Particulate and soluble hexavalent chromium are cytotoxic and genotoxic to human lung epithelial cells

Particulate hexavalent chromium (Cr(VI)) is a well-established human lung carcinogen. It is currently a major public health concern, there is widespread exposure to it in occupational settings and to the general public. However, despite the potential widespread exposure and the fact that the lung is its target organ, few studies have considered the toxic effects of particulate Cr(VI) in human lung cells. Accordingly, we used lead chromate as a model particulate Cr(VI) compound and determined its cytotoxicity and genotoxicity in cultured human bronchial epithelial cells, using BEP2D cells as a model cell line. We found that lead chromate induced concentration-dependent cytotoxicity in BEP2D cells after a 24 h exposure. Specifically, the relative survival was 78, 59, 53, 46 and 0% after exposure to 0.5, 1, 5, 10 and 50 μg/cm² lead chromate, respectively. Similarly, the amount of chromosome damage increased with concentration after 24 h exposure to lead chromate. Specifically, 0.5, 1, 5 and 10 μg/cm² damaged 10, 13, 20 and 28% of metaphase cells with the total amount of damage reaching 11, 15, 24 and 36 aberrations per 100 metaphases, respectively. Lead chromate (50 μg/cm² lead chromate) induced profound cell cycle delay and no metaphases were found. In addition we investigated the effects of soluble hexavalent chromium, sodium chromate, in this cell line. We found that 1, 2.5, 5 and 10 μM sodium chromate induced 66, 35, 0 and 0% relative survival, respectively. The amount of chromosome damage increased with concentration after 24 h exposure to sodium chromate. Specifically, 1, 2.5 and 5 μM damaged 25, 34 and 41% of metaphase cells with the total amount of damage reaching 33, 59 and 70 aberrations per 100 metaphases, respectively. Ten micromolar sodium chromate induced profound cell cycle delay and no metaphases were found. Overall the data clearly indicate that hexavalent Cr(VI) is cytotoxic and genotoxic to human lung epithelial cells.     

Distribution, transformation and bioavailability of trivalent and hexavalent chromium in contaminated soil

The purpose of this study was to investigate solid-phase distribution, transformation, and bioavailability of Cr in Cr(III) and Cr(VI) contaminated soils. The effects of EDTA treatment on solid-phase distribution of Cr in soils were also examined. The results show that Cr in both initially Cr(III)- and Cr(VI)-contaminated soils was mainly present in the organic matter bound fraction. Chromium had similar solid-phase distribution and similar overall binding intensity in both Cr(III)- and Cr(VI)-contaminated soils after a growing season. Transformation between Cr(III) and Cr(VI) took place in both Cr(III)- and Cr(VI)-treated soils. Chromium in the Cr(III)-contaminated soils was mostly present as Cr(III), while Cr in Cr(VI)-treated soils was mainly transformed into Cr(III). About 2% of Cr in native non-treated soils was found as Cr(VI). EDTA treatment increased Cr in soluble and exchangeable fraction in Cr(III)-treated soils. In both Cr(III)- and Cr(VI)-contaminated soils, Cr in oxide bound and organic matter bound     

Rats retain chromium in tissues following chronic ingestion of drinking water containing hexavalent chromium

Humans have sometimes been exposed to as much as 10 ppm Cr(VI) in drinking water from contaminated wells. The risks to these individuals are not well understood because the digestive tract reduces some of the Cr(VI) to the less bioavailable Cr(III) prior to absorption, and the disposition of the remaining Cr(VI) has not been well studied. We determined tissue Cr concentrations in rats after chronic ingestion of Cr(VI) in drinking water at concentrations relevant to human exposure levels. Adult male and female Fischer 344 rats consumed ad libitum 0, 0.5, 3, or 10 ppm Cr(VI) as K₂CrO₄ in drinking water for 44 wk. Rats then were given deionized water 4–6 d prior to sample collection. Females given 3 or 10 ppm Cr(VI) consumed more Cr(VI) per unit of body weight than did males. Bone Cr concentrations were significantly elevated in rats that drink 10 ppm Cr(VI). Renal Cr concentrations were significantly elevated in male rats that drink 3 or 10 ppm Cr(VI) and in female rats dosed with 10 ppm Cr(VI). Female rats had elevated liver Cr concentrations after drinking 3 or 10 ppm Cr(VI). Testicular Cr concentrations were slightly elevated in rats that drank 10 ppm Cr(VI). Brain, ovarian, and whole-blood Cr concentrations were below detection limits in all exposure groups. Although tissue Cr accumulation may have resulted from absorption of Cr(III), it is poorly absorbed. Therefore, the increased tissue retention may also have resulted, in part, from increased absorption of Cr(VI) and its subsequent uptake from the systemic circulation.     

