Water relations and photosynthesis in the cryptoendolithic microbial habitat of hot and cold deserts

Two cryptoendolithic microbial communities, lichens in the Ross Desert of Antarctica and cyanobacteria in the Negev Desert, inhabit porous sandstone rocks of similar physical structure. Both rock types adsorb water vapor by physical mechanisms unrelated to biological processes. Yet the two microbial communities respond differently to water stress: cryp-toendolithic lichens begin to photosynthesize at a matric water potential of –46.4 megaPascals (MPa) [70% relative humidity (RH) at 8°C], resembling thallose desert lichens. Cryptoendolithic cyanobacteria, like other prokaryotes, photosynthesize only at very high matric water potentials [> –6.9 MPa, 90% RH at 20°C].     

Grazing in a porous environment: 1. The effect of soil pore structure on C and N mineralization

The porous soil environment constrains grazing of microorganisms by microbivorous nematodes. In particular, at matric potentials at which water-filled pore spaces have capillary diameters less than nematode body diameters the effect of grazing, e.g. enhanced mineralization, should be reduced ('exclusion hypothesis') because nematodes cannot access their microbial forage. We examined C and N mineralization, microbial biomass C (by fumigation-extraction), the metabolic quotient (C mineralization per unit biomass C), nematode abundance, and soil water content in intact soil cores from an old field as a function of soil matric potential (−3 to −50 kPa). We expected, in accordance with the exclusion hypothesis, that nematode abundance, N and C mineralization would be reduced as matric potential decreased, i.e. as soils became drier. N mineralization was significantly greater than zero for −3 kPa but not for −10, −20 and −50 kPa. Microbial biomass C was less at −50 kPa than at −10 kPa, but not significantly different from biomass C at −3 and −20 kPa. The metabolic quotient was greatest at −50 kPa than any of the other matric potentials. From the exclusion hypothesis we expected significantly fewer nematodes to be present at −50 and −20 kPa representing water-filled capillary pore sizes less than 6 and 15 μm, respectively, than at −3 and −10 kPa. Microbivorous (fungivorous+bacterivorous) nematode abundance per unit mass of soil was not significantly different among matric potentials. Body diameters of nematodes ranged from 9 μm to 40 μm. We discuss several alternatives to the exclusion hypothesis, such as the 'enclosure hypothesis' which states that nematodes may become trapped in large water-filled pore spaces even when capillary pore diameters (as computed from matric potential) are smaller than body diameters. One of the expected outcomes of grazing in enclosures is the acceleration of nutrient cycling. This revised version was published online in June 2006 with corrections to the Cover Date.     

Colonization of Wheat Roots by an Exopolysaccharide-Producing Pantoea agglomerans Strain and Its Effect on Rhizosphere Soil Aggregation

The effect of bacterial secretion of an exopolysaccharide (EPS) on rhizosphere soil physical properties was investigated by inoculating strain NAS206, which was isolated from the rhizosphere of wheat (Triticum durum L.) growing in a Moroccan vertisol and was identified as Pantoea aglomerans. Phenotypic identification of this strain with the Biotype-100 system was confirmed by amplified ribosomal DNA restriction analysis. After inoculation of wheat seedlings with strain NAS206, colonization increased at the rhizoplane and in root-adhering soil (RAS) but not in bulk soil. Colonization further increased under relatively dry conditions (20% soil water content; matric potential, −0.55 MPa). By means of genetic fingerprinting using enterobacterial repetitive intergenic consensus PCR, we were able to verify that colonies counted as strain NAS206 on agar plates descended from inoculated strain NAS206. The intense colonization of the wheat rhizosphere by these EPS-producing bacteria was associated with significant soil aggregation, as shown by increased ratios of RAS dry mass to root tissue (RT) dry mass (RAS/RT) and the improved water stability of adhering soil aggregates. The maximum effect of strain NAS206 on both the RAS/RT ratio and aggregate stability was measured at 24% average soil water content (matric potential, −0.20 MPa). Inoculated strain NAS206 improved RAS macroporosity (pore diameter, 10 to 30 μm) compared to the noninoculated control, particularly when the soil was nearly water saturated (matric potential, −0.05 MPa). Our results suggest that P. agglomerans NAS206 can play an important role in the regulation of the water content (excess or deficit) of the rhizosphere of wheat by improving soil aggregation.     

