Equisetum arvense as a silica fertilizer.

The aim was to use the agricultural weed and silica (Si) hyperaccumulator Equisetum arvense as Si fertilizer in plant cultivation. We investigated (1) the Si uptake in various Equisetum species, (2) where Si accumulates in the Equisetum plant, (3) processing methods to release as much Si as possible from dried, ground E. arvense plants and (4) which treatment yields gives the highest uptake of Si in young wheat plants cultivated in soil containing ground E. arvense. The results showed that E. arvense containes 22% Si and was among the best Si accumulators. Equisetum arvense accumulates Si as both soluble and firmly bound fractions. Amorphous silica (SiO2) accumulates in the outer cell walls of epidermis of the entire plant. Regarding the processing method, a longer treatment time, greater concentration of Equisetum, boiling, and the addition of sodium bicarbonate increased the Si availability in ground, dried E. arvense. The addition of untreated, ground, dried E. arvense to the soil, corresponding to 160 kg Si ha-1, increased the available Si in the soil and the Si uptake in wheat plants by five-fold, compared with the control. Boiling the ground E. arvense increased the Si uptake by 10 times, and the of sodium bicarbonate increased the availability and uptake by 40 times, compared with the control. In conclusion, dried, ground E. arvense can be used as a Si fertilizer as is, after boiling for a slightly better effect, or with sodium bicarbonate (up to a similar amount as the ground material) for best effect.

Removal of PFAS from water by aquatic plants.

We have found that aquatic plants can reduce the content of perfluorinated alkyl substances (PFAS) within a short period of time. The aim of this study was to determine the variation in the uptake of PFAS from contaminated water by various wetland plant species, investigate the effect of biomass on PFAS removal, and determine whether laccases and peroxidases are involved in the removal and degradation of PFAS. Seventeen emergent and one submerged wetland plant species were screened for PFAS uptake from highly contaminated lake water. The screening showed that Eriophorum angustifolium, Carex rostrata, and Elodea canadensis accumulated the highest levels of all PFAS. These species were thereafter used to investigate the effect of biomass on PFAS removal from water and for the enzyme studies. The results showed that the greater the biomass per volume, the greater the PFAS removal effect. The plant-based removal of PFAS from water is mainly due to plant absorption, although degradation also occurs. In the beginning, most of the PFAS accumulated in the roots; over time, more was translocated to the shoots, resulting in a higher concentration in the shoots than in the roots. Most PFAS degradation occurred in the water; the metabolites were thereafter taken up by the plants and were accumulated in the roots and shoots. Both peroxidases and laccases were able to degrade PFAS. We conclude that wetland plants can be used for the purification of PFAS-contaminated water. For effective purification, a high biomass per volume of water is required.

Phenomic profiling to reveal tolerance mechanisms and regulation of ascorbate-glutathione cycle in wheat varieties (Triticum aestivum L.) under arsenic stress.

The potential of arsenic (As) tolerant and sensitive varieties of wheat (Triticum aestivum L.) has yet to be explored despite of alarming situation of arsenic toxicity. To fill this gap, the study aimed to explore the role of antioxidants, phytochelatins, and ascorbate-glutathione for As tolerance in wheat. A total of eight varieties were exposed to different arsenate treatments (0, 1, 5, 10, 50, 100, 200, 500, 1000, 2000, and 10,000 µM) initially to screen effective treatment as well as contrasting varieties via Weibull distribution frequency for further analysis. The Weibull analysis found 200 µM as the most effective treatment in the present study. Selected varieties were analyzed for accumulation of total As and As speciation, oxidative stress (malondialdehyde, hydrogen peroxide), antioxidants (superoxide dismutase, catalase, peroxidase), phytochelatins, and ascorbate-glutathione cycle (glutathione-S-transferase, glutathione reductase, glutathione peroxidase, ascorbate peroxidase). Tolerant varieties showed less accumulation and translocation of total As, arsenate, and arsenite to the shoots compared with sensitive varieties under 200 µM treatment. Low concentration in tolerant varieties correlated with better growth and development response. Tolerant varieties showed higher induction of metabolites (glutathione, phytochelatins) compared to sensitive ones. Furthermore, tolerant varieties showed better performance of antioxidant and ascorbate-glutathione cycle enzymes in response to As exposure. The findings of the present study provided great insight into the wheat tolerance mechanism upon As exposure between contrasting varieties.

