the world experience high rates of land-use change and this has a major influence on terrestrial carbon (C) pools and the global C cycle. We assessed land-use change from agriculture to reforested plantings (with endemic species), up to 33 years of age, using 10 paired sites in the wet tropics, Australia. We determined the impacts on 0-50 cm below-ground C (soil organic C (SOC), charcoal C, humic organic C, particulate organic C, resistant organic C), C stored in roots (fine and coarse), C stored in living above-ground biomass and debris C pools. Reforested areas accumulated ecosystem C at a rate of 7.4 Mg ha-1 yr-1. Reforestation plantings contained, on average, 2.3 times more ecosystem C than agricultural areas (102 Mg ha-1 and 233 Mg ha-1, respectively). Most of the C accumulation was in living above-ground and below-ground biomass (60 and 30%, respectively) with a smaller amount in debris pools (16%). Apart from C in roots, soil C accumulation was not obvious across sites ranging from 8 to 33 years since reforestation, relative to the agricultural baseline. Differences in SOC (and associated SOC pools) to a depth of 50 cm, did exist between reforested areas and adjacent agriculture at some sites, however there was not a consistent trend in SOC associated with reforestation. Local site-based factors (e.g. soil texture and mineralogy, land-use history and microbial activity) appear to have a strong influence on the direction of the change in SOC. While reforestation in the tropics has great potential to accumulate C in biomass in living vegetation, and debris pools, it is likely to take approximately 50 years before C stocks of reforested areas resemble natural ecosystems. Accumulation of SOC through reforestation is difficult to achieve, highlighting the need to conserve carbon pools in remnant forests in the tropics.

Dataset on seed details of wheat genotypes, solution treatments to measure seedling emergence force and the relation between seedling force and strain.

The seed details (weight, vigor) and germination rate of 16 wheat (Triticum aestivum) genotypes in a non-limiting conditions were measured. The dataset presents seed germination rate and seed vigor of 16 wheat genotypes. The dataset also presents the concentrations of the cations to create solution treatments of various sodium adsorption ratio (SAR) and ionic strength (I). Finally, dataset presented a figure of the experimental design to measure seedling emergence force of wheat genotypes. The image of the setup and the relation between strain and force have been presented here to convert the strain of the beam into seedling emergence force. This dataset has been used in research work titled 'Greater emergence force and hypocotyl cross sectional area may improve wheat seedling emergence in sodic conditions' (Anzooman et al., 2018) [1].

Greater emergence force and hypocotyl cross sectional area may improve wheat seedling emergence in sodic conditions.

Surface crusting of sodic soils is a major problem in the semi-arid tropics when rapid drying after sowing follows light showers, leading to reduced seedling emergence and grain yield. The magnitude of the force exerted by germinating seeds affects the ability of the seedlings to rupture the crust and emerge. This study aimed to determine whether the seed germination and seedling emergence force of wheat (Triticum aestivum L.) seedlings vary among different genotypes at different sodicity levels. Germination and emergence force of seedlings of four wheat genotypes was determined in assays using four solutions with sodium adsorption ratio (SAR) values ranging from 0 to 60. Seed germination and seedling emergence force varied between genotypes at different sodicity levels, with the emergence force of the coleoptile correlated to the cross sectional area of the hypocotyl. The results suggest that the selection of wheat genotypes with rapid germination, higher seedling emergence force and larger hypocotyl cross sectional area, offers a strategy to improve seedling emergence in crusted sodic soils.

Soil Organic Carbon Stabilization: Mapping Carbon Speciation from Intact Microaggregates.

The clearing of land for agricultural production depletes soil organic carbon (OC) reservoirs, yet despite their importance, the mechanisms by which C is stabilized in soils remain unclear. Using synchrotron-based infrared microspectroscopy, we have for the first time obtained in situ, laterally resolved data regarding the speciation of C within sections taken from intact free microaggregates from two contrasting soils (Vertisol and Oxisol, 0-20 cm depth) impacted upon by long-term (up to 79 y) agricultural production. There was no apparent gradient in the C concentration from the aggregate surface to the interior for any of the three forms of C examined (aliphatic C, aromatic C, and polysaccharide C). Rather, organo-mineral interactions were of critical importance in influencing overall C stability, particularly for aliphatic C, supporting the hypothesis that microaggregates form through organo-mineral interactions. However, long-term cropping substantially decreased the magnitude of the organo-mineral interactions for all three forms of C. Thus, although organo-mineral interactions are important for OC stability, C forms associated with the mineral phases are not entirely resistant to degradation. These results provide important insights into the underlying mechanisms by which microaggregates form and the factors influencing the persistence of OC in soils.