Assessment of the phytotoxicity of chromium in soils using the selective ion exchange resin extraction method

Chromium in soils is present in the form of Cr(VI) oxyanions or Cr(III) cations. The toxicity and mobility of Cr(VI) are higher than those of Cr(III), thus it is essential that the availability of Cr(VI) in soils be accurately estimated in order to assess the phytotoxicity of Cr and its resultant health hazards to animals and humans. In this study, the Cu-saturated selective ion exchange resin (DOWEX M4195) was used as an infinite sink to test the feasibility of using the resin for extracting available Cr(VI) from soil. In the experiments, the results show that the resin had a high affinity for Cr(VI) and that Cr(VI) adsorbed by resins could be desorbed by using 10% NaCl (pH 4). In addition, the adsorption and desorption of Cr(VI) were not affected by pH levels, the forms of Cr(VI) or the presence of major anions in the soil solution. The above results indicate that the Cu-saturated resin can selectively adsorb Cr(VI) from solution. In the soil extraction experiments, three Cr(VI)-spiked soils were processed using the Cu-saturated resin extraction method. The results show that amounts of soil Cr extractable by the resin had a significant negative correlation to the height of wheat seedlings grown in the Neubauer test. Comparing this to the commonly used extractant, 0.1 M HCl, the amount of soil Cr, extractable by the resin, had a higher correlation to plant height. The results suggest that the selective ion exchange resin method developed in this study is useful in evaluating the quantities of plant-available Cr(VI) in soil and can, therefore, assess the phytotoxicity of Cr in soil.     

Histopathological and ultrastructural changes in the antennal gland,midgut, hepatopancreas,and gill of grass shrimp following exposure to hexavalent chromium

Grass shrimp, Palaemonetes pugio, were exposed for 1 month to subacute concentrations of hexavalent chromium (0.5, 1.0, 2.0, 4.0 ppm) after which the gills, midgut, hepatopancreas, and antennal glands were examined for histopathological and ultrastructural changes. Pathological changes were greatest in the antennal glands, followed by hepatopancreas, gills, and midgut. Severe changes occurred in some shrimp, even at 0.5 ppm chromium. Cells of all tissues frequently had both swollen mitochondria and rough endoplasmic reticulum. Small, spherical or ring-like intranuclear inclusions, possibly indicative of cellular hyperactivity or manifestions of chromium and/or protein complexes, were most prevalent in the hepatopancreas and antennal glands but also occurred in the midgut and gills. Other major degenerative changes in the antennal glands were restricted to the labyrinth and included diminution of basal plasmalemmal infoldings and cytoplasmic density, nuclear hypertrophy followed by widespread nuclear pyknosis and epithelial desquamation. In severely altered hepatopancreas hypertrophy was indicated for the basal laminae, nuclei, and possibly for the nucleoli. There was an apparent reduction in mitotic events and many observed mitotic nuclei were abnormal. Abnormal midgut hypertrophy was present in only 8 of 20 examined shrimp, exposed to 0.5 and 1 ppm chromium. Further, the gills of only 10 of the 40 examined chromium-exposed shrimp possessed abnormal features detectable with ligh microscopy. Ultrastructural analysis of the latter indicated an increase in lysosomes and a decrease in cytoplasmic density. In addition, there was a pronounced diminution in the degree of lamellar, subcuticular plasmalemmal infolding. This latter feature is postulated to be a mechanism for the regulation of chromium influx. Possible explanations for most observed alterations in the above tissues are proposed.     