Denitrification responses to increasing cadmium exposure in Baltic Sea sediments.

Benthic ecosystems have come under intense pressure, due to eutrophication-driven oxygen decline and industrial metal contamination. One of the most toxic metals is Cadmium (Cd), which is lethal to many aquatic organisms already at low concentrations. Denitrification by facultative anaerobic microorganisms is an essential process to transform, but also to remove, excess nitrate in eutrophied systems. Cd has been shown to decrease denitrification and sequester free sulfide, which is available when oxygen is scarce and generally inhibits complete denitrification (i.e. N2O to N2). In polluted sediments, an interaction between oxygen and Cd may influence denitrification and this relationship has not been studied. For example, in the Baltic Sea some sediments are double exposed to both Cd and hypoxia. In this study, we examined how the double exposure of Cd and fluctuations in oxygen affects denitrification in Baltic Sea sediment. Results show that oxygen largely regulated N2O and N2 production after 21 days of exposure to Cd (ranging from 0 to 500 µg/L, 5 different treatments, measured by the isotope pairing technique (IPT)). In the high Cd treatment (500 µg/L) the variation in N2 production increased compared to the other treatments. Increases in N2 production are suggested to be an effect of 1) enhanced nitrification that increases NO3- availability thus stimulating denitrification, and 2) Cd successfully sequestrating sulfide (yielding CdS), which allows for full denitrification to N2. The in situ field sediment contained initially high Cd concentrations in the pore water (∼10 µg/L) and microbial communities might already have been adapted to metal stress, making the effect of low Cd levels negligible. Here we show that high levels of cadmium pollution might increase N2 production and influence nitrogen cycling in marine sediments.

Different response to Cd stress in domesticated and wild safflower (Carthamus spp.).

Cadmium (Cd) can stress plants by affecting various physiological functions. Cd stress-response mechanisms were investigated in two genotypes of domesticated safflower (Carthamus tinctorius) and a population of wild safflower (Carthamus oxycantous) to explore potential differences in tolerance mechanisms of these species. A hydroponic experiment was conducted with 6-day-old safflower plants. Genotypes AC-Sterling (tolerant) and Saffire (semi-tolerant) from C. tinctorius, and Arak (sensitive) a population from C. oxycantouswere subjected to three concentrations of Cd (i.e., 0, 1, and 20 µM CdCl2). Genotypic differences were detected in Cdtolerance index, Cd concentration in shoots and roots, Cd translocation to shoots, Cd bound to cell walls, superoxide dismutase (SOD) activity, lipid peroxidation, and phytochelatins accumulation in safflower plants upon exposure to CdCl2. Results indicate that genotypic differences were more obvious in the presence of low (i.e., 1 µM) rather than high (i.e., 20 µM) CdCl2 concentrations. Comparing genotypes, root and shoot Cd accumulation was highest in the semi-tolerant genotype. Cadmium translocation to shoots was increased with increasing tolerance. The percentage of Cd bound to root cell walls was higher in the tolerant genotype, but only with low CdCl2 addition. Furthermore, in the tolerant genotype, SOD activity was lowest in both roots and shoots with low CdCl2 addition but highest with high CdCl2 addition, while the opposite was found for phytochelatins. Lipid peroxidation was decreased with Cd tolerance at both CdCl2 concentrations. We conclude that safflower relies mainly on binding Cd to the cell walls and the formation of phytochelatins in root and shoot tissues, in order to handle the Cd stress, evidenced by lessening Cd-induced lipid peroxidation.

Silicon Influences Soil Availability and Accumulation of Mineral Nutrients in Various Plant Species.