Involvement of Nitrate Reductase and Pyoverdine in Competitiveness of Pseudomonas fluorescens Strain C7R12 in Soil

Involvement of nitrate reductase and pyoverdine in the competitiveness of the biocontrol strain Pseudomonas fluorescens C7R12 was determined, under gnotobiotic conditions, in two soil compartments (bulk and rhizosphere soil), with the soil being kept at two different values of matric potential (−1 and −10 kPa). Three mutants affected in the synthesis of either the nitrate reductase (Nar⁻), the pyoverdine (Pvd⁻), or both (Nar⁻ Pvd⁻) were used. The Nar⁻ and Nar⁻ Pvd⁻ mutants were obtained by site-directed mutagenesis of the wild-type strain and of the Pvd⁻ mutant, respectively. The selective advantage given by nitrate reductase and pyoverdine to the wild-type strain was assessed by measuring the dynamic of each mutant-to-total-inoculant (wild-type strain plus mutant) ratio. All three mutants showed a lower competitiveness than the wild-type strain, indicating that both nitrate reductase and pyoverdine are involved in the fitness of P. fluorescens C7R12. The double mutant presented the lowest competitiveness. Overall, the competitive advantages given to C7R12 by nitrate reductase and pyoverdine were similar. However, the selective advantage given by nitrate reductase was more strongly expressed under conditions of lower aeration (−1 kPa). In contrast, the selective advantage given by nitrate reductase and pyoverdine did not differ in bulk and rhizosphere soil, indicating that these bacterial traits are not specifically involved in the rhizosphere competence but rather in the saprophytic ability of C7R12 in soil environments.     

A Stable Bioluminescent Construct of Escherichia coli O157:H7 for Hazard Assessments of Long-Term Survival in the Environment

A chromosomally lux-marked (Tn5 luxCDABE) strain of nontoxigenic Escherichia coli O157:H7 was constructed by transposon mutagenesis and shown to have retained the O157, H7, and intimin phenotypes. The survival characteristics of this strain in the experiments performed (soil at −5, −100, and −1,500 kPa matric potential and artificial groundwater) were indistinguishable from the wild-type strain. Evaluation of potential luminescence was found to be a rapid, cheap, and quantitative measure of viable E. coli O157:H7 Tn5 luxCDABE populations in environmental samples. In the survival studies, bioluminescence of the starved populations of E. coli O157:H7 Tn5 luxCDABE could be reactivated to the original levels of light emission, suggesting that these populations remain viable and potentially infective to humans. The attributes of the construct offer a cheap and low-risk substitute to the use of verocytotoxin-producing E. coli O157:H7 in long-term survival studies.     

Influence of arbuscular mycorrhizal fungi on soil structure and soil water characteristics of vertisols