Risk of Silver Transfer from Soil to the Food Chain Is Low after Long-Term (20 Years) Field Applications of Sewage Sludge.

The increasingly widespread usage of silver (Ag) nanoparticles has raised concerns regarding their environmental risk. The behavior of Ag and its transfer risk to the food chain were investigated using a long-term field experiment that commenced in 1942 in which Ag-containing sewage sludge was repeatedly applied to the soil (25 applications during 20 years). The speciation of the Ag in both the sludge and the soils retrieved from the long-term experimental archive was examined using synchrotron-based X-ray absorption spectroscopy, and extractable Ag concentrations from soils were determined using 0.01 M Ca(NO3)2 and 0.005 M DTPA. The total Ag in the sludge during the time period of 1942-1961 ranged from 155 to 463 mg kg-1. These values are 1-2 orders of magnitude higher than those in currently produced sludge (ca. 0.5-20 mg kg-1). Long-term repeated applications of these sludges resulted in an increase of Ag in soils from 1.9 mg kg-1 in the control to up to 51 mg kg-1. The majority (>80%) of the Ag in both the sludge and the sludge-treated soils was present as insoluble Ag2S, thereby markedly reducing the bioavailability of this Ag. Concentrations of Ag in the archived crop samples were generally <0.70 mg kg-1 in edible tissues, much less than those in diets that may cause an adverse effects in animals and humans (>100 mg kg-1). These data indicate that the transfer of Ag (derived from both traditional Ag industry and current nano Ag industry) to the terrestrial food chain is limited.

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

The present study investigated the role of trichomes in absorption of foliar-applied zinc fertilizers in soybean and tomato. Using synchrotron-based X-ray fluorescence microscopy for in situ analyses of hydrated leaves, we found that upon foliar application of ZnSO4, Zn accumulated within 15 min in some non-glandular trichomes in soybean, but not in tomato. However, analyses of cross-sections of soybean leaves did not show any marked accumulation of Zn in tissues surrounding trichomes. Furthermore, when near-isogenic lines of soybean differing 10-fold in trichome density were used to compare Zn absorption, it was found that foliar Zn absorption was not related to trichome density. Therefore, it is suggested that trichomes are not part of the primary pathway through which foliar-applied Zn moves across the leaf surface in soybean and tomato. However, this does not preclude trichomes being important in other plant species, as they are known to be highly diverse. We also compared the absorption of Zn when supplied as either ZnSO4, nano-ZnO, or bulk-ZnO, and found that absorption from ZnSO4 was about 10-fold higher than from nano- and bulk-ZnO, suggesting that it was mainly absorbed as soluble Zn. This study improves our understanding of the absorption of foliar-applied nutrients.

Defining appropriate methods for studying toxicities of trace metals in nutrient solutions.

The use of inappropriate experimental conditions for examining trace metal phytotoxicity results in data of questionable value. The present study aimed to identify suitable parameters for study of phytotoxic metals in nutrient solutions. First, the literature was reviewed to determine the concentration of six metals (Cd, Cu, Hg, Ni, Pb, and Zn) from solution of contaminated soils. Next, the effects of pH, P, Cl, NO3, and four Fe-chelators were investigated by using thermodynamic modelling and by examining changes in root elongation rate of soybean (Glycine max cv. Bunya). The literature review identified that the solution concentrations of metals in soils were low, ranging from (µM) 0.069-11Cd, 0.19-15.8 Cu, 0.000027-0.000079 Hg, 1.0-8.7 Ni, 0.004-0.55 Pb, and 0.4-36.3 Zn. For studies in nutrient solution, pH should generally be low given its effects on solubility and speciation, as should the P concentration due to the formation of insoluble phosphate salts. The concentrations of Cl, NO3, and various chelators also influence metal toxicity through alteration of metal speciation. The nutrient solutions used to study metal toxicity should consider environmentally-relevant conditions especially for metal concentrations, with concentrations of other components added at levels that do not substantially alter metal toxicity.