Micro-PIXE investigations of apoplastic iron in freeze-dried root cross-sections of soil grown barley

Abstract

Freeze-dried cryo-sections of barley roots ( Hordeum vulgare L. cv. Alexis) were used to investigate the possible role of the root apoplast as an iron-storage pool for plants; this possibility has been a matter of controversy. Micro-PIXE analyzes in pixel mode were complemented by the STIM technique. Data were analyzed using the new Heidelberg software package biopixe, which provides true elemental maps of inhomogeneous samples such as freeze-dried cross-sections of roots. The maps clearly show a high heterogeneity of the iron distribution in roots between adjacent cell layers. Accumulations of iron were observed in the cell walls of the outermost cell layers of the roots and at the endodermis. Based on the correlation between iron and soil related elements like titanium, aluminum and silicon, most of the iron located at the root surface could be attributed to soil contamination. It could also be shown that these soil contaminations lead to an overestimation of the apoplastic iron concentrations determined by methods commonly used in the botanical field. Besides this, low accumulations of iron were observed in the cell walls of the outmost cell layers of the roots. This may indicate that the root apoplast may have a minor function in iron nutrition.