The influence of redox conditions in soils on extraplasmatic Fe-loading of plant roots

Loading of extraplasmatic Fe, as a potential storage pool for Fe nutrition, was studied in roots of maize grown under hypoxic conditions in soil culture. Extraplasmatic Fe loading was investigated depending on (i) duration of flooding (0, 1, 2 or 4 days) and (ii) microbial activity as affected by graduated addition of carbon sources (0, 2 or 10 g each starch and cellulose kg^?1 soil). Maize plants were grown in a soil culture system with root systems enclosed in membrane bags to avoid Fe contamination of the root surface by soil particles. Due to the high redox buffer capacity of the Haplic Luvisol employed for the experiments, flooding treatments induced only moderately reducing conditions (～?300 mV) and a slight increase of extraplasmatic Fe loading (41\to165 mg kg^?1 d.m.). Strongly reducing conditions (?100 mV) associated with a high Fe²⁺ concentration in the soil solution and a significant increase of extraplasmatic Fe (1190 mg kg^?1 d.m.) were obtained only after application of high amounts of organic carbon (10 g starch and 10 g cellulose kg^?1 soil), which accompanied by unrealistic reducing conditions due to intense stimulation of microbial growth. The expression of effects only under extremely high application level of organic carbon (～?33 t C ha^?1) suggest that similar to aerobic conditions, extraplasmatic Fe-loading under transient hypoxia is probably of limited ecological significance for the iron nutrition of higher plants, at least in soils with a high redox buffer capacity as employed in the present study. Abbreviations: DHA – dehydrogenase activity; d.m. – dry matter; DOC – dissolved organic carbon; Eh – redox potential; PIXE – proton-induced X-ray emission; STIM – scanning transmission ion microscopy.