Bioremoval of hexavalent chromium from water by a salt tolerant bacterium, Exiguobacterium sp. GS1

Pollution of terrestrial surfaces and aquatic systems by hexavalent chromium, Cr(VI), is a worldwide public health problem. A chromium resistant bacterial isolate identified as Exiguobacterium sp. GS1 by 16S rRNA gene sequencing displayed high rate of removal of Cr(VI) from water. Exiguobacterium sp. GS1 is 99% identical to Exiguobacterium acetylicum. The isolate significantly removed Cr(VI) at both high and low concentrations (1–200 μg mL⁻¹) within 12 h. The Michaelis–Menten K _m and V _max for Cr(VI) bioremoval were calculated to be 141.92 μg mL⁻¹ and 13.22 μg mL⁻¹ h⁻¹, respectively. Growth of Exiguobacterium sp. GS1 was indifferent at 1–75 μg mL⁻¹ Cr(VI) in 12 h. At initial concentration of 8,000 μg L⁻¹, Exiguobacterium sp. GS1 displayed rapid bioremoval of Cr(VI) with over 50% bioremoval in 3 h and 91% bioremoval in 8 h. Kinetic analysis of Cr(VI) bioremoval rate revealed zero-order in 8 h. Exiguobacterium sp. GS1 grew and significantly reduced Cr(VI) in cultures containing 1–9% salt indicating high salt tolerance. Similarly the isolate substantially reduced Cr(VI) over a wide range of temperature (18–45  °C) and initial pH (6.0–9.0). The T _opt and initial pH_opt were 35–40  °C and 7–8, respectively. Exiguobacterium sp. GS1 displayed a great potential for bioremediation of Cr(VI) in diverse complex environments.     

Kinetics and modeling of hexavalent chromium reduction in Enterobacter cloacae

Kinetics of bacterial reduction of toxic hexavalent chromium (chromate: CrO(4) (2-)) was investigated using batch and fedbatch cultures of Enterobacter cloacae strain HO1. In fedbatch cultures, the CrO(4) (2-) feed was controlled on the basis of the rate of pH change. This control strategy has proven to be useful for avoiding toxic CrO(4) (2-) overload. A simple mathematical model was developed to describe the bacterial process of CrO(4) (2-) reduction. In this model, two types of bacterial cells were considered: induced, CrO(4) (2-)-resistant cells and uninduced, sensitive ones. Only resistant cells were assumed to be able to reduce CrO(4) (2-). These fundamental ideas were supported by the model predictions which well approximated all experimental data. In a simulation study, the model was also used to optimize fed-batch cultures, instead of lengthy and expensive laboratory experiments. (c) 1993 John Wiley & Sons, Inc.     

Effects of the trivalent and hexavalent chromium on the physicochemical properties of mammalian cell nucleic acids and synthetic polynucleotides

The effects of trivalent (chromium chloride) and hexavalent (potassium dichromate) chromium have been studied on the nucleic acids of cultured mammalian cells (BHK hamster fibroblast line), commercial DNA and RNA, and synthetic polynucleotides of known base composition. Modifications of UV absorption spectra and alterations of thermal denaturation and renaturation patterns have been observed by directly treating purified nucleic acids, as well as by examining nucleic acids extracted from cells treated with chromium compounds.Cr(III) interacts with nucleic acid bases, mostly guanine and cytosine, but also with phosphate groups, leading to deprotonation of bases as well as intramolecular cross-links, sandwich complexes between bases and chelation between bases and phosphates. Such interactions destabilize the DNA structure. On the contrary, stabilization of RNA, due to intramolecular metal bonds between nitrogen bases in GC-rich regions, is mainly produced. The kind of interaction of Cr(III) with nucleic acids is not significantly different when intact BHK cells are treated.Cr(VI) interacts similarly with DNA and RNA giving instead different effects when purified nucleic acids or intact cells are treated. Treatment of purified DNA produces breakages in the polynucleotide chain due to the oxidizing power of Cr(VI). In intact cell treatments, changes in the properties of DNA are observed. These could result from the combined action of Cr(III), produced by the intracellular reduction of Cr(VI) and the oxidizing activity of residual Cr(VI).The relevance of Cr(VI) and Cr(III) interactions to the mechanisms of chromium (carcino)genic action is summarized. It is stressed that Cr(VI), if not completely reduced to Cr(III) by extracellular and endoplasmic constituents, can reach the cell nucleus and directly interact with DNA.     

