Selenium distribution in ryegrass and its antioxidant role as affected by sulfur fertilization

Selenium (Se) essentiality to plants has not been demonstrated although evidence indicates that it plays a significant role as antioxidant in higher plants. Research concerning to the uptake and allocation of Se in plant tissues is reported in numerous works. However, the effect of sulfur (S) on both the distribution and the antioxidant ability of Se in selenite-treated plant remains unclear. In this work the effect of S application (0–100 mg S kg⁻¹ soil) on shoot Se concentration of Lolium perenne cv. Aries was studied. Se distribution into different fractions of plants supplied with selenite (2 mg Se kg⁻¹ soil) and the state of the antioxidative system were determined. Results showed that shoot Se concentration decreased at least 33% by S application. Plants supplied with S registered the lowest GSH-Px activity and the highest lipid peroxidation. Most of Se was incorporated into the organic fraction of the plant tissue irrespective of the S treatment. However, a significant decrease of both the soluble protein and the amino acid fraction occurred, and the residual Se fraction seemed to increase at expense of the organic-Se soluble fraction. Although no essential selenoproteins have been clearly identified in vascular plants, the decrease of the soluble protein fraction and the different pattern of protein synthesized (SDS-PAGE analysis) may explain the observed reduction of the GSH-Px activity.     

Selenium uptake and its influence on the antioxidative system of white clover as affected by lime and phosphorus fertilization

Selenium (Se) is regarded as an antioxidant in animal and human nutrition, but its biological role in plants needs to be clarified. Plants vary considerably in their ability to tolerate Se, and their biochemical response to Se may be affected by liming or P fertilization. Two greenhouse experiments were conducted with white clover (Trifolium repens L.) to evaluate Se accumulation, tolerance, and the antioxidant response at increasing selenite supply levels (from 0 to 60 g Se ha⁻¹) and the effect of lime and P on both the Se uptake and the antioxidant activity of plants treated with 0, 20 and 40 g Se ha⁻¹. Selenium concentration in plant tissues was increased by Se applications, and plant growth was reduced at Se supply levels above 20 g ha⁻¹. At shoot concentration up to 200 μg kg⁻¹ DW, Se promoted antioxidant ability by increasing the free radical scavenging activity and by inhibiting lipid peroxidation (TBARS), whereas above this level TBARS accumulation increased. Significant changes in the activities of peroxidase (POD) and ascorbate peroxidase (APX) enzymes were also observed as a consequence of the increase in shoot Se concentration. The application of lime and P improved the plant nutrition, which increased the dry matter yield and enhanced the plant’s antioxidative system. Under different combinations of soil acidity and P fertilization a differential uptake of Se by the plant occurred. These factors appear to be responsible for beneficial or detrimental effects of Se in terms of lipid peroxidation of biological membranes and the activation of POD and APX in white clover.     

Selenium as an anti-oxidant and pro-oxidant in ryegrass

Selenium is an essential element for antioxidation reactions in human and animals. In order to study its biological role in higher plants, ryegrass (Lolium perenne) was cultivated in a soil without Se or amended with increasing dosages of H₂SeO₄ (0.1, 1.0, 10.0 and 30.0 mg Se kg⁻¹). Ryegrass was harvested twice and the yields were analyzed for antioxidative systems and growth parameters. Selenium exerted dual effects: At low concentrations it acted as an antioxidant, inhibiting lipid peroxidation, whereas at higher concentrations, it was a pro-oxidant, enhancing the accumulation of lipid peroxidation products. The antioxidative effect was associated with an increase in glutathione peroxidase (GSH-Px) activity, but not with superoxide dismutase (SOD) and αα-tocopherol, which was the only tocopherol detected. In the second yield, the diminished lipid peroxidation due to a proper Se addition coincided with promoted plant growth. The oxidative stress found at the Se addition level ≥ 10 mg kg⁻¹ resulted in drastic yield losses. This result indicates that the toxicity of Se can be attributed, in addition to metabolic disturbances, to its pro-oxidative effects. Neither the growth-promoting nor the toxic effect of Se could be explained by the changes in the total chlorophyll concentration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparative effects of selenite and selenite on the glutathione-related enzymes activity in pig blood platelets

