Ecological relationships of wild rice, Zizania spp. 11. Electron microscopy study of iron plaques on the roots of northern wild rice (Zizania palustris)

The deposition, composition, and structure of iron plaques formed on the roots of northern wild rice (Zizania palustris L.) from five wetlands across Ontario, Canada, were examined through a scanning electron microscope and energy-dispersive X-ray analyzer. Iron was oxidized within the rhizosphere resulting in the precipitation of ferric hydroxide onto root surfaces, visible as an orange-brown plaque. The northern wild rice samples analyzed were found to form substantial iron root plaques in a variety of surface water and sediment chemistries. Iron plaques ranged structurally from thin to crust plaques (<1 mu m to 14 mu m thick) and were composed mainly of Fe, O, Al, and K. Iron plaque was found within and on root epidermal cells, occasionally filling epidermal cells and penetrating into the root cortex. Rounded grooves were observed in the plaque surface of two samples and were reasoned to have been caused by iron-reducing bacteria. Observations on the anatomy of northern wild rice roots are presented along with a model of iron plaque development.