Contamination level and potential health risk assessment of hexavalent chromium in soils from a coal chemical industrial area in Northwest China

Abstract

This study aimed to evaluate the total Cr and Cr(VI) contamination levels and assess the possible health risk of Cr(VI) in soils from a coal chemical industrial area in Northwest China. The contamination factor (CF) was used to calculate the total Cr and Cr(VI) contamination levels in soils from the study area. The highest concentration of Cr(VI) (69.58?mg/kg) was found in the top soil (0–20?cm) with the distance of 10 m to the coal cinder heap. The carcinogenic risk (CR) and hazard quotient (HQ) were estimated for health risk to workers by using “Chinese Technical Guidelines for Risk Assessment of Contaminated Sites (HJ 25.3-2014).” The results showed that the soils from the study area were moderately polluted by the total Cr and Cr(VI). There was no serious non-CR (HQ < 1). However, the CR values of Cr(VI) were significantly higher than the threshold value, indicating that workers are facing serious threat of Cr(VI). Inhalation (70.32%) was the main exposure pathway to CR, followed by dermal contact (20.64%), and then ingestion (9.04%). These results provide basic information of Cr(VI) pollution control and environment management in coal chemical industrial areas.     

固磷基质(无定形铁)淋失特征及其与磷素淋失的关系

采用淋洗试验研究了水田土壤固磷基质（无定形铁）淋失特征及其与磷素淋失的关系。结果表明,添加柠檬酸和葡萄糖培养处理对无定形铁、络合态铁的淋失具有极显著效应,水分和温度等因素对无定形铁和络合态铁的淋失交互效应显著;相关分析表明,固磷基质（无定形铁）淋失与磷素淋失的相关关系显著,说明对磷素淋失有一定的影响。     

Transport of gases into leaves

Abstract. Transport of gases between the intercellular spaces of plant leaves and the surrounding air is analysed in terms of multicomponent collision processes through an isothermal, porous septum. Interaction of diffusing species with each other and with the pore walls is described using a modified Stefan–Maxwell equation and an equation relating the pressure gradient to the sum of the diffusive fluxes, weighted by their appropriate Knudsen diffusivities. Viscous How arising from an excess pressure within the leaf is also considered.
Equations are derived which describe the flux densities of water vapour and CO₂ through the stomata. The analysis is general and is applicable to trace gases other than CO₂. A simple conductance is defined for water vapour to relate the flux and mol fraction difference across the stomata, viz. N_w =− g_w , δ x_w / x_a . A simple conductance cannot be defined for CO₂ because the flux of water vapour has a significant influence on the CO₂ gradient. The equation derived for the intercellular mol fraction of CO₂ is in terms of the fluxes of CO₂ and water vapour and represents a 'large-pore' ( d > μm) approximation which requires no information about stomalal geometry. Analogous equations are developed for transfer of gases through the leaf boundary layer. Sample calculations are presented to illustrate the effect of neglecting the interaction of water vapour and CO₂ on the calculated intercellular and surface concentrations of CO₂. Equations for computing water vapour and CO₂ flux densities from leaf chamber measurements are also presented.     

Influences of the petroleum-recovering activity on the arsenic level in groundwater of Kuitun,Xinjiang, China

Increasing demands of groundwater in petroleum-recovering regions could elevate the level and mobility of arsenic in groundwater as a result of the enhanced dissolution of arsenic-bearing iron or manganese oxide due to the accelerated sulfate reduction by microorganisms in a reductive environment. To substantiate this possibility, groundwater samples were collected from 220 water wells in the nearby petroleum wells in Kuitun. Dissolved arsenic, iron, manganese, and sulfate levels and pH in groundwater samples were analyzed. The dissolved arsenic levels in groundwater varied from <2.3 to 789.4 μg·L^?1, in which approximately 96.4% of the measured values exceeded the allowed limits of the World Health Organization. An inverse relation existed between dissolved arsenic and sulfate levels. Most of the high arsenic-level samples (>300 μg·L^?1) were found in wells at close proximity to petroleum wells where a high iron or manganese level was also detected. The oil-exploring activity in the study region seemed to have enhanced the microbial reduction of sulfate in underground environment and hence the level of arsenic in groundwater. The microbial sulfate reduction coupled with the reduction of arsenic-bearing iron oxides in the groundwater environment may explain the spatial heterogeneity of the arsenic level in groundwater.     