The effects of inorganic selenium (Se) compounds (sodium selenite and selenate) on the activities of glutathione-related enzymes (glutathione peroxidase, glutathione-S-transferase [GST] and glutathione reductase [GR]) in pig blood platelets were investigated in vitro. GST activity in blood platelets treated with 10⁻⁴ M of selenite was reduced to 50%, whereas no decrease GST activity was observed after the treatment of platelets with the same dose of selenate. In platelets incubated with physiological doses (10⁻⁷, and 10⁻⁶ M) of Se compounds, the activity of glutathione peroxidase (GSH-Px) was enhanced (about 20%). GR activity after the exposure of platelets to tested Se compounds was unaffected.     

Regulation of sulphur assimilation in lettuce plants in the presence of selenium

Selenium (Se) is considered an essential trace element for animals because of its nutritional and clinical value, including its special relevance in cancer prevention, and thus Se is at present used in biofortification programmes. However, possible effects of Se application on S metabolism and plant growth are still not clear. Thus, we analysed the effect that Se application in two different forms (selenate versus selenite) exerts on the S metabolism in lettuce plants grown for 66 days. Our results indicate that the application of selenite as opposed to selenate does not affect the foliar concentration of S. With respect to different enzymes in charge of sulphate (SO₄²⁻) assimilation, the ATP-sulphurylase activity varies only with the application of different rates of selenate, while the activity of O-acetylserine(thiol)lyase (OAS-TL) and serine-acetyltransferase (SAT) increase in activity mainly when selenite is applied. Finally, the concentration of cysteine (Cys) and total thiols (SH-total), fundamentally in the selenate treatments, increased with shoot biomass. In conclusion, this study confirms that the form and application rate of Se affects S assimilation, selenate being the more suitable form to improve effectiveness of the biofortification programme with this trace element.     

Effect of byproduct,nitrogen fertilizer,and zeolite on phosphate rock dissolution and extractable phosphorus in acid soil

The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively. In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A nutrient solution study was conducted to compare the availability of selenite and selenate to perennial ryegrass (Lolium perenne L. cv. Evening Shade) and strawberry clover (Trifolium fragiferrum L. cv. O'Conner) at basal concentrations of SO₄ (0.5 mM) and PO₄ (2 μM) similar to those found in soil solution. Concentrations up to 5 mM SO₄ and 200 μM PO₄ allowed quantitative comparison of the efficacy of the competing ions. In both species, selenite was more phytotoxic than selenate, especially for shoot growth. Selenate was less toxic, and tended to preferentially inhibit root growth. Translocation percentages were much higher with selenate (≥84%) than with selenite (≤47%). A 10-fold increase in sulfate decreased uptake from selenate by >90% in both species. In ryegrass, 10-fold increases in phosphate caused 30% to 50% decreases in Se accumulation from selenite, but in clover such decreases only occurred in the roots. Sulfate-selenate antagonisms were thus stronger than phosphate-selenite antagonisms. Nonetheless, conventional nutrient solutions with millimolar phosphate will significantly underestimate Se availability from selenite, and moderate levels of sulfate salinity can inhibit selenate uptake sufficiently to reverse the apparent relative availability of the two forms of Se. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen-Use Efficiency in Relation to Different Forms and Application Rates of Se in Lettuce Plants

The main objective of this work was to determine whether nitrogen-use efficiency (NUE) was affected by the application of different forms and dosages of selenium (Se), to ascertain the influence of this trace element in a biofortification program in lettuce plants. The parameters analyzed were biomass, NO₃ ⁻ concentration, and total reduced N as well as those defining NUE in plants: total nitrogen accumulation (TNA), nitrogen efficient ratio (NER), nitrogen-utilization efficiency (NUtE), and nitrogen-uptake efficiency (NUpE). According to our results, application of Se affected NUE in lettuce plants. With the application of selenite as well as selenate NO₃ ⁻ uptake was reduced, thus diminishing the NUpE and the foliar concentration of this anion. In addition, selenate application at a rate of 20 μM and selenite at 5 μM induced N utilization, reflected by an increase in NER and NUtE; this result coincides with augmented biomass production. Notably, our results indicate that when Se is applied at high rates, selenite is far more phytotoxic, this being associated with a higher reduction of NUE in these plants.     