The influence of inoculation with arbuscular mycorrhizal fungi (AM fungi) on soil water characteristics of fast and slowly wetted vertisol samples was studied. Vertisols characteristically have a low stability to wetting, and the disruption of their larger pores when they swell leads to reduced water infiltration and thereby to runoff. The degree of aggregate breakdown determines the ability of the soil to drain. A vertisol was used in this pot experiment with four treatments: T₁: Pasteurized soil, T₃: Pasteurized soil, with plants, T₄: Inoculated, pasteurized soil, with plants, T₅: Unpasteurized soil, with plants. A treatment using inoculated, pasteurized soil (T₂) was included in a related study (Bearden and Petersen, 2000) comparing aggregate stability, and the present study follows the same numbering to aid in comparison of experiments. After fast, disruptive wetting, the soil inoculated with AM fungi (T₄) was found to have a lower soil water content than did the soils from the other treatments at matric potentials lower than –3.92 kPa. This indicates greater drainage from pores smaller than 75 m for the soil inoculated with AM fungi, and the greater drainage appears to be directly related to a characteristic pore range between 67 and 75 m. The soil without plants (T₁), when wetted fast, had a lower soil water content at matric potentials higher than –3.92 kPa than soils from the other treatments, which indicates less pore volume due to pores larger than 75 m in the treatment without plants. The pore indexes, calculated as the ratio between the slope of the fast and the slope of the slowly-wetted water characteristics, generally had the highest values for the soil inoculated with AM fungi (T₄) from matric potential 0.00 to –0.29 kPa. In this matric potential range, the pore indexes were less than one. The unpasteurized soil with naturally present AM fungi (T₅) generally had the highest pore indexes from matric potential –0.49 to –3.92 kPa, and the pore indexes in this matric potential range were above one. These results indicate the smallest loss of very large pores in the soil inoculated with AM fungi (T₄) and the largest gain of smaller sized pores in the unpasteurized soil (T₅). This suggests that the resistance to breakdown of the largest pores is related to the presence of roots, and that the gain of groups of smaller pores is related to the presence of hyphae.     

(Bio-)remediation of VCHC contaminants in a Technosol under unsaturated conditions

The remediation of dense non-aqueous phase liquids has always been a concern of both public and scientific interest groups. In this research work a modified physical concept of (bio)remediation of a volatile chlorinated hydrocarbon (VCHC) contamination was elaborated under laboratory conditions and modeled with HYDRUS-2D. In field dechlorination is influenced by both physicochemical and hydraulic properties of the substrate, e.g. texture, pore size distribution, pore liquid characteristics, e.g. viscosity, pH, surface tension, and dependent on the degree of saturation of the vadose zone. Undisturbed soil cores (100 cm³) were sampled from a Spolic Technosol. Considering hydraulic properties and functions, unsaturated percolation was performed with vertically and horizontally structured samples. VCHC concentrations were calculated prior, during, and after each percolation cycle. According to laboratory findings, microemulsion showed the most efficient results with regard to flow behavior in the unsaturated porous media and its accessibility for bacteria as nutrient. The efficiency of VCHC remediation could be increased by the application of a modified pump-and-treat system: the injection of bacteria Dehalococcoides ethanogenes with microemulsion, and extraction at a constant matric potential level of ?6 kPa. Achieved data was used for HYDRUS-2D simulations, modeling in situ conditions, demonstrating the practical relevance (field scale) of performed unsaturated percolation (core scale), and in order to exclude capillary barrier effects.     

Physical properties of two-component growth media based on Sphagnum peat and their implications for plant-available water and aeration

The physical properties, in particular the water retention characteristics, of two-component growth media based on low-humified Sphagnum peat were studied. The high water retention of pure peat, which is further increased by shrinkage of the medium at desorption, yielded low air-filled porosity at high matric potentials ( –1 kPa). The addition of coarse perlite to peat decreased the shrinkage markedly and also tended to increase the low saturated hydraulic conductivity of peat, which had initially been rather low. In all media studied, the amount of water that is easily available to plants (water content retained between –1 and –10 kPa matric potential) was relatively high. In peat that contained half repellent rockwool or hydrogel, this water retention was, however, markedly lower. Between –10 and –50 kPa matric potential, water retention was rather low in all media (< 10%). Within the lowest matric potential range studied (–50 to –1500 kPa), water retention was considerably elevated in peat that contained half hydrogel. The implications of the physical properties of the media for plant-available water and aeration in the media are discussed.     