Isolation and characterization of a highly effective bacterium Bacillus cereus b-525k for hexavalent chromium detoxification

The chromate resistant Gram-positive Bacillus cereus strain b-525k was isolated from tannery effluents, demonstrating optimal propagation at 37 °C and pH 8. The minimum inhibitory concentration (MIC) test showed that B. cereus b-525k can tolerate up to 32 mM Cr⁶⁺, and also exhibit the ability to resist other toxic metal ions including Pb²⁺ (23 mM), As³⁺ (21 mM), Zn²⁺ (17 mM), Cd²⁺ (5 mM), Cu²⁺ (2 mM), and Ni²⁺ (3 mM) with the resistance order as Cr ⁶⁺ > Pb²⁺ > As³⁺ >Zn²⁺ >Cd²⁺ >Ni²⁺ >Cu²⁺. B. cereus b-525k showed maximum biosorption efficiency (q) of 51 mM Cr⁶⁺/g after 6 days. Chromate stress elicited pronounced production of antioxidant enzymes such as catalase (CAT) 191%, glutathione transferase (GST) 192%, superoxide dismutase (SOD) 161%, peroxidase (POX) 199%, and ascorbate peroxidase (APOX) (154%). Within B. cereus b-525k, the influence of Cr⁶⁺ stress (2 mM) did stimulate rise in levels of GSH (907%) and non-protein thiols (541%) was measured as compared to the control (without any Cr⁶⁺ stress) which markedly nullifies Cr⁶⁺ generated oxidative stress. The pilot scale experiments utilizing original tannery effluent showed that B. cereus b-525k could remove 99% Cr⁶⁺ in 6 days, thus, it could be a potential candidate to reclaim the chromate contaminated sites.     

The use of low-quality water on the transport and transformation of waste water in irrigation soils

Waste water use for agricultural production in the semi-arid and arid climate areas is increasingly regarded as a solution for water shortage. Still today many questions remain unanswered about the mid- and long-term effect of irrigation by waste water on plant, soil and human health. In this paper the effect of the use of waste water on the soil hydraulic properties, the solute transport and transformation behaviour is being studied conducting steady state and non-steady state waste water application treatments on undisturbed soil columns and a field plot. Detailed spatial and temporal information on the propagation of water and solute is obtained using tensiometers, soil solute extraction and time domain reflectometry probes. The experimental data are processed using the HYDRUS-2D modeling tool. After calibration, the model is used in a predictive way as to simulate the mid- and long-term effects of the use of treated waste water as irrigation water.     

Effects of combining biological treatment and activated carbon on hexavalent chromium reduction

The objectives of the present work were: (a) to analyze the Cr(VI) removal by combining activated sludge (AS) with powdered activated carbon (PAC), (b) to analyze the effect of PAC and Cr(VI) on the growth kinetics of activated sludge, and (c) to determine if the combined method (AS-PAC) for Cr(VI) removal can be considered additive or synergistic with respect to the individual processes. Chromate removal was improved by increasing PAC concentrations in both PAC and AS-PAC systems. Cr(VI) removal using the AS-PAC system was higher than using AS or PAC. The increase of Cr(VI) caused longer lag phase and lower observed specific growth rate (μ_obs), biomass yield (Y_X/S), and specific growth substrate consumption rate (q_S) of activated sludge; additionally, PAC did not enhance the growth kinetic parameters (μ_obs, Y_X/S, q_S). Cr(VI) reduction in AS-PAC system was the result of the additive effect of each individual Cr(VI) removal process.     

Transport of bacterial inoculants through intact cores of two different soils as affected by water percolation and the presence of wheat plants

Abstract Water flow-innduced transport of Burkholderia cepacia strain P2 and Pseudomonas fluorescens strain R2f cells through intact cores of loamy sand and silt loam field soils was measured for two percolation regimes, 0.9 and 4.4 mm h⁻¹, applied daily during 1 hour. For each strain, transport was generally similar between the two water regimes. Translocation of B. cepacia , with 4.4 mm h⁻¹, did occur initially in both soils. In the loamy sand soil, no change in the bacterial distribution occurred during the experiment (51 days). In the silt loam, B. cepacia cell numbers in the lower soil layers were significantly reduced, to levels at or below the limit of detection. Transport of P. fluorescens in both soils also occurred initially and was comparable to that of B. cepacia . Later in the experiment, P. fluorescens was not detectable in the lower soil layers of the loamy sand cores, due to a large decrease in surviving cell numbers. In the silt loam, the inoculant cell distribution did not change with time. Pre-incubation of the inoculated cores before starting percolation reduced B. cepacia inoculant transport in the loamy sand soil measured after 5 days, but not that determined after 54 days. Delayed percolation in the silt loam soil affected bacterial transport only after 54 days. The presence of growing wheat plants overall enhanced bacterial translocation as compared to that in unplanted soil cores, but only with percolating water. Percolation water from silt loam cores appeared the day after the onset of percolation and often contained inoculant bacteria. With loamy sand, percolation water appeared only 5 days after the start of percolation, and no inoculant bacteria were found. The results presented aid in predicting the fate of genetically manipulated bacteria in a field experiment.     

