The effects of lead-resistant pseudomonads on the growth of Triticum Aestivum seedlings under lead stress

Five Pseudomonad strains, SPb-1, SPb-2, SPb-3, (from a water sample), SPb-4, and SPb-5 (from rhizosphere), which could tolerate lead acetate up to 1000 microm ml(-1), were isolated from an industrially polluted area around Kasoor, Pakistan. Only SPb-5 harbours a lead-resistant plasmid. Triticum aestivum seeds inoculated with strains SPb-4 and SPb-5 were germinated and grown under different concentrations (0, 1, 5, 10, 15 and 20 mM) of lead acetate for ten days. Germination and seedling growth were adversely affected. Poor growth was associated with increased lead content of seedlings. With bacterial inoculation, germination and seedling growth were improved, as compared with non-inoculated treatments. Stimulation in the growth of seedlings was accompanied by a decrease in lead content, as compared with non-inoculated treatments.     

Both trivalent and hexavalent chromium strongly alter in vitro germination and ultrastructure of kiwifruit pollen

Due to its widespread industrial use, chromium is considered a dangerous environmental pollutant. It is known to inhibit plant growth and development. The present study provides the first evidence of the toxicity of this metal on the male haploid generation of a higher plant. Both Cr(III) and Cr(VI) species, supplied as CrCl(3) and CrO(3), respectively, exerted a strong dose-dependent inhibitory effect on kiwifruit pollen tube emergence and growth. Cr(III) resulted more effective than Cr(VI) in the 16-75microM interval; moreover, complete inhibition of germination was attained at much lower doses than Cr(VI). Also tube morphology was affected. While the plasma membrane was still undamaged in the large majority of the treated pollen grains, dramatic ultrastructural alterations were induced by chromium including chromatin condensation, swelling of mitochondria, cytoplasmic vacuolization, and perturbed arrangement of endoplasmic reticulum cisternae. Thus, it seems that the impact of the two chromium species on kiwifruit pollen may result in severe compromission of both essential structures and functions of the male gametophyte.     

Uptake of chromate in sulfate deprived wheat plants

Terrestrial plants have been proposed for the removal of chromate from waste waters. Since chromate seems to be absorbed in roots by the same transport system as sulfate, sulfate-deprivation pretreatment and sulfate absence during chromate uptake were tested in wheat in order to increase chromium uptake efficiency. At 1 and 5 microg CrO(2-)(4) ml(-1) the highest chromate uptake was observed when plants suffered 5 days of sulfate deprivation pretreatment and absence of sulfate during chromate uptake. However, only at the lower concentration chromate net uptake was rapidly replaced by net efflux presumably due to toxic effects. At 5 microg CrO(2-)(4) ml(-1), the uptake of chromate was also enhanced by sulfate-deprivation pretreatment alone or lack of sulfate-competition. We conclude that sulfate is a strong inhibitor of chromate uptake, and when plants are going to be used to remove chromate from waste waters, sulfate-chromate interaction should be considered.     