Fate of polycyclic aromatic hydrocarbons (PAH) in the rhizosphere and mycorrhizosphere of ryegrass

Polycyclic aromatic hydrocarbons (PAH) can be degraded in the rhizosphere but may also interact with vegetation by accumulation in plant tissues or adsorption on root surface. Previous studies have shown that arbuscular mycorrhizal (AM) fungi contribute to the establishment and maintenance of plants in a PAH contaminated soil. We investigated the fate of PAH in the rhizosphere and mycorrhizosphere including biodegradation, uptake and adsorption. Experiments were conducted with ryegrass inoculated or not with Glomus mosseae P2 (BEG 69) and cultivated in pots filled with soil spiked with 5 g kg⁻¹ of anthracene or with 1 g kg⁻¹ of a mixture of 8 PAH in a growth chamber. PAH were extracted from root surfaces, root and shoot tissue and rhizosphere soil and were analysed by GC-MS. In both experiments, 0.006 – 0.11‰ of the initial extractable PAH concentration were adsorbed to roots, 0.003 – 0.16‰ were found in root tissue, 0.001‰ in shoot tissue and 36 – 66% were dissipated, suggesting that the major part of PAH dissipation in rhizosphere soil was due to biodegradation or biotransformation. With mycorrhizal plants, anthracene and PAH were less adsorbed to roots and shoot tissue concentrations were lower than with non mycorrhizal plants, which could contribute to explain the beneficial effect of AM fungi on plant survival in PAH contaminated soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Differential Effects of Sodium Selenite and Nano-Se on Growth Performance,Tissue Se Distribution,and Glutathione Peroxidase Activity of Avian Broiler

The present research evaluated differential effects of sodium selenite and nano-Se on growth performance, tissue Se distribution, and glutathione peroxidase (GSH-Px) activity of avian broiler. Broilers were randomly segregated into 12 groups so that three replicates were available for each of the three treatments (T-1, T-2, and T-3) and control groups. The control groups were fed basal diets without Se addition. T-1, T-2, and T-3 were fed with diets containing 0.2 mg kg⁻¹ sodium selenite, 0.2 mg kg⁻¹ nano-Se, and 0.5 mg kg⁻¹ nano-Se, respectively. Compared with the control, Se supplementation remarkably improved daily weight gain and survival rate and decreased feed conversion ratio (P < 0.05). However, no significant difference was observed between T-1, T-2, and T-3. The tissue Se content was significantly higher (P < 0.05) in Se-supplemented groups than the control, and T-3 showed the highest. Furthermore, higher Se content was observed in liver, and there was a significant difference (P < 0.05) compared with that in muscle. As for serum and hepatic GSH-Px activities, Se supplementation remarkably improved GSH-Px activity (P < 0.05), and there was no significant difference (P > 0.05) between treatments (T-1, T-2, and T-3).     

Variation in selenium tolerance and accumulation among 19 Arabidopsis thaliana accessions