Silicon (Si) effects on mineral nutrient status in plants are not well investigated. It is known that Si has a beneficial effect on plants under stressed conditions. The aim was to make a state of the art investigation of the Si influence: (1) on nutrient availability in four different soil types, namely clayish, sandy, alum shale and submerged soil; and (2) on accumulation of various nutrients in maize, lettuce, pea, carrot and wheat growing in hydroponics. Soil was treated with K2SiO3 corresponding to 80 and 1000 kg Si ha-1 and the nutrient medium with 100, 500, 1000 and 5000 µM Si. In general, Si effects were similar in all analyzed plant species and in all soil types tested. Results showed that, in soil, Si increased the availability of Ca, P, S, Mn, Zn, Cu and Mo and that of Cl and Fe tended to increase. The availability of K and Mg was not much affected by Si. Uptake from solution of S, Mg, Ca, B, Fe, and Mn increased; N, Cu, Zn and K decreased; P decreased/increased; and Cl and Mo was not influenced. Translocation to shoot of Mg, Ca, S, Mn, and Mo increased; Fe, Cu and Zn decreased; and K, P, N, Cl and B was not affected. It was concluded that, if plants had been cultivated in soil, Si-maintained increased availability of nutrients in the soil solution would probably compensate for the decrease in tissue concentration of those nutrient elements. The study shows that Si also influences the nutrient uptake in non-stressed plants.

Phytofiltration of arsenic by aquatic moss (Warnstorfia fluitans).

This work investigates whether aquatic moss (Warnstorfia fluitans) originating from an arsenic (As)-contaminated wetland close to a mine tailings impoundment may be used for phytofiltration of As. The aim was to elucidate the capacity of W. fluitans to remove As from arsenite and arsenate contaminated water, how nutrients affect the As uptake and the proportion of As adsorption and absorption by the moss plant, which consists of dead and living parts. Arsenic removal from 0, 1, or 10% Hoagland nutrient solution containing 0-100 µM arsenate was followed over 192 h, and the total As in aquatic moss after treatment was analysed. The uptake and speciation of As in moss cultivated in water containing 10 µM arsenate or arsenite were examined as As uptake in living (absorption + adsorption) and dead (adsorption) plant parts. Results indicated that W. fluitans removed up to 82% of As from the water within one hour when 1 µM arsenate was added in the absence of nutrients. The removal time increased with greater nutrient and As concentrations. Up to 100 µM As had no toxic effect on the plant biomass. Both arsenite and arsenate were removed from the solution to similar extents and, independent of the As species added, more arsenate than arsenite was found in the plant. Of the As taken up, over 90% was firmly bound to the tissue, a possible mechanism for resisting high As concentrations. Arsenic was both absorbed and adsorbed by the moss, and twice as much As was found in living parts as in dead moss tissue. This study revealed that W. fluitans has potential to serve as a phytofilter for removing As from As-contaminated water without displaying any toxic effects of the metalloid.

Comparison of silicon nanoparticles and silicate treatments in fenugreek.

Silicon (Si) fertilization improves crop cultivation and is commonly added in the form of soluble silicates. However, most natural plant-available Si originates from plant formed amorphous SiO2 particles, phytoliths, similar to SiO2-nanoparticles (SiNP). In this work we, therefore, compared the effect by sodium silicate and that of SiNP on Si accumulation, activity of antioxidative stress enzymes catalase, peroxidase, superoxide dismutase, lignification of xylem cell walls and activity of phenylalanine ammonia-lyase (PAL) as well as expression of genes for the putative silicon transporter (PST), defensive (Tfgd 1) and phosphoenolpyruvate carboxykinase (PEPCK) and protein in fenugreek (Trigonella foenum-graecum L.) grown in hydroponics. The results showed that Si was taken up from both silicate and SiNP treatments and increasing sodium silicate addition increased the translocation of Si to the shoot, while this was not shown with increasing SiNP addition. The silicon transporter PST was upregulated at a greater level when sodium silicate was added compared with SiNP addition. There were no differences in effects between sodium silicate and SiNP treatments on the other parameters measured. Both treatments increased the uptake and accumulation of Si, xylem cell wall lignification, cell wall thickness, PAL activity and protein concentration in seedlings, while there was no effect on antioxidative enzyme activity. Tfgd 1 expression was strongly downregulated in leaves at Si addition. The similarity in effects by silicate and SiNP would be due to that SiNP releases silicate, which may be taken up, shown by a decrease in SiNP particle size with time in the medium.