Water Relations of Pachysandra Leaves during Freezing and Thawing : Evidence for a Negative Pressure Potential Alleviating Freeze-Dehydration Stress

The evergreen herb Pachysandra terminalis becomes moderately frost-hardy in winter. The water relations of its frost-hardy leaves were studied during a freeze-thaw cycle. Leaf water potentials, measured by psychrometry at subfreezing temperatures, were identical with those of ice, indicating equilibrium freezing. Microscopic observations showed extracellular freezing of tissue water. As evidenced by thermal analysis, the freezing process starts with the crystallization of a minor volume which was identified as apoplasmic water. The following long-lasting exotherm indicated slow export of water from the protoplasts driven by extracellular crystallization. In partially frozen leaves, the fractions of liquid water were measured at several subfreezing temperatures by nuclear magnetic resonance spectroscopy. They were consistently greater than those calculated from the osmotic potentials of cellular fluid, and the differences increased with decreasing temperature. About 50% of the differences could be abolished by freeze-killing of the leaf and was thus ascribed to the effect of a (negative) pressure reinforcing the osmotic potential. The persistent part of the differences may have reflected a matric component. At −7°C, the absolute values of both potentials were −1.7 megapascals each. The water relations of Pachysandra leaves clearly indicate nonideal equilibrium freezing where negative pressures and matric potentials contribute to the leaf water potential and thus alleviate freeze-dehydration of the tissue.     

Chemical and Sensory Changes Associated with Microbial Flora of Mediterranean Boque (Boops boops) Stored Aerobically at 0, 3, 7, and 10°C

The development of a microbial population and changes in the physicochemical and sensorial characteristics of Mediterranean boque (Boops boops), called gopa in Greece, stored aerobically at 0, 3, 7, and 10°C were studied. Pseudomonads and Shewanella putrefaciens were the dominant bacteria at the end of the storage period, regardless of the temperature tested. Enterobacteria and Brochothrix thermosphacta also grew, but their population density was always 2 to 3 log₁₀ CFU g⁻¹ less than that of pseudomonads. The concentration of potential indicators of spoilage, glucose and lactic acid, decreased while that of the α-amino groups increased during storage. The concentrations of these carbon sources also decreased on sterile fish blocks inoculated with strains isolated from fish microbial flora. The organic acid profile of sterile fish blocks inoculated with the above-mentioned bacteria and that of naturally spoiled fish differed significantly. An excellent correlation (r = −0.96) between log₁₀ counts of S. putrefaciens or Pseudomonas bacteria with freshness was observed in this study.     

Soil textures rather than root hairs dominate water uptake and soil–plant hydraulics under drought

Although the role of root hairs (RHs) in nutrient uptake is well documented, their role in water uptake and drought tolerance remains controversial. Maize (Zea mays) wild-type and its hair-defective mutant (Mut; roothairless 3) were grown in two contrasting soil textures (sand and loam). We used a root pressure chamber to measure the relation between transpiration rate (E) and leaf xylem water potential (ψ_{leaf_x}) during soil drying. Our hypotheses were: (1) RHs extend root–soil contact and reduce the ψ_{leaf_x} decline at high E in dry soils; (2) the impact of RHs is more pronounced in sand; and (3) Muts partly compensate for lacking RHs by producing longer and/or thicker roots. The ψ_{leaf_x}(E) relation was linear in wet conditions and became nonlinear as the soils dried. This nonlinearity occurred more abruptly and at less negative matric potentials in sand (ca. −10 kPa) than in loam (ca. −100 kPa). At more negative soil matric potentials, soil hydraulic conductance became smaller than root hydraulic conductance in both soils. Both genotypes exhibited 1.7 times longer roots in loam, but 1.6 times thicker roots in sand. No differences were observed in the ψ_{leaf_x}(E) relation and active root length between the two genotypes. In maize, RHs had a minor contribution to soil–plant hydraulics in both soils and their putative role in water uptake was smaller than that reported for barley (Hordeum vulgare). These results suggest that the role of RHs cannot be easily generalized across species and soil textures affect the response of root hydraulics to soil drying.