Selenium (Se) is an essential element for many organisms but also toxic at higher levels. The objective of this study was to identify accessions from the model species Arabidopsis thaliana that differ in Se tolerance and accumulation. Nineteen Arabidopsis accessions were grown from seed on agar medium with or without selenate (50 microM) or selenite (20 microM), followed by analysis of Se tolerance and accumulation. Tissue sulfur levels were also compared. The Se Tolerance Index (root length+Se/root length control) varied among the accessions from 0.11 to 0.44 for selenite and from 0.05 to 0.24 for selenate. When treated with selenite, the accessions differed by two-fold in shoot Se concentration (up to 250 mgkg(-1)) and three-fold in root Se concentration (up to 1000 mgkg(-1)). Selenium accumulation from selenate varied 1.7-fold in shoot (up to 1000 mgkg(-1)) and two-fold in root (up to 650 mgkg(-1)). Across all accessions, a strong correlation was observed between Se and S concentration in both shoot and root under selenate treatment, and in roots of selenite-treated plants. Shoot Se accumulation from selenate and selenite were also correlated. There was no correlation between Se tolerance and accumulation, either for selenate or selenite. The F(1) offspring from a cross between the extreme selenate-sensitive Dijon G and the extreme selenate-tolerant Estland accessions showed intermediate selenate tolerance. In contrast, the F(1) offspring from a cross between selenite-sensitive and -tolerant accessions (Dijon GxCol-PRL) were selenite tolerant. The results from this study give new insight into the mechanisms of plant selenium (Se) tolerance and accumulation, which may help develop better plants for selenium phytoremediation or as fortified foods.     

Uptake and transport of selenite and selenate by soybean seedlings of two genotypes

In an attempt to address the role of biological behavior on Se uptake by soybean crop and the genotype effects, experiments with time and concentration sequences of Se uptake by seedlings in Hoagland solution are conducted using selenite and selenate respectively. Two soybean cultivars Tong-ai 405 (TA) and Qidong Green-skin (QG) are used as different genotypes. In presence of selenite, Se uptake by both roots and shoots exhibited a linear increase with the growing time at 5 M and with the solution Se concentrations. However, in presence of selenate, the linear response to growing time is only valid before 24 h of growing. While root Se uptake is much slower under selenate than under selenite in the time sequence experiment, shoot Se levels are similar between the two different Se form treatments. Nevertheless, in the experiment of concentration sequence, either root Se or shoot Se responses linearly to solution Se concentration regardless of the Se forms supplied. A big discrepancy of root Se level with a similarity of shoot Se between the two cultivars is observed in the concentration sequence experiment. This supports a much faster passive uptake of selenite but more or less an active uptake of selenate by soybean seedlings. Comparatively, cultivars TA have a consistently higher Se concentration than QG both in roots and shoots under selenate, while no difference of concentration ratio of shoot to root is recognized between them. The higher Se level in seed grains, therefore, may be accounted for not by Se transport form root to shoot but by greater ability of Se uptake and retention under selenate by the former cultivar. Therefore, not only forms of Se supply but also genotype difference affects the Se bioavailability by different soybean cultivars. This should be taken into account for screening the high Se-efficiency plants or cultivars to improve the Se supply of the food chain.     

Evaluation of the Potential of Peas (Pisum sativum L.) to Be Used in Selenium Biofortification Programs Under Mediterranean Conditions

Selenium (Se), which has antioxidant, anticancer, and antiviral properties, is an essential micronutrient for humans and animals. This micronutrient is found in high quantity in legumes. Peas have an ever-increasing importance in Spain, and to increase their nutritional value, two foliar Se fertilizers: sodium selenate and sodium selenite, at five different rates: 0, 10, 20, 40, 80 g?ha^?1, were studied during the 2010/2011 crop season on semiarid Mediterranean conditions. Sodium selenate was much more effectively taken up by plants compared to sodium selenite. There was a strong linear relationship between the total Se content and Se rate in both sodium selenate and selenite. For each gram of Se fertilization as either sodium selenate or sodium selenite, the increase of total Se concentration in the grain was 148 and 19 μg Se?kg^?1 dry weight, respectively. Ingestion of 100 g of peas previously fertilized with 10 g of sodium selenate per hectare would result in an intake of 179 μg of Se. This is almost 90 % of the daily recommended dose needed to reduce the chance of some cancers and about 179 % of the minimum concentration required to prevent Se deficiency diseases in animals. The pea has shown to have a strong ability to uptake and accumulate Se under Mediterranean conditions; therefore, this would make it a very strong candidate for inclusion in biofortification programs aiming to increase Se in the food chain.     