Arsenic concentrations in local aromatic and high-yielding hybrid rice cultivars and the potential health risk: a study in an arsenic hotspot.

The presence of high levels of arsenic (As) in rice fields has negative effects on the health of those consuming rice as their subsistence food. This study determined the variation in total As concentration in local aromatic rice (LAR) (kalijira) and two high-yielding varieties (HYVs) (BRRI dhan 32 and BRRI dhan 28) grown in paddy fields in Matlab, Bangladesh, an As hotspot with elevated As levels in groundwater. Mature rice grain samples and soil samples were collected from different paddy fields, and the As concentrations in both the de-husked grains and the husks of the three rice cultivars were analysed to identify the safest of the three cultivars for human consumption. The results showed that the total As concentration was higher (0.09-0.21 mg As kg-1) in the de-husked grains of LAR than in the husks, while the opposite was found for the HYV rice. Moreover, the As concentration in soil samples was 2 to 5-fold higher for the LAR than for the HYVs, but the As accumulation factor (AF) was lower in the LAR (0.2-0.4%) than in the HYVs (0.9-1%). Thus, LAR can be considered the safest of the three cultivars for human consumption owing to its low AF value. Furthermore, due to the low AF, growing LAR instead of HYVs in soils with slightly elevated As levels could help improve the food safety level in the food chain.

Silicate reduces cadmium uptake into cells of wheat.

Cadmium (Cd) is a health threat all over the world and high Cd content in wheat causes high Cd intake. Silicon (Si) decreases cadmium content in wheat grains and shoot. This work investigates whether and how silicate (Si) influences cadmium (Cd) uptake at the cellular level in wheat. Wheat seedlings were grown in the presence or absence of Si with or without Cd. Cadmium, Si, and iron (Fe) accumulation in roots and shoots was analysed. Leaf protoplasts from plants grown without Cd were investigated for Cd uptake in the presence or absence of Si using the fluorescent dye, Leadmium Green AM. Roots and shoots of plants subjected to all four treatments were investigated regarding the expression of genes involved in the Cd uptake across the plasma membrane (i.e. LCT1) and efflux of Cd into apoplasm or vacuole from the cytosol (i.e. HMA2). In addition, phytochelatin (PC) content and PC gene (PCS1) expression were analysed. Expression of iron and metal transporter genes (IRT1 and NRAMP1) were also analysed. Results indicated that Si reduced Cd accumulation in plants, especially in shoot. Si reduced Cd transport into the cytoplasm when Si was added both directly during the uptake measurements and to the growth medium. Silicate downregulated LCT1 and HMA2 and upregulated PCS1. In addition, Si enhanced PC formation when Cd was present. The IRT1 gene, which was downregulated by Cd was upregulated by Si in root and shoot facilitating Fe transport in wheat. NRAMP1 was similarly expressed, though the effect was limited to roots. This work is the first to show how Si influences Cd uptake on the cellular level.

Novel Field Data on Phytoextraction: Pre-Cultivation With Salix Reduces Cadmium in Wheat Grains.

