Increased groundwater levels cause iron toxicity in Glyceria fluitans (L.)

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After restoration of the hydrology in a eutrophic wetland meadow seepage strongly increased and the helophyte Glyceria fluitans intensively developed brown necrotic spots on 67% of the leaves at a site where groundwater table was highest. Sites where plants intensively developed brown necrotic spots and sites where plants did not develop these spots were compared using soil pore water chemistry, root plaque content and plant nutrient concentration. At affected sites, iron concentrations in the soil pore water were 3.2 times higher (2616 μmol l −1) resulting in 9.4 times higher iron plaque concentrations on the roots (473 μmol g −1 DW), 3.4 times higher iron concentrations in the youngest unaffected leaves (6.2 μmol g −1 DW) and 18.2 times higher iron concentrations in the affected leaves (32.8 μmol g −1 DW). No such differences were found for other elements. The location is known to be an iron-rich site with iron-rich groundwater in the subsoil. Furthermore, it is known that the reduction of insoluble Fe(III) oxides to Fe(II) in waterlogged soils is stimulated. It is therefore likely that the site with the highest groundwater table possessed the highest iron concentration in the soil pore water and that this relatively high iron concentration is responsible for the development of brown necrotic leaf spots after restoration of the hydrology. This is the first observation of brown necrotic leaf spot development, caused by iron toxicity, for wild plants growing under field conditions.