The effect of phytophagous beetles on plant succession in an old—field community in southeastern Ontario was investigated over a 5—yr period. An experimental comparison of insecticide treated and untreated plots indicates that grazing by Trirhabda spp. on the goldenrod Solidago canadensis resulted in a reduction in precent cover of this species (from 40—70% to °1%), and a significant increase in percent cover of a number of earlier stage successional species.

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Roots of Typha latifolia L. exposed to Fe 2 + under reduced conditions in solution culture developed visible coatings (plaques) of an oxidized Fe compound that extended as much as 15-17 /tm into the rhizosphere. Iron concentrations were significantly less and discoloration was not apparent on the surface of roots exposed to Fe-(BPDSh, Fe>, Fe-EDDHA, and Fe-EDT A. The extent of plaque formation increased with the concentration of Fe 2 + in solution and with pH of the solution in the range of 3.0 to 4.6. Above pH 4.6, oxidation of Fe 2 + in the culture solution may have reduced precipitation of Fe on the root surface. Plaque development was most extensive approximately 1.0 cm from the root tip, but all root surfaces showed some Fe staining. Scanning electron micrographs of plaqued roots, grown both in solution culture and in the field, provided support for a model of cast formation by oxidation and precipitation of Fe on external cell surfaces. 1

Deposition of an iron oxide plaque or coating on roots of rice (Oryza sativa L. cv. M-201 84 Biggs) ameliorated the toxic effects of copper (Cu), nickel (Ni), and Cu + Ni on plant growth and affected patterns of metal uptake and accumulation. Plaque was formed on plants during 24 h in an anaerobic solution containing 20 mg ∙ L−1 iron at pH 5.5. Subsequently two additional coatings were deposited after new growth of roots. Plants were grown hydroponically in one-quarter strength Hoagland's solution and exposed to Cu (0.5 mg ∙ L−1), Ni (2.0 mg ∙ L−1), or Cu + Ni; exposure to metals lasted 21 – 45 days. At the end of the experimental period growth was estimated as total biomass, dry weight of root, dry weight of shoot, length of leaf, and length of root. After harvesting, Fe, Cu, and Ni concentrations in plaque were measured using a dithionite–citrate–bicarbonate extraction, and internal Fe, Cu, and Ni concentrations were measured in roots and leaves using atomic absorption spectrophotometry. Formation of the palque did not affect growth of plants in control solutions. When exposed to Cu, plants with plaque had significantly larger biomass, dry weight of roots, dry weight of leaves, and growth increments of root and leaf than plants without plaque. When exposed to Ni, plants with plaque had signifiantly higher increment in leaf growth than plants without plaque. When exposed to Ni + Cu, plants with plaque had significantly higher biomass than plants without plaque. Toxic symptoms were not observed on plants with plaque grown with excess copper, while all other plants exposed to metals showed chlorosis, necrosis, or both. Metals were detected on the surface of roots, whether plants were allowed to develop a plaque or not. Cu, Ni, and Fe were concentrated in plaque and in roots. Lower concentrations of Cu and Ni were found in leaves of plants with a plaque than in plants without plaque. Comparisons of concentrations of Fe, Cu, and Ni with those of other species grown under field conditions are made. Key words: rice, iron plaque, toxicity of Cu and Ni.

Replicate samples of Phragmites communis roots coated with hydrous iron oxide were analyzed for internal and external Fe using the dithionite‐citrate‐bicarbonate technique (hot DCB), a modified DCB technique (cold DCB), Ethylenediamine tetraacetic acid (EDTA), Diethylenetriamine pentaacetic acid (DTPA), and a total Fe analysis. EDTA and DTPA were found to be inefficient in extracting FeOOH while both DCB techniques were efficient. Concentrations of Fe extracted (8.93% and 9.37% by weight) and percent of total Fe extracted (98.3% and 98.6%) by the hot and cold DCB techniques (respectively) were not significantly different. Results of the total Fe analysis (8.58% Fe by weight) were the same as extracted Fe and total Fe determined by the DCB techniques. This may reflect the large external Fe : internal Fe ratio of the sample roots. Despite equivalent results, the cold DCB technique is preferred over the hot DCB technique due to a reduced likelihood of structural damage to roots.