Antioxidative and growth-promoting effect of selenium on senescing lettuce

In human and animal cells, Se plays an essential role in antioxidation and exerts an antiaging function but it is toxic at high dietary intake. To increase its intake in forage and foodstuffs, Se fertilization is adopted in some countries where soils are low in bioavailable Se, even though higher plants are regarded not to require Se. To test its ability to counteract senescence-related oxidative stress in higher plants, a pot experiment was carried out with lettuce (Lactuca sativa) cultivated with increasing amounts of H₂SeO₄. The yields harvested 7 or 14 weeks after sowing revealed that a low Se dosage (0.1 mg kg^–1 soil) stimulated the growth of senescing seedlings (dry weight yield by 14%) despite a decreased chlorophyll concentration. The growth-promoting function was related to diminished lipid peroxidation. In young and senescing plants, the antioxidative effect of Se was associated with the increased activity of glutathione peroxidase (GSH-Px). In the senescing plants, the added Se strengthened the antioxidative capacity also by preventing the reduction of tocopherol concentration and by enhancing superoxide dismutase (SOD) activity. When no Se was added, tocopherols and SOD activity diminished during plant senescence. The higher Se dosage (1.0 mg kg^–1 soil) was toxic and reduced the yield of young plants. In the senescing plants, it diminished the dry weight yield but not the fresh weight yield.     

Tolerance of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to high soil and solution selenium levels

The fertilisation of wheat crops with Se is a cost-effective method of enhancing the concentration of organic Se in grain, in order to increase the Se intake of animals and humans. It is important to avoid phytotoxicity due to over-application of Se. Studies of phytotoxicity of Se in wheat grown in Australia, where rainfall and grain yield are usually relatively low, have not been reported previously, and overseas studies have had varied results. This study used trials conducted in the field, glasshouse and laboratory to assess Se phytotoxicity in wheat. In field trials that used rates of up to 120 g ha^–1Se as selenate, and in pilot trials that used up to 500 g ha^–1 Se soil-applied or up to 330 g ha^–1 Se foliar-applied, with soils of low S concentrations (2–5 mg kg^–1), no Se toxicity symptoms were observed. In pot trials of four weeks duration, the critical tissue level for Se toxicity was around 325 mg kg^–1 DW, a level attained by addition to the growth medium of 2.6 mg kg^–1 Se as selenate. Solution concentrations above 10 mg L^–1 Se inhibited early root growth of wheat in laboratory studies, with greater inhibition by selenite than selenate. For selenite, Se concentrations around 70 mg L^–1 were required to inhibit germination, while for selenate germination % was unaffected by a solution concentration of 150 mg L^–1 Se. Leaf S concentration and content of wheat increased three-fold with the addition of 1 mg kg^–1 Se as selenate to the growth medium. This effect is probably due to the induction of the S deficiency response of the main sulphate transporter. This study found wheat to be more Se-tolerant than did earlier studies of tobacco, soybeans and rice. We conclude that Se phytotoxicity in wheat will not be observed at the range of Se application rates that would be used to increase grain Se for human consumption (4–200 g ha^–1 Se as selenate, which would result in soil and tissue levels well below those seen in the above studies), even when – as is common in Australia – soil S concentration and grain yield are low.     