Cadmium (Cd) is a health hazard, and up to 43% of human Cd intake comes from wheat products, since Cd accumulates in wheat grains. Salix spp. are high-accumulators of Cd and is suggested for Cd phytoextraction from agricultural soils. We demonstrate, in field, that Salix viminalis can remove Cd from agricultural soils and thereby reduce Cd accumulation in grains of wheat subsequently grown in a Salix-treated field. Four years of Salix cultivation reduce Cd concentration in the soil by up to 27% and in grains of the post-cultivated wheat by up to 33%. The higher the plant density of the Salix, the greater the Cd removal from the soil and the lower the Cd concentration in the grains of post-cultivated wheat, the Cd reduction remaining stable several years after Salix cultivation. The effect occurred in both sandy and clayey soil and in winter and spring bread wheat cultivars. Already one year of Salix cultivation significantly decrease Cd in post grown wheat grains. With this field experiment we have demonstrated that phytoextraction can reduce accumulation of a pollutant in post-cultivated wheat and that phytoextraction has no other observed effect on post-cultivated crops than reduced uptake of the removed pollutant.

Silicon modifies root anatomy, and uptake and subcellular distribution of cadmium in young maize plants.

BACKGROUND AND AIMS: Silicon (Si) has been shown to ameliorate the negative influence of cadmium (Cd) on plant growth and development. However, the mechanism of this phenomenon is not fully understood. Here we describe the effect of Si on growth, and uptake and subcellular distribution of Cd in maize plants in relation to the development of root tissues. METHODS: Young maize plants (Zea mays) were cultivated for 10 d hydroponically with 5 or 50 µm Cd and/or 5 mm Si. Growth parameters and the concentrations of Cd and Si were determined in root and shoot by atomic absorption spectrometry or inductively coupled plasma mass spectroscopy. The development of apoplasmic barriers (Casparian bands and suberin lamellae) and vascular tissues in roots were analysed, and the influence of Si on apoplasmic and symplasmic distribution of (109)Cd applied at 34 nm was investigated between root and shoot. KEY RESULTS: Si stimulated the growth of young maize plants exposed to Cd and influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root. Si did not affect the distribution of apoplasmic and symplasmic Cd in maize roots, but considerably decreased symplasmic and increased apoplasmic concentration of Cd in maize shoots. CONCLUSIONS: Differences in Cd uptake of roots and shoots are probably related to the development of apoplasmic barriers and maturation of vascular tissues in roots. Alleviation of Cd toxicity by Si might be attributed to enhanced binding of Cd to the apoplasmic fraction in maize shoots.

Increased metal tolerance in Salix by nicotinamide and nicotinic acid.

We have earlier shown that nicotinamide (NIC) and nicotinic acid (NiA) can induce defence-related metabolism in plant cells; e.g. increase the level of glutathione. Here we investigated if NIC and NiA could increase the metal tolerance in metal sensitive clones of Salix viminalis and whether this would be mediated via increased glutathione level. Salix clones, sensitive or tolerant to zinc (Zn), copper (Cu) and cadmium (Cd) were grown in the presence of heavy metals (Cd, Cu or Zn) or NIC and NiA as well as in combination. In addition, the influence of N-acetyl-cystein (NAC) and l-2-oxothiazolidine 4-carboxylate (OTC), stimulators of reduced glutathione (GSH) biosynthesis, and the glutathione biosynthesis inhibitor buthionine sulfoximine (BSO) was analysed. Tolerance was measured as effects on root and shoot dry weight, and the glutathione and metal concentrations in the tissues were analysed. Results showed that NIC and NiA decreased the toxic effects of Cd, Cu and Zn on growth significantly in sensitive clones, but also to some extent in tolerant clones. However, the glutathione level and metal concentration did not change by NIC or NiA addition. Treatment with NAC, OTC or BSO did not per se influence the sensitivity to Cd, although the glutathione level increased in the presence of NAC and OTC and decreased in response to BSO. The results suggest that NIC and NiA increased the defence against heavy metals but not via glutathione formation per se.

Cadmium uptake and interaction with phytochelatins in wheat protoplasts.