Heavy metal contamination in the vicinity of an industrial area near Bucharest

BACKGROUND, AIMS AND SCOPE: Chromium enters into the aquatic environment as a result of effluent discharge from steel works, electroplating, leather tanning industries and chemical industries. As the Cr(VI) is very harmful to living organisms, it should be quickly removed from the environment when it happens to be contaminated. Therefore, the aim of this laboratory research was to develop a rapid, simple and adaptable solvent extraction system to quantitatively remove Cr(VI) from polluted waters. METHODS: Aqueous salt-solutions containing Cr(VI) as CrO4(2-) at ppm level (4-6 ppm) were prepared. Equal volumes (5 ml) of aqueous and organic (2-PrOH) phases were mixed in a 10 ml centrifuge tube for 15 min, centrifuged and separated. Concentrations of Cr(VI), in both the aqueous and organic phases, were determined by atomic absorption spectrometry. The effects of salt and acid concentrations, and phase-contact time on the extraction of Cr(VI) were investigated. In addition, the extraction of Cr(VI) was assessed in the presence of tetramethylammonium chloride (TMAC) in 2-PrOH phase. Effects of some other metals, (Cd(II), Co(II), Cu(II), Ni(II) and Zn(II)), on the extraction of Cr(VI) were also investigated. RESULTS AND DISCUSSION: The Cr(VI) at ppm level was extracted quantitatively by salting-out the homogeneous system of water and 2-propanol(2-PrOH) using chloride salts, namely CaCl2 or NaCl, under acidic chloride media. The extracted chemical species of Cr(VI) was confirmed to be the CrO3Cl-. The ion-pair complex extracted into the organic phase was rationalized as the solvated ion-pair complex of [2-PrOH2+, CrO3Cl-]. The complex was no longer stable. It implied the reaction between extracted species. Studies revealed that salts and acid directly participated in the formation of the above complex. Use of extracting agents (TMAC) didn't show any significant effect on the extraction of Cr(VI) under high salting-out conditions. There is no significant interference effect on the extraction of Cr(VI) by the presence of other metals. The Cr(VI) in the organic phase was back-extracted using an aqueous ammonia solution (1.6 mol dm(-3)) containing 3 mol dm(-3) NaCl. The extraction mechanism of Cr(VI) is also discussed. CONCLUSIONS: Salting-out of homogeneous mixed solvent of 2-propanol can be employed to extract Cr(VI) quantitatively, as an ion-pair of [2-PrOH2+ * CrO3Cl-] solvated by 2-PrOH molecules. Then, the complex becomes 'solvent-like' and is readily separated into the organic phase. The increase of Cl- ion concentration in the aqueous phase favors the extraction. The 2-PrOH, salts and acid play important roles in the extraction process. There is no need to use an extracting agent at a high salting-out condition. RECOMMENDATIONS AND PERSPECTIVES: Chromium(VI) must be quickly removed before it enters into the natural cycle. As the 2-PrOH is water-miscible in any proportion, ion-pairing between 2-PrOH2+ and CrO3Cl- becomes very fast. As a result, Cr(VI) can easily be extracted. Therefore, the method is recommended as a simple, rapid and adaptable method to quickly separate Cr(VI) from aqueous samples.     

Interaction of ozone exposure and Fusarium subglutinans inoculation on growth and disease development of loblolly pine seedlings

Seedlings from ten half-sib families of loblolly pine (Pinus taeda) were exposed in open-top chambers to carbon-filtered air (CF), non-filtered air (NF), or air amended with ozone to 1.7 or 2.5 times ambient. After 105 days of exposure, half the seedlings within each family were wounded but not inoculated and half were wounded and inoculated with the pitch canker fungus, Fusarium subglutinans, to which five families were relatively resistant. After an additional 50 days of ozone treatment, seedling growth and canker development were recorded. Cankers were significantly (sigma < or = 0.05) smaller among resistant compared to susceptible families, and were significantly larger among seedlings receiving the highest (2.5) compared to the ambient (NF) ozone treatment. The wound scars of non-inoculated seedlings were also significantly larger among seedlings receiving the 2.5 compared to the NF treatment, but these dimensions did not differ significantly with seedling family or resistance. The weights of needles and large roots were significantly smaller at the 2.5 compared to the 1.7 ozone treatment for inoculated but not for non-inoculated seedlings; this resulted in a significant interaction for ozone and inoculation effects. Among resistant families, root weights were significantly smaller for inoculated seedlings. Diameter growth and dry weights of needles were significantly smaller among inoculated compared to non-inoculated seedlings, but did not differ between NF and 2.5 ozone treatments.     

