Mathematical modelling of iron uptake by graminaceous species as affected by iron forms in soil and phytosiderophore efflux

Graminaceous species excrete nonproteoinogenic amino acids (phytosiderophores) as an adaptive mechanism to iron (Fe) deficiency. According to Marschner and Romheld, these organic molecules ran mobilize Fe in soil and are taken up as Fe(III)- phytosiderophore complexes with a high rate. The quantitative effect of the efflux of mugineic acid (MA) as a representative of the phytosiderophores on the influx of chemically mobilized Fe into the roots was calculated depending on different Fe forms in soil, such as goethite, ferrihydrite and humic-Fe complexes. The calculations show that MA mobilizes Fe from goethite at a low rate. As a consequence, the calculated influx of mobilized Fe from soil with goethite as the only Fe form was very low compared to the demand of the plants. If ferrihydrite was the principal Fe form in soil, the influx of chemically mobilized Fe was much higher and met the required demand of the plants if the MA efflux was medium to high, according to the data of Romheld and Marschner (1990). With humic-Fe complexes as the only Fe form in soil, the influx of chemically mobilized Fe was much higher compared to soil with inorganic Fe forms. The reason for this result is that Fe from humic-Fe complexes is easily mobilized by MA even if the efflux of the phytosiderophore is low. The calculations show the strong importance of Fe bound by humic substances for the Fe acquisition by strategy II plants.