In order to investigate the role of phytochelatins in short-time uptake of Cd(2+) into the cytosol of wheat protoplasts, a new method was applied, using fluorescence microscopy and the heavy metal-specific fluorescent dye, 5-nitrobenzothiazole coumarin, BTC-5N. The uptake of Cd(2+) into protoplasts from 5- to 7-day-old wheat seedlings (Triticum aestivum, L. cv. Kadett) was lower in protoplasts from seedlings raised in the presence of 1 microM CdCl(2), than in the absence. Presence of CdCl(2) in the cultivation medium increased the content of phytochelatins (PCs) in the protoplasts. When seedlings were raised in the presence of both Cd(2+) and buthionine sulfoximine (BSO), an inhibitor of glutathione (GSH) synthesis, only little PC was found in the protoplasts. Pre-treatment with BSO alone did not affect the content of PC, but inhibited that of GSH. The inhibition of GSH was independent of pre-treatment with Cd(2+). Unidirectional flux analyses, using (109)Cd(2+), showed approximately the same uptake pattern of Cd(2+) as did the fluorescence experiments showing the cytosolic uptake of Cd(2+). Thus, the diminished uptake of Cd(2+) into protoplasts from cadmium-pre-treated plants was not depending on PCs. Instead, it is likely that pre-treatment with Cd(2+) causes a down-regulation of the short-term Cd(2+) uptake, or an up-regulation of the Cd(2+) extrusion. Moreover, since addition of Cd(2+) to protoplasts from control plants caused a cytosol acidification, it is likely that a Cd(2+/)H(+)-antiport mechanism is involved in the extrusion of Cd(2+) from these protoplasts.

A new method to detect cadmium uptake in protoplasts.

The mechanism for cadmium (Cd2+) uptake into the cytosol of protoplasts from 5- to 7-day-old wheat seedlings (Triticum aestivum L. cv. Kadett) was investigated by a new method, using fluorescence microscopy and the heavy metal-specific fluorescent dye, 5-nitrobenzothiazole coumarin, BTC-5N. Cadmium fluorescence gradually increased in the cytosol of shoot and root protoplasts upon repeated additions of CdCl2 to the external medium, reflecting an uptake of Cd2+. The uptake was inhibited by calcium and potassium chloride, as well as by Verapamil and tetraethylammonium (TEA), inhibitors of calcium and potassium channels, respectively. Calcium competitively inhibited the cadmium uptake. The metabolic inhibitors vanadate and dinitrophenol partly inhibited the uptake, suggesting it was dependent on membrane potential. The results indicate that cadmium is taken up by channels permeable to both calcium and potassium. The total uptake of cadmium into the protoplasts was also detected by unidirectional flux analyses using 109Cd2+, and showed approximately the same maximal concentration of Cd2+ as the fluorescence measurements. By combining the two methods it is possible to detect both uptake into the cytosol and into the vacuole.

Interclonal variation of heavy metal interactions in Salix viminalis.

In the complex chemistry of soil, interactions between metals can be expected and these affect the uptake of the metals by the plants. The role of the metal-metal interaction may vary between different plants. This study was performed to investigate if variations exist in the interactions between Cd, Cu, and Zn on toxicity and accumulation of these metals in different clones of Salix viminalis. Two studies were performed. First, to study interaction at uptake, 10 clones with high or low accumulation capacity of Cd, Cu, and Zn, respectively, were treated with 0.3 microM Cd, 0.1 microM Cu, and 3 microM Zn (all three metals at the same time or separately). Second, to study the effect of one of the metals on the sensitivity of the plant to the other metals, three clones with high or low sensitivity to each of the three metals were used in a modified Weibull analysis. Examination of the results shows that interclonal variation exists in effects of metal interaction on metal accumulation and sensitivity exists. The uptake experiment showed that accumulation of Cu was decreased by the other metals, but only in clones with high Cu-accumulating properties because of decreased net uptake of Cu. The accumulation of Zn in roots was increased two- to threefold in all clones in the presence of the other metals because of a decreased translocation of Zn to the shoot. The accumulation of Cd was not changed by the presence of the other metals in any of the clones. The second experiment showed that the effect of interactions between the different metals on metal toxicity was present in all clones but appeared most frequently in the clone with high Zn resistance. Synergistic effects between Cu and Zn in the Zn-resistant clone suggested that this clone had evolved an additional site of toxic action that was absent in the other clones.