Synthesis of zero-valent copper-chitosan nanocomposites and their application for treatment of hexavalent chromium

This study used ionotropic crosslinking to synthesize chitosan-tripolyphosphate chelating resin beads, which are used to fabricate zero-valent copper-chitosan nanocomposites. The copper nanoparticles were dispersed on chitosan-tripolyphosphate beads, and were thus able to maintain appropriate dispersion and stability, which greatly improves their applicability. The fabrication process contains two steps: using chitosan-tripolyphosphate beads to adsorb Cu(II) ions, followed by chemical reduction to reduce Cu(II) ions to zero-valent copper. This study explored the adsorption of synthesized chitosan-tripolyphosphate beads to Cu(II) ions, and used SEM/EDS, XPS, and TEM to examine the properties of zero-valent copper-chitosan nanocomposites. The results showed that, the adsorption behavior of hexavalent chromium from aqueous solution onto fabricated nanocomposites has better adsorption capacity than that of the chitosan-tripolyphosphate beads.     

土-水介质中低放核素污染物的生物修复

各种人为因素使人类生态环境的放射性核素本底值不断增加,这些放射性核素一旦进入水-土介质,可通过各种途径产生污染危害,常规的化学或物理方法不适用于水-土介质中低放核素污染物的治理,人类企图开发出新的修复技术以对付低放核素的污染问题,生物修复以其低成本,环境搅动性少等优点而成为关注的对象,本文介绍了环境中低放核素污染物的来源和低放核素生物修复的概念,并就国内外低放核素生物修复研究状况作一归纳和评述,在此基础上提出低放核素生物修复未来研究的方向。     

Effect of growth and subsequent decomposition of blue-green algae on the transformation of iron and manganese in submerged soils

N₂-fixing blue-green algae (Cyanobacteria), besides enriching soils with N and organic carbon, may modify a number of chemical and electro-chemical properties of the soils resulting in a change in availability of some micronutrient elements. Keeping this in view, an experiment was conducted to study the effects of growth and subsequent decomposition of blue-green algae on changes in the different forms of Fe and Mn in four soils under submerged condition. A mixed algal culture containing Anabaena, Nostoc, Cylindrospermum, and Tolypothrix was used as inoculum. It was allowed to grow for 2 months, after which the soils were sequentially extracted with (i) M NH₄OAc (pH 7.0), (ii) M K₄P₂O₇, (iii) 0.1 M NH₂OH.HCl (pH 2.0), (iv) 0.2 M (NH₄)₂C₂O₄ (pH 3.0) and (v) 0.1 M ascorbic acid to obtain water-soluble plus exchangeable, organically bound, easily reducible, amorphous oxides-and crystalline oxides-bound forms of Fe and Mn, respectively, both during the growth as well as the subsequent in-situ decomposition of the algal biomass in soils. Iron and Mn in the extracts were estimated by atomic absorption spectrophotometry.The results showed that growth of blue-green algae in submerged rice soils caused a decrease in the NH₄OAc-extractable forms of Fe and Mn with concomitant increases in all the other four determined forms of the elements. Such decreases and/or increases in different forms of Fe and Mn in soils were explained as being due to release of O₂, addition of organic matter and liberation of extracellular organic compounds by the blue-green algae during their growth. The decomposition of algal biomass resulted in an increase in the NH₄OAc-, K₄P₂O₇- and (NH₄)₂C₂O₄-extractable forms of Fe and Mn with a simultaneous decrease in the NH₂OH · HCl- and ascorbic acid-extractable forms. Development of strong reducing conditions and formation of organic acids with chelating properties were suggested as being the cause of the above changes. The implication of these changes in the forms of Fe and Mn for the Fe and Mn nutrition of rice plants were discussed.