Microbial removal of uranium in uranium-bearing black shale

The effects of several conditional factors on efficiency of U bioleaching using an iron-oxidizer, Acidithiobacillus ferrooxidans, from U-bearing black shale (349 mg kg-1 of U) were investigated. When batch-type reactors containing black shale were initially inoculated with the cells, lower pH, higher redox potential and higher amount of aqueous Fe3+ than those of non-inoculated reactor were observed until 200 h. Such development of condition, which was facilitated by microbial activity, can enhance the rate and extent of U leaching from the solid substrate. However, under the condition of enough nutrients and energy source (Fe2+) supplied, indigenous Fe-oxidizers in the non-inoculated black shale were activated over time. They exerted almost same influence on the leaching efficiency with the inoculated samples after 250 h. Low initial Fe2+ supply (5 g l-1) and no addition of inorganic nutrients resulted in nearly identical extent of U leaching with that of 9 g l-1 of initial Fe2+ and nutrients supply. The results indicate that, in a practical process of bioleaching, the expenses for Fe2+ and nutrients addition can be reduced.     

Improvement of the hydrocarbon phytoremediation rate by Cyperus laxus Lam. inoculated with a microbial consortium in a model system

Hydrocarbon phytoremediation by Cyperus laxus Lam. growing on perlite and inoculated with hydrocarbon-degrading microorganisms was evaluated. Total petroleum hydrocarbons (TPH) were extracted from weathered soil (60.7 g of TPH kg(-1) of dry soil) and spiked on perlite at initial concentration of 5 g of TPH kg(-1) of dry perlite. Phenological characteristics, total microbial viable counts, hydrocarbon degraders and residual hydrocarbons were determined through 180 days of culture. Phenological characteristics of inoculated plants were improved as compared with non-inoculated plants: root biomass was 1.6 times greater, flowering time was reduced (13%), and the number of inflorescences was 1.5 times higher. The rhizospheric bacterial and fungi counts were higher for planted treatments (inoculated and not inoculated) than for unplanted pots. The maximum phytoremediation rate (0.51 mg of TPH g(-1) of dry plant d(-1)) for inoculated plants was reached at 60 days of culture, and was two times higher than for non-inoculated plants (55% TPH removal). Similar hydrocarbon phytoremediation extent values for inoculated (90%) and non-inoculated (85%) plants were obtained at 180 days of culture. The present study demonstrated that mutual benefits between C. laxus and inoculated hydrocarbon-degrading microorganisms are improved during phytoremediation. It is pertinent to note that this is the first report of hydrocarbon phytoremediation by Cyperus laxus Lam., a native plant growing in highly contaminated swamps.     

The ability of Azolla caroliniana to remove heavy metals (Hg(II), Cr(III), Cr(VI)) from municipal waste water

The aim of this paper was to investigate the capacity of a small water fern, Azolla caroliniana Willd. (Azollaceae), to purify waters polluted by Hg and Cr. Many plants are capable of accumulating heavy metals (called hyperaccumulators) and one of them is the water fern A. caroliniana. During 12 days of the experiment the fern was grown on the nutrient solution containing Hg2+, Cr3+ and CrO4(2-) ions, each in a concentration 0.1, 0.5 and 1.0 mg dm(-3). The presence of these ions caused a 20-31% inhibition of A. caroliniana growth, the highest in the presence of Hg(II) ions, in comparison to the control. After day 12 of the experiment, metal contents the solution decreased to 0-0.25 mg dm(-3), and this decrease comprised between 74 (Cr3+ 1.0 mg dm(-3) treatment) and 100% (CrO4(2-) 0.1 mg dm(-3) treatment). The fern took a lesser quantity of the metals from 0.1 mg dm(-3) treatments compared to 0.5 and 1.0 mg dm(-3) treatments. In the A. caroliniana tissues the concentration of heavy metals under investigation ranged from 71 to 964 mg kg(-1) dm; the highest level being found for Cr(III) containing nutrient solution.     

