Roots plaques

St-Cyr L, Crowder AA. 1990. Manganese and copper in the root plaque of Phragmites australis (Cav.) Trin. ex Steudel. Soil Science 149 191-198. 

St-Cyr, L. and Campbell, P.G.C. (1996) Metals (Fe, Mn, Zn) in the root plaque of submerged aquatic plants collected in situ relations with metal concentrations in the adjacent sediments and in the root tissue, Biogeochemistry 33, 45-76. 

The rhizosphere is home to a diverse microbial community, including aerobic heterotrophs (Gilbert and Frenzel, 1998), methane oxidizers (Bosse and Frenzel, 1997 Calhoun and King, 1997), and ammonium and nitrite oxidizers (Bodelier et al., 1996 Arth et al., 1998). Microscopy has also shown that microbial cells are associated with Fe plaque (Trolldenier, 1988 St-Cyr et al., 1993), but visual examinations alone cannot determine if cells are responsible for plaque formation. Trolldenier (1988) demonstrated that rusty-colored colonies formed when root plaque was inoculated into an iron-containing medium, but further characterization of these colonies was not attempted. 

There was a moderate adsorption of Al to Fe plaque, and this resulted in reduced Al uptake in plant tissue of Phragmites (Batty et al., 2002). Copper is concentrated in Fe plaque but will preferentially adsorb to Mn if Mn is present in the plaque (St-Cyr and Crowder, 1990 Ye et al., 2001). The sequestration of Cu in the root plaque appears to reduce uptake of Cu by the plant. However, as Cu concentrations build up on the Fe plaque, the amount of Cu transported into the plant tissue can increase (Ye et al., 2001). Zinc can also be concentrated in Fe plaque. For Typha, this effect is more pronounced in the field than in artificial laboratory microcosms (Ye et al., 1998). Several reports have shown... 

Geochemical analyses of the contaminated sediments in the root zone using sequential chemical extractions showed that greater than half of the arsenic is strongly adsorbed (Keon et al. 2000, 2001). A mixture of arsenic oxidation states and associations was observed and supported by bulk XANES and EXAFS data collected at the SSRL. Arsenic in the upper 40 cm of the wetland, which contains the peak corresponding to maximum deposition, appears to be controlled by iron phases, with a small contribution from sulfidic phases. The results suggest that iron oxide phases may be present in the otherwise reducing wetland sediments as a substrate onto which arsenic can adsorb, perhaps due to cattail root plaque formation. 

The dominant and most reported component of root plaques is various oxidized compounds of iron. Microscopic observations of root plaques show a highly heterogenous morphology composed mostly of an amorphous material dispersed throughout nodules (50-300 nm in diameter), needles (50-100 nm in length), and filaments with variable lengths. This iron plaque formation on roots results from diffusion of Fe + toward the root zone in response to concentration gradients at the interface (similar to those observed at the soil-floodwater interface). The oxidized rhizosphere functions as a sink for Fe + and other reduced substances. 

FIGURE 7.30 (a) Conceptual diagram showing possible root plaque interference with phosphorous uptake... 

Wetland plants provide an oxygenated environment in the root zone, which can fnnction as an effective sink by oxidizing dissolved Fe(II) and Mn(ll). Several stndies have shown root plaque formation on snrface roots are due to the precipitation of Fe(lll) and Mn(IV) oxides (see Section 10.7 for additional details). 

Soluble Fe(ll) and Mn(II) are oxidized and precipitated on root surfaces, resulting in plaque formation. Although the actual site for Fe(II) and Mn(ll) oxidation in the root zone varies with plant species, the plaque formation typically follows oxygen release from roots. Details of plaque formation in the root zone of wetland plants are discussed in Chapter 7. In this section, consequences of root plaque formation will be discussed. Factors controlling iron plaque formation on roots have been reviewed in detail by Mendelssohn et al. (1995). Plaque on root surfaces can have both positive and negative effects on plants. These include... 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

Metal cations in the soil solution may be immobilized by sorption onto iron plaque on root surfaces in submerged soils, in the same way that solubilized Zn + was re-adsorbed on ferric oxide in the experiments in Figure 6.22. Sequestering of metals on the external surfaces of wetland roots in this way limits uptake... 

Hansel CM, Fendorf S, Sutton S, NewviUe M. 2001. Characterization of Fe plaque and associated metals on the roots of mine-waste impacted aquatic plants. Environmental Science Technology 35 3863-3868. 

Otte ML, Rozema 1, Koster L, Haarsma MS, Broekman RA. 1989. Iron plaque in roots of Aster tripolium L. interaction with zinc uptake. New Phytologist 111 309-317. 

Ye ZH, Baker AIM, Wong MH, Willis AJ. 1997a. Copper and nickel uptake, accumulation and tolerance in Typha latifolia with and without iron plaque on the root surface. New Phytologist 136 481-488. 

Beighton D and Lynch E (1995) Comparison of selected microflora of plaque and underlying carious dentine associated with primary root caries lesions. Caries Res 29, 154-158. 

Schiipbach P, Osterwalder V and Guggenheim B (1995) Human root caries microbiota in plaque covering sound, carious and arrested root surfaces. Caries Res 29, 382-395. 

The excretion of proteolytic enzymes by plaque microorganisms (Suido et al., 1986) probably accounts for the proteolytic activity observed in carious dentin (Larmas et al.,1968 Larmas, 1972). Proteases may also derive from the crevicular fluid (Cimasoni et al., 1977), when the root... 

Chabbi, A. (1999) Juncus bulbosus as a pioneer species in acidic lignite mining lakes interactions, mechanism and survival strategies. NewPhytol. 144 133-142 Chabbi, A. Hines, M.E. Rumpel, C. (2001) The role of organic carbon excretion by bulbous rush roots and its turnover and utilization by bacteria under iron plaques in extremely acid sediments. Environ. Experimental Botany 46 237-245... 

Zhang, X. Zhang, F. Mao, D. (1999) Effect of iron plaque outside roots on nutrient uptake by rice Oryza sativa L.) Phosphate uptake. Plant and Soil 209 187-192 Zhang,Y Charlet, L. Schindler, P.W. (1992) Adsorption of protons, Fe(II) and Al(IIl) on lepidocrocite (y-FeOOH). Colloids Surfaces 63 259-268... 

In dentistry, they are used for sterilization of certain instruments and prevention and treatment of dental plaque and peridental diseases. They are also used in root canal therapy (RCT), treatment of acute necrotizing gingivitis and other infective oral conditions. Antiseptics and disinfectants are also used as ingredient in various dentifrices. 

In water logged soils radial oxygen loss from the root raises the redox potential in the rhizosphere as a consequence of which iron oxide plaques are seen to develop on root surfaces. Bacha and Hossner (1977) removed the coatings on rice roots growing under anaerobic conditions. The coatings were composed primarily of the iron oxide mineral lepidocrocite (y-FeOOH) as the only crystalline component. St-Cyr and Crowder (1990) studied the iron oxide plaque on roots of Phragmites and detected both Fe and Mn. The Fe Mn ratio of the plaque resembled the ratio of Fe Mn in substrate carbonates. The plaque material also contained Cu. 

Maigolis HC, Zhang YP, Gewirtz A, van Houte J, Moreno EC Cariogenic potential of pooled plaque fluid from exposed root surfaces in humans. Arch Oral Biol 1993 38 131-138. 

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