Root hairs of maize do not show evident contribution to root growth, water uptake, and soil–plant hydraulics, whereas soil textures affect the response of root hydraulics to soil drying.     

In Situ Biosurfactant Production by Bacillus Strains Injected into a Limestone Petroleum Reservoir

Biosurfactant-mediated oil recovery may be an economic approach for recovery of significant amounts of oil entrapped in reservoirs, but evidence that biosurfactants can be produced in situ at concentrations needed to mobilize oil is lacking. We tested whether two Bacillus strains that produce lipopeptide biosurfactants can metabolize and produce their biosurfactants in an oil reservoir. Five wells that produce from the same Viola limestone formation were used. Two wells received an inoculum (a mixture of Bacillus strain RS-1 and Bacillus subtilis subsp. spizizenii NRRL B-23049) and nutrients (glucose, sodium nitrate, and trace metals), two wells received just nutrients, and one well received only formation water. Results showed in situ metabolism and biosurfactant production. The average concentration of lipopeptide biosurfactant in the produced fluids of the inoculated wells was about 90 mg/liter. This concentration is approximately nine times the minimum concentration required to mobilize entrapped oil from sandstone cores. Carbon dioxide, acetate, lactate, ethanol, and 2,3-butanediol were detected in the produced fluids of the inoculated wells. Only CO₂ and ethanol were detected in the produced fluids of the nutrient-only-treated wells. Microbiological and molecular data showed that the microorganisms injected into the formation were retrieved in the produced fluids of the inoculated wells. We provide essential data for modeling microbial oil recovery processes in situ, including growth rates (0.06 ± 0.01 h⁻¹), carbon balances (107% ± 34%), biosurfactant production rates (0.02 ± 0.001 h⁻¹), and biosurfactant yields (0.015 ± 0.001 mol biosurfactant/mol glucose). The data demonstrate the technical feasibility of microbial processes for oil recovery.     

Effect of Nematodes on Rhizosphere Colonization by Seed-Applied Bacteria

There is much interest in the use of seed-applied bacteria for biocontrol and biofertilization, and several commercial products are available. However, many attempts to use this strategy fail because the seed-applied bacteria do not colonize the rhizosphere. Mechanisms of rhizosphere colonization may involve active bacterial movement or passive transport by percolating water or plant roots. Transport by other soil biota is likely to occur, but this area has not been well studied. We hypothesized that interactions with soil nematodes may enhance colonization. To test this hypothesis, a series of microcosm experiments was carried out using two contrasting soils maintained under well-defined physical conditions where transport by mass water flow could not occur. Seed-applied Pseudomonas fluorescens SBW25 was capable of rhizosphere colonization at matric potentials of −10 and −40 kPa in soil without nematodes, but colonization levels were substantially increased by the presence of nematodes. Our results suggest that nematodes can have an important role in rhizosphere colonization by bacteria in soil.     