Interaction of bioaccumulation of heavy metal chromium with water relation, mineral nutrition and photosynthesis in developed leaves of Lolium perenne L

Contamination by chromium (Cr) is widespread in agricultural soils and industrial sites. This heavy metal represents a risk to human health. In order to gain fundamental insights into the nature of the adaptation to Cr excess, the characterisation of physiological indices, including responses of photosynthetic gas exchange and chlorophyll a fluorescence along with changes in mineral nutrient contents and water status were studied in ray grass (Lolium perenne L.). Increased concentrations of Cr(VI) (0-500 microM Cr) in the Co?c and Lessaint nutrient solution were applied. The growth of Lolium perenne is decreased by chromium and the leaves have lost their pigments. Chromium accumulation was greater in roots than in leaves and reached 2450 and 210 microg g(-1) DW, respectively with 500 microM Cr(VI) in nutrient medium. The physiological parameters were severely reduced by this heavy metal. Cr induced toxicity arising from 100 microM Cr(VI) and resulted in a modification of mineral content in roots and leaves, especially for Ca, Mg and Fe. The chromium stress decreased CO2 assimilation rates mainly due to stomatal closure, which reduced water loss by transpiration without decreasing the cellular available CO2. The fluorescence parameters associated with photosystem II (PSII) activity and the photochemical activity are modified by chromium. Non-radiative energy dissipation mechanisms were triggered during stress since non-photochemical quenching was increased and efficiency of excitation capture by open centers was reduced.     

Effect of humic substance on thermal treatment of chromium(VI)-containing latosol soil

Latosol soils contaminated with chromium(VI) [Cr(VI)], which is hazardous, can be recycled as raw materials for porcelain and construction sectors if a proper thermal stabilization process is implemented. This study investigates how thermal treatment affects Cr behavior during the sintering of latosol and deorganic latosol samples; both samples are artificially contaminated with CrO3. Approaches including X-ray absorption spectroscopy, scanning electron microscopy, N2-based Brunauer Emmett Teller surface analyzer, thermogravimetric analyzer/differential scanning calorimeter, and the toxicity characteristic leaching procedure promulgated by Taiwan Environmental Protection Administration are used in this study. After drying the Cr(VI)-contaminated latosol (i.e., containing 37,120 mg of Cr/kg sample) at 105 degrees C, approximately 80% of the doped CrO3 is chemically reduced to Cr(OH)3 by a humic substance naturally existing in the soil. In contrast, in the organics-free CrO3-contaminated latosol dried at 105 degrees C, only 9% of the doped CrO3 is reduced to Cr(OH)3. Heating the samples at 500 and 1100 degrees C transforms hazardous Cr(VI) into Cr(III) that is negligibly toxic; Cr2O3, which is insoluble, is detected as the most abundant Cr species. Moreover, formation of Cr2SiOs, which is suggested to relate to low Cr leaching, is only detected in the sample heated at 1100 degrees C. Surface morphology, surface area, and thermogravimetric analyzer/differential scanning calorimeter results demonstrate that thermal treatment at 1100 degrees C can incur considerable soil sintering/ melting if the humic substance in the soil has been heated off previously. Finally, Cr concentrations in the toxicity characteristic leaching procedure leachates collected from the samples thermally treated at 1100 degrees C for 4 hr are < or =0.21 mg of Cr L(-1) that are much less than the Taiwan Environmental Protection Administration regulatory limit (<5 mg of Cr L(-1)); consequently, these two samples are nonhazardous, and they have the potential for resource recycling. Conversely, Cr concentrations in the leachates from all 500 degrees C and 105 degrees C samples are in the 25.6-1279 mg L(-1) range.     

Growth of Lygeum spartum in acid mine tailings: response of plants developed from seedlings, rhizomes and at field conditions

Lygeum spartum is a native species in semiarid Mediterranean areas that grows spontaneously on acid mine tailings. We aimed to study the suitability of this plant for phytostabilization. L. spartum was grown from both seeds and rhizomes in acid mine tailings with various fertilizer and lime treatments. Untreated soils had a solution pH of 2.9 with high concentrations of dissolved salts (Electrical Conductivity 25 dS m(-1)) and Zn (3100 mg L(-1)). Plants grown on untreated soil had high shoot metal concentrations (>4000 mg kg(-1)Zn). Liming increased the solution pH to 5.5 and reduced the dissolved salts by more than 75%, resulting in lower shoot metal accumulation. Plants grown from rhizomes accumulated less metal than those grown from seeds. Plants collected in the field had metal concentrations an order of magnitude less than plants raised in the growth chamber. These differences may be due to the higher moisture content and homogeneous nature of the soils used in the pot experiment.     

