Spatio-temporal patterns of chemical gradients around roots investigated with µXRF and X-ray CT

Abstract

Chemical gradients around roots are formed by water uptake and selective uptake of elements and thereby triggered radial transport processes. Gradients on different root segments are expected to vary in magnitude, e.g. root age determines duration of root-soil-contact and thus the dimension of depletion or accumulation zones. Current knowledge with respect to chemical rhizosphere gradients is primarily based on linearized (compartmentalized) or pseudo-linearized (rhizobox) systems, which do not represent the radial geometry of transport to and from roots. Within the DFG-funded Priority Program 2089 we developed a new targeted sampling on undisturbed samples containing different root segments to overcome these shortcomings.In order to evaluate the temporal change of root system architecture, we apply X-ray computed tomography (X-ray CT) and advanced tools of image analysis and registration, as the direct observation of roots in a 3D system is hindered by the non-transparency of soil.This allows a targeted sampling of specific root ages/types by extracting intact subsamples (ø 1.6 cm) from larger pots (ø 7 cm), in which the plants were grown. To investigate the influence of soil texture and root age on the formation of chemical gradients, this new subsampling protocol was first tested in a pot experiment with two Zea mays L. genotypes  (the wild-type (WT) and the corresponding mutant defective in root hair elongation (rth3)) grown for three weeks in two different textures (sand vs. loam). Resin embedded subsamples containing either segments of the primary root or young roots were imaged with micro X-ray fluorescence (μXRF) to evaluate element distributions as a function of distance to the root surfaces. First results show a higher precipitation of calcium and sulfur in the vicinity of the primary root than in the vicinity of young roots indicating an age effect. Magnitude and extend of the gradient differs between sand and loam.