Bacterial Activity at −2 to −20°C in Arctic Wintertime Sea Ice

Arctic wintertime sea-ice cores, characterized by a temperature gradient of −2 to −20°C, were investigated to better understand constraints on bacterial abundance, activity, and diversity at subzero temperatures. With the fluorescent stains 4′,6′-diamidino-2-phenylindole 2HCl (DAPI) (for DNA) and 5-cyano-2,3-ditoyl tetrazolium chloride (CTC) (for O₂-based respiration), the abundances of total, particle-associated (>3-μm), free-living, and actively respiring bacteria were determined for ice-core samples melted at their in situ temperatures (−2 to −20°C) and at the corresponding salinities of their brine inclusions (38 to 209 ppt). Fluorescence in situ hybridization was applied to determine the proportions of Bacteria, Cytophaga-Flavobacteria-Bacteroides (CFB), and Archaea. Microtome-prepared ice sections also were examined microscopically under in situ conditions to evaluate bacterial abundance (by DAPI staining) and particle associations within the brine-inclusion network of the ice. For both melted and intact ice sections, more than 50% of cells were found to be associated with particles or surfaces (sediment grains, detritus, and ice-crystal boundaries). CTC-active bacteria (0.5 to 4% of the total) and cells detectable by rRNA probes (18 to 86% of the total) were found in all ice samples, including the coldest (−20°C), where virtually all active cells were particle associated. The percentage of active bacteria associated with particles increased with decreasing temperature, as did the percentages of CFB (16 to 82% of Bacteria) and Archaea (0.0 to 3.4% of total cells). These results, combined with correlation analyses between bacterial variables and measures of particulate matter in the ice as well as the increase in CFB at lower temperatures, confirm the importance of particle or surface association to bacterial activity at subzero temperatures. Measuring activity down to −20°C adds to the concept that liquid inclusions in frozen environments provide an adequate habitat for active microbial populations on Earth and possibly elsewhere.     

Miscibility of Sphingomyelins and Phosphatidylcholines in Unsaturated Phosphatidylcholine Bilayers

Polyunsaturated phospholipids are common in biological membranes and affect the lateral structure of bilayers. We have examined how saturated sphingomyelin (SM; palmitoyl and stearoyl SM (PSM and SSM, respectively)) and phosphatidylcholine (PC; dipalmitoyl PC and 1-palmitoyl-2-stearoyl PC (DPPC and PSPC, respectively)) segregate laterally to form ordered gel phases in increasingly unsaturated PC bilayers (sn-1: 16:0 and sn-2: 18:1...22:6; or sn-1 and sn-2: 18:1…22:6). The formation of gel phases was determined from the lifetime analysis of trans-parinaric acid. Using calorimetry, we also determined gel phase formation by PSM and DPPC in unsaturated PC mixed bilayers. Comparing PSM with DPPC, we observed that PSM formed a gel phase with less order than DPPC at comparable bilayer concentrations. The same was true when SSM was compared with PSPC. Furthermore, we observed that at equal saturated phospholipid concentration, the gel phases formed were less ordered in unsaturated PCs having 16:0 in sn-1, as compared to PCs having unsaturated acyl chains in both sn-1 and sn-2. The gel phases formed by the saturated phospholipids in unsaturated PC bilayers did not appear to achieve properties similar to pure saturated phospholipid bilayers, suggesting that complete lateral phase separation did not occur. Based on scanning calorimetry analysis, the melting of the gel phases formed by PSM and DPPC in unsaturated PC mixed bilayers (at 45 mol % saturated phospholipid) had low cooperativity and hence most likely were of mixed composition, in good agreement with trans-parinaric acid lifetime data. We conclude that both interfacial properties of the saturated phospholipids and their chain length, as well as the presence of 16:0 in sn-1 of the unsaturated PCs and the total number of cis unsaturations and acyl chain length (18 to 22) of the unsaturated PCs, all affected the formation of gel phases enriched in saturated phospholipids, under the conditions used.     

Noninvasive Quantitative Measurement of Bacterial Growth in Porous Media under Unsaturated-Flow Conditions