Molecular study of thermal immobilization of chromium(VI) with clay

Clay that contains kaolinite has been used extensively as a raw material for manufacturing of bricks and china at 900-1100 degrees C. This study used clay to stabilize the contaminant chromium(VI) [Cr(VI)] through a heating process at 500-1100 degrees C. X-ray absorption spectroscopic results indicated that the 500-900 degrees C heating process transformed hazardous Cr(VI) to nontoxic Cr(III); Cr2O3 was the species detected as most abundant. The 1100 degrees C heating process caused the formation of Cr2SiO5, which was not detected in the samples heated at 500-900 degrees C. Fourier transformed extended X-ray absorption fine structure spectra were fitted by use of WinXAS software. Phase shifts and backscatter(ing) amplitudes for specific atom pairs, based on the crystallographic data for CrO3 and Cr2O3, were theoretically calculated with the FEFF software. The processed XAS data show that the first shell coordination numbers were similar to each other as the temperature was increased from 500 to 900 degrees C and 1100 degrees C, implying that their Cr(III) crystallite size was relatively similar. The interatomic distance between the target center element and the first shell for the 500-1100 degrees C samples was 1.98 A. The Debye-Waller factor for the 1100 degrees C sample was increased compared with the 500 and 900 degrees C samples and probably indicates the formation of Cr2SiO5.     

Excess chromium alters uptake and translocation of certain nutrients in citrullus

Citrullus plants were grown in refined sand with varying levels of chromium to determine their tolerance limit to excess chromium. The plants were maintained in control nutrient solution for 24 days and on the 25th day chromium as dichromate was added at 0.05, 0.1, 0.2, 0.3 and 0.4 mM. A control set of plants was grown in the same nutrient solution without chromium. At chromium levels >0.2 mM plants showed growth depression, with chlorosis and loss of turgor of middle leaves. Affected leaves had narrow lamina; tendrils were thin, short and did not have coiling property. Later chlorosis became severe and changed to necrosis in patches. Petiole along with lamina became wilted, rugged and hung down due to complete loss of water. At lower chromium concentration, (0.05, 0.1 and 0.2 mM) only depression in growth was observed. With increase in chromium concentration of nutrient solution accumulation of chromium in different parts of Citrullus was increased. Increase in concentrations of phosphorus, manganese and decrease in iron, copper, zinc and sulphur were observed in leaves. Toxicity of chromium was greater at 0.2-0.4 mM, compared to lower concentrations. Threshold of toxicity and toxicity of Cr in old leaves were, respectively, 0.9 and 3.9 microg g(-1) dry matter of citrullus.     

Uptake studies of environmentally hazardous (51)Cr in Mung beans

Attempt has been made to study the accumulation behaviour of a common plant, Mung bean (Vigna radiata) towards Cr(III) and Cr(VI) to have an insight on the migration and bio-magnification of Cr. For this purpose healthy germinated Mung bean seeds were sown in the sand in the presence of Hoagland's nutrient solution containing measured amount of K(2)(51)Cr(2)O(7) and (51)Cr(NO(3))(3).9H(2)O. Growth rate was also studied in the presence and absence of phosphate salts in the medium. It has been found that the transfer of chromium from soil to plant is significantly low (maximum 5% for both Cr(III) and Cr(VI)). Maximum accumulation of Cr occurs in the root with respect to the total chromium accumulation by the plant. Other parts of the Mung bean plant, e.g. cotyledons, shoot and leaves, show negligible accumulation. Therefore, the chance of direct intake of Cr through food as well as through the grazing animals to human body is less.     

