Many trace metal and metalloids elements are essential for life, but excessive accumulation in food crops can have adverse health effects. Thus, it is important to understand the mechanisms of uptake and accumulation. Knowledge of the metal distribution and metal binding to understand the trace metal trafficking and storage, can be obtained using X-ray fluorescence techniques. Synchrotron X-rays in tomography and microspectroscopy provide such information on shock frozen tissues with spatial resolution of some hundreds of nanometers to the micrometer range. However, only very small sections of leaves and roots can be studied. Without additional information, it remains unknown whether the obtained results are representative of the entire leaf or only a feature of a certain region. Due to the high counting time needed for a good signal to noise ratio, only duplicates, and sometimes triplicates can be studied.

Using a benchtop X-ray fluorescence machine 2D raster scans of an entire leaf or bigger sections of a root can be obtained. Furthermore, a custom-designed measuring chamber allows for measurements of intact leaves and roots. The combination of the two approaches (synchrotron and benchtop) allows to quantify the variation within the same leaf, and among a bigger set of replicates. In this study tomograms were collected at DESY P06 using the Maia detector. The large solid angle of Maia allows faster data collection with a relatively low dose, which diminishes the risk of beam damage. Raster scans of the fluorescence of whole leaves and roots sections were collected in a customized Bruker M4 TORNADO machine equipped with two large area XFLASH SSD detectors.

We show two case studies. The non-accumulator Noccaea ochroleucum exposed to Turnip Yellow Mosaic Virus (TYMV) was studied to understand how the pathogen infection changes the metal distribution in these plants. Both µXRF tomograms and XANES were measured on control and virus-infected leaves. The XANES spectra were compared with model solutions bearing all ligations possible for Zn speciation. XANES revealed a different pattern for the accumulation of Zn for the virus infected plant. In the second case we studied element distribution along the root tips of soybean under Zn deficient and Zn excess conditions. The roots were measured in vivo both with and without desorption of elements in the apoplast.