Glucose-dependent growth of the luxCDABE reporter bacterium Pseudomonas fluorescens HK44 was monitored noninvasively in quartz sand under unsaturated-flow conditions within a 45- by 56- by 1-cm two-dimensional light transmission chamber. The spatial and temporal development of growth were mapped daily over 7 days by quantifying salicylate-induced bioluminescence. A nonlinear model relating the rate of increase in light emission after salicylate exposure to microbial density successfully predicted growth over 4 orders of magnitude (r² = 0.95). Total model-predicted growth agreed with growth calculated from the mass balance of the system by using previously established growth parameters of HK44 (predicted, 1.2 × 10¹² cells; calculated, 1.7 × 10¹² cells). Colonization expanded in all directions from the inoculation region, including upward migration against the liquid flow. Both the daily rate of expansion of the colonized zone and the population density of the first day's growth in each newly colonized region remained relatively constant throughout the experiment. Nonetheless, substantial growth continued to occur on subsequent days in the older regions of the colonized zone. The proportion of daily potential growth that remained within the chamber declined progressively between days 2 and 7 (from 97 to 13%). A densely populated, anoxic region developed in the interior of the colonized zone even though the sand was unsaturated and fresh growth medium continued to flow through the colonized zone. These data illustrate the potential of a light transmission chamber, bioluminescent bacteria, and sensitive digital camera technology to noninvasively study real-time hydrology-microbiology interactions associated with unsaturated flow in porous media.     

Water deficits and reproduction in maize : response of the reproductive tissue to water deficits at anthesis and mid-grain fill

Reproductive development in maize (Zea mays L.) is vulnerable to plant water deficits during anthesis but becomes less sensitive as reproduction progresses. To determine whether changes in tissue water status correlated with the change in sensitivity, we examined the water potential (Ψ_w), osmotic potential (Ψ_s), and turgor of reproductive tissues during a short-term water deficit imposed at anthesis or mid-grain fill. Plants were grown in controlled environments in soil. At anthesis, leaf, husk, silk, and ovary Ψ_w of control plants was similar (−0.5 to −0.65 megapascal) at midday. When water was withheld, Ψ_w decreased to −1.75, −1.3, −1.2, and −1.0 megapascal in these tissues. Net water uptake by the ovaries was inhibited, but final dry weight, solute content, and total extractable carbohydrates were similar to the controls. At mid-grain fill, leaf, husk, grain, and embryo Ψ_w of control plants were −0.55, −0.35, −0.75, and −0.80 megapascal at midday. When water was withheld, leaf and husk Ψ_w decreased to −2.4 and −1.4 megapascal within 6 days. However, grain and embryo Ψ_w remained within 0.15 megapascal of control values. The grain continued to accumulate dry matter despite a net loss of water and a reduction in total solute content. These results indicate that the response of the reproductive tissues to plant water deficits varies with stage of grain development. The maintenance of a favorable water status only after grain filling is under way may explain, at least in part, the high sensitivity to plant water deficits early in reproductive development and the decrease in sensitivity as reproduction progresses.     

Proton and copper adsorption to maize and soybean root cell walls

A surface complexation model which has been used to describe inner-sphere complexation on metal oxide surfaces was applied to the adsorption of Cu by isolated cell walls of 4-day and 28-day-old maize (Zea mays L. cv WF9 × Mo17) and 21-day-old soybean (Glycine max [L.] Merr. cv Dare) roots. Concentration dependence of the titration data prevented the determination of unique pK and capacitance values for the 4-day maize cell walls, though mean values obtained for the intrinsic pK of the titratable carboxyl groups were 3.0 (4-day maize), 3.6 (28-day maize), and 3.0 (21-day soybean) as determined by potentiometric titration with either NaOH or HCl in 20 millimolar NaCl. The constant capacitance model was applied to Cu sorption data from rapid batch equilibrium experiments in an ionic medium of 20 millimolar NaClO₄. Speciation calculations indicated that the formation of a bidentate surface complex was sufficient to describe the experimental data for all three types of plant material, with only one value for the pK and capacitance density. The intrinsic constants of Cu complexation by a neutral site are: log K = −0.3 ± 0.1, −0.2 ± 0.3, and 0.9 ± 0.1 for 4-day and 28-day maize, and 21-day soybean, respectively. The integral capacitance density parameter, which describes the relationship between surface charge density and electrical potential, is several times larger than for crystalline mineral surfaces. This result indicates that the surface electrical potential remains low even when the surface charge density is high. Such behavior is characteristic of gels and porous oxides.