Spectrophotometric determination of sulphur dioxide

Sulphur dioxide, an important industrial gas and air pollutant, is usually estimated using mercury salts. The authors have developed a method in which hazardous mercury salts are avoided. Sulphur dioxide is trapped in aqueous morpholine and mixed with the excess of dichromate solution in acidic medium. The hexavalent chromium in dichromate is reduced to trivalent chromium by sulphur dioxide and the excess of hexavalent chromium is determined with diphenylcarbazide which yields a soluble red-violet complex with an absorption maximum at 540 nm. The decrease in the absorbance values of the red-violet complex formed after reduction, when compared to that of a reagent control, is proportional to the concentration of sulphur dioxide used for reduction. Beer's law operates between 0.4 and 4 microg ml(-1) concentration of sulphur dioxide in solution.     

Effect of lead, mercury and cadmium on a sulphate-reducing bacterium

A sulphate-reducing bacterial strain isolated from the south-west coast of India resembling Desulfosarcina in its physiology was tested for its behaviour towards HgCl(2), CdSO(4) and Pb(NO(3))(2). The order of toxicity to growth of these metal salts in a lactate-based medium at 50 microg ml(-1) concentrations was Cd>Pb>Hg and to respiration Pb>Cd>Hg. Inhibitory concentrations (viz. 100 microg ml(-1) of HgCl(2) and 200 microg ml(-1) of Pb(NO(3)(2)) had a stimulatory effect when the substrate was changed to acetate. With sodium acetate at 0.1% concentration, Hg and Pb had maximum stimulatory effect for growth and sulphide production. Experiments conducted directly with sediment slurries amended with lactate showed that all three metals (at levels below their inhibitory concentrations, i.e. 50 microg ml(-1) of metal salt for Cd and Hg and 100 microg ml(-1) for Pb) inhibited sulphate-reducing activity (SRA) with Pb decreasing the peak production by 68%. The order of toxicity in both lactate and acetate-amended slurry was Pb>Cd>Hg and Pb>Hg>Cd, respectively. With acetate, SRA in the presence of Cd and Hg was stimulated 110% and 27%, respectively. Pb inhibited SRA by 11%. There is a general reduction in the inhibition of sulphide production in slurries as compared with pure culture of the isolate.     

Effects of atmospheric ammonia and ammonium sulphate on Douglas fir (Pseudotsuga menziesii)

Three-year-old Douglas firs (Pseudotsuga menziesii) were fumigated with 180 microg m(-3) NH3 or clean (charcoal-filtered) air. During these fumigations the plants received 15 mm artificial rain weekly, supplemented with 20, 500 or 2500 micromol litre(-1) (NH4)2SO4. Exposure to NH3 and NH4+ for 14 weeks resulted in a change of the nutrient status of the needles. The most remarkable effect was the increase in the N/K ratio, due to both uptake of N and leaching of K. The action of NH3 was stronger than that of NH4+. Both NH3 and (NH4)2SO4 affected the epicuticular wax layer and decreased mycorhiza frequency. Following fumigation and artificial rain treatments, needles were incubated for 8 h in a medium containing 0, 50, 250, 500 and 2500 micromol litre(-1) (NH4)2SO4. Almost no exchange of Ca, Mg and K for NH4+ was found. Therefore this ion exchange probably explains only a minor part of the changes in nutrient status of the whole trees.     

Chromium accumulation by the hyperaccumulator plant Leersia hexandra Swartz

Leersia hexandra Swartz (Gramineae), which occurs in Southern China, has been found to be a new chromium hyperaccumulator by means of field survey and pot-culture experiment. The field survey showed that this species had an extraordinary accumulation capacity for chromium. The maximum Cr concentration in the dry leaf matter was 2978 mg kg(-1) on the side of a pond near an electroplating factory. The average concentration of chromium in the leaves was 18.86 times as that in the pond sediment, and 297.41 times as that in the pond water. Under conditions of the nutrient solution culture, it was found that L. hexandra had a high tolerance and accumulation capacity to Cr(III) and Cr(VI). Under 60 mg l(-1) Cr(III) and 10 mg l(-1) Cr(VI) treatment, there was no significant decrease of biomass in the leaves of L. hexandra (p>0.05). The highest bioaccumulation coefficients of the leaves for Cr(III) and Cr(VI) were 486.8 and 72.1, respectively. However, L. hexandra had a higher accumulation capacity for Cr(III) than for Cr(VI). At the Cr(III) concentration of 10 mg l(-1) in the culture solution, the concentration of chromium in leaves was 4868 mg kg(-1), while at the same Cr(VI) concentration, the concentration of chromium in leaves was only 597 mg kg(-1). These results confirmed that L. hexandra is a chromium hyperaccumulator which grows rapidly with a great tolerance to Cr and broad ecological amplitude. This species could provide a new plant resource that explores the mechanism of Cr hyperaccumulation, and has potential for usage in the phytoremediation of Cr-contaminated soil and water.     