Removing Selenate from Groundwater with a Vegetable Oil-Based Biobarrier

Vegetable oil–based permeable reactive biobarriers (PRBs) were evaluated as a method for remediating groundwater containing unacceptable amounts of selenate. PRBs formed by packing laboratory columns with sand coated with soybean oil were used. In an initial 24-week study a simulated groundwater containing 10 mg L⁻¹ selenate-Se was supplied to three soil columns and the selenate and selenite content of the effluent waters monitored. Two of the soil columns were effective at removing selenate and, during the final 21 weeks of the study, effluents from these columns contained almost no selenate or selenite. Almost all (95%) of the selenate removed was recovered as immobilized selenium sequestered in the solid matrix of the column. For unknown reasons, the third column failed to reduce selenate. A second study looked at the ability of PRBs to remove selenate when nitrate was present. As was done in the first study, three columns were evaluated but this time the water supplied to the columns contained 20 mg L⁻¹ nitrate-N and 10 mg L⁻¹ selenate-Se. Nitrate quickly disappeared from the effluents of these columns and during the final 23 weeks of the study, the nitrate content of the effluent water averaged less than 0.03 μg ml⁻¹ nitrate-N. Selenate was also removed by these columns but at a slower rate than observed with nitrate. In the final 6 weeks of the study, about 95% of the selenate applied to the columns was removed. In situ PRBs containing soybean oil might be used to remediate groundwater contaminated with both selenate and nitrate.     

Comparative effects of selenite and selenate on growth and selenium accumulation in lettuce plants under hydroponic conditions

The effects of different concentrations of selenite (2–30 μM) and selenate (2–60 μM) on biomass production, leaf area, and concentrations of photosynthetic pigments in lettuce plants were investigated. On the basis of the obtained results, the threshold of toxicity for the selenite and selenate has been designated. The toxicity thresholds for selenite and selenate were determined at concentrations of 15 and 20 μM, respectively. Next, four selenium (Se) concentrations (2, 4, 6 or 15 μM), below or near the toxicity boundary, have been selected for the lettuce biofortification experiment. In the biofortified plants, the oxidant status (levels of lipid peroxidation and H₂O₂ concentrations), as well as Se and sulphur (S) accumulation were analysed. In the edible parts of the lettuce, the Se concentration was higher for selenate presence compared to selenite; however, this difference was not as obvious as it was noted in the case of the roots, where selenite application caused the high accumulation of Se. An application of 15 μM Se as selenite caused a decline in the biomass and an intensification of prooxidative processes in the plant’s tissues and as toxic should be excluded from further biofortification experiments. These results indicate that an application of either selenate or selenite to the nutrient solution at concentrations below 15 μM can be used for biofortification of lettuce with Se, evoking better plant growth and not inducing significant changes in the oxidant status, the concentration of assimilation pigments and S accumulation.     

Overexpression of cystathionine-γ-synthase enhances selenium volatilization in<Emphasis Type="Italic"> Brassica juncea</Emphasis>

Selenium (Se) can be assimilated and volatilized via the sulfate assimilation pathway. Cystathionine--synthase (CGS) is thought to catalyze the synthesis of Se-cystathionine from Se-cysteine, the first step in the conversion of Se-cysteine to volatile dimethylselenide. Here the hypothesis was tested that CGS is a rate-limiting enzyme for Se volatilization. Cystathionine--synthase from Arabidopsis thaliana (L.) Heynh. was overexpressed in Indian mustard [Brassica juncea (L.) Czern & Coss], and five transgenic CGS lines with up to 10-fold enhanced CGS levels were compared with wild-type Indian mustard with respect to Se volatilization, tolerance and accumulation. The CGS transgenics showed 2- to 3-fold higher Se volatilization rates than wild-type plants when supplied with selenate or selenite. Transgenic CGS plants contained 20–40% lower shoot Se levels and 50–70% lower root Se levels than the wild type when supplied with selenite. Furthermore, CGS seedlings were more tolerant to selenite than the wild type. There were no differences in Se accumulation or tolerance from selenate, in agreement with the earlier finding that selenate-to-selenite reduction is rate-limiting for selenate tolerance and accumulation. In conclusion, CGS appears to be a rate-limiting enzyme for Se volatilization. Overexpression of CGS offers a promising approach for the creation of plants with enhanced capacity to remove Se from contaminated sites in the form of low-toxic volatile dimethylselenide.Abbreviations CGS cystathionine--synthase - DMSe dimethylselenide - SeCys selenocysteine - WT wild type     

Effect of Selenium Pre-treatment on Antioxidative Enzymes and Lipid Peroxidation in Cd-exposed Suckling Rats

