Overestimation of apoplastic Fe in roots of soil grown plants

The concentration of apoplastic Fe in roots of soil-grown plants was determined in order to evaluate the possible significance of the root apoplast as storage pool for Fe. Roots were grown in membrane bags maintaining contact with the soil solution but without direct contact with the soil. It could be shown that the concentration of apoplastic Fe in roots grown in most soils was lower than 50 mg kg(-1) root dry matter. This concentration is much lower than previously reported from nutrient solution and soil-grown cultures (about 1000 mg kg(-1) dry matter). The concentration of apoplastic Fe in strategy I and strategy II plants was little affected by the different soils used. Direct contact of roots with soil led to soil contamination at the root surface and so to an overestimation of the apoplastic Fe pool. Soil contamination of roots grown in direct contact with soil was detected by measuring the soil markers Ti, Si and Al and was further shown by PIXE. In conclusion, the low concentration of apoplastic Fe in roots demonstrates that the root apoplast is less significant as storage pool for Fe than previously assumed.