Influence of plant growth promoting bacteria and Cr6+ on the growth of Indian mustard

The Cr(6+) resistant plant growth promoting bacteria (PGPB), Pseudomonas sp. PsA4 and Bacillus sp. Ba32 were isolated from heavy metal contaminated soils and their plant growth promoting activity on the Indian mustard (Brassica juncea) were assessed with different concentrations of Cr(6+) in soil. Production of siderophores and the solubilization of phosphate were observed in both strains, PsA4 and Ba32. Production of IAA was only observed in strain PsA4. Inoculation of PsA4 or Ba32 promoted the growth of plants at 95.3 and 198.3 microg of Cr(6+)g(-1) soil. The maximum growth was observed in the plants inoculated with strain PsA4. Both strains, PsA4 and Ba32 did not influence the quantity of accumulation of chromium in root and shoot system. The present observations showed that the strains PsA4 and Ba32 protect the plants against the inhibitory effects of chromium, probably due to the production of IAA, siderophores and solubilization of phosphate.     

Effects of rhizobium, arbuscular mycorrhizal fungi and anion content of simulated rain on subterranean clover

An experiment was conducted to determine the extent to which rhizobia, mycorrhizal fungi, and anions in simulated rain affect plant growth response to acid deposition. Germinating subterranean clover seeds were planted in steam-pasteurized soil in pots and inoculated with Rhizobium leguminosarum, Glomus intraradices, Glomus etunicatum, R. leguminosarum + G. intraradices, R. leguminosarum + G. etunicatum, or no microbial symbionts. Beginning 3 weeks later, plants and the soil surface were exposed to simulated rain in a greenhouse on 3 days week(-1) for 12 weeks. Rain solutions were deionized water amended with background ions only (pH 5.0) or also adjusted to pH 3.0 with HNO3 only, H2SO4 only, or a 50/50 mixture of the two acids. Glomus intraradices colonized plant roots poorly, and G. intraradices-inoculated plants responded like nonmycorrhizal plants to rhizobia and rain treatments. Variation in plant biomass attributable to different rain formulations was strongest for G. etunicatum-inoculated plants, and the effect of rain formulation differed with respect to nodulation by rhizobia. The smallest plants at the end of the experiment were noninoculated plants exposed to rains (0.38 g mean dry weight total for 3 plants pot(-1)). Among nonnodulated plants infected by G. etunicatum, those exposed to HNO3 rain were largest, followed by plants exposed to HNO3 + H2SO4, pH 5.0, and H2SO4 rain, in that order. Among plants inoculated with both R. leguminosarum + G. etunicatum, however, the greatest biomass occurred with pH 5.0 rains, resulting in the largest plants in the study (1.00 g/3 plants). Treatment-related variation among root and shoot biomass data reflected those for whole-plant biomass. Based on quantification of biomass and N concentrations in shoot and root tissues, total N content of plants inoculated with G. etunicatum alone and exposed to the HNO3 + H2SO4 rains was approximately the same as plants inoculated with R. leguminosarum + G. entunicatum and exposed to pH 5 rains. Thus, the acid-mixture rains and rhizobia under no acid deposition provided approximately equal amounts of N in biomass. The significant interactions among rain formulation and the symbiotic status of the plants suggest that conclusions concerning the impact of acid deposition on plants in the environment cannot be considered reliable because most experiments on which such assessments are based have not tested confounding influences of microorganisms and precipitation characteristics.