Since there are no data about the protective role of selenium (Se) against cadmium (Cd)-induced oxidative damage in early life, we studied the effect of Se supplementation on antioxidative enzyme activity and lipid peroxidation (through thiobarbituric acid reactive substances; TBARS) in suckling Wistar rats exposed to Cd. Treated animals received either Se alone for 9 days (8 μmol, i.e., 0.6 mg Se as Na₂SeO₃ kg⁻¹ b.w., daily, orally; Se group), Cd alone for 5 days (8 μmol, i.e., 0.9 mg Cd as CdCl₂ kg⁻¹ b.w., daily, orally; Cd group), or pre-treatment with Se for 4 days and then co-treatment with Cd for the following 5 days (Se + Cd group). Our results showed that selenium supplementation, with and without Cd, increased SOD activity in the brain and kidney, but not in the liver and GSH-Px activity across all tissues compared to control rats receiving distilled water. Relative to the Cd group, Se + Cd group had higher kidney and brain SOD and GSH-Px activity (but not the liver), while in the liver caused increased and in the brain decreased TBARS level. These results suggest that Se stimulates antioxidative enzymes in immature kidney and brain of Cd-exposed rats and could protect against oxidative damage.     

Influence of arbuscular mycorrhiza on clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons

 The effect of arbuscular mycorrhiza (AM) on white clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons (PAH) was assessed in a pot experiment. The soil was spiked with 500 mg kg^–1 anthracene, 500 mg kg^–1 chrysene and 50 mg kg^–1 dibenz(a,h)anthracene, representing common PAH compounds with three, four and five aromatic rings, respectively. Three treatments and two harvest times (8 and 16 weeks) were imposed on plants grown in spiked soil: no mycorrhizal inoculation, mycorrhizal inoculation (Glomus mosseae P2, BEG 69) and mycorrhizal inoculation and surfactant addition (Triton X-100). Pots without PAH were also included as a control of plant growth and mycorrhizal colonization as affected by PAH additions. The competitive ability of clover vis-à-vis ryegrass regarding shoot and root growth was enhanced by AM, but reduced by PAH and the added surfactant. This was reflected by mycorrhizal root colonization which was moderate for clover (20–40% of total root length) and very low for ryegrass (0.5–5% of total root length). Colonization of either plant was similar in spiked soil with and without the added surfactant, but the PAH reduced colonization of clover to half that in non-spiked soil. P uptake was maintained in mycorrhizal clover when PAH were added, but was reduced in non-mycorrhizal clover and in mycorrhizal clover that received surfactant. Similar effects were not observed on ryegrass. These results are discussed in the context of the natural attenuation of organic pollutants in soils. Accepted: 12 June 2000     

Plant availability of soil selenate additions and selenium distribution within wheat and ryegrass

Selenate fertilization is an effective way to secure selenium (Se) nutrition in Se-poor areas but the cycling of the added selenate in the soil-plant system requires further clarification. We examined the Se uptake efficiency of wheat and ryegrass and Se distribution within these plants in two pot experiments. The behaviour of added selenate in a sand soil under wheat was monitored by sequential extractions during a ten-week growing period. In addition, the relationship between Se uptake of ryegrass and the salt extractable and ligand exchangeable Se in a sand and silty clay soil were studied. The added selenate remained mainly salt soluble in the soil throughout the monitoring. Se uptake by wheat comprised 12% of the soluble Se pool in soil and extended over the whole period of growth. In wheat, over 50% of Se accumulated in grains. The Se uptake of ryegrass comprised, on average, 40% of the soil salt soluble Se. In ryegrass, over 80% of the Se accumulated in roots. The distribution pattern of Se in plants can clearly have a major influence on both the Se cycle in soil and the nutritional efficiency of Se fertilization. The simple salt extraction showed fertilization-induced changes in the soluble soil Se pool, whereas the ligand exchangeable Se fraction reflected the difference in the nonlabile Se status between the two soils.