Improving Visualisation of In-Vivo Root Anatomy and Fluid Transport in Soil Using Iodinated Contrast in Synchrotron XCT and XRF

Synchrotron X-ray computed tomography (SRXCT) imaging is a technique now commonly deployed for non-destructive 3D visualisation of root morphology in soil environments. However, visualising the internal anatomy of roots in soil using SRXCT can be difficult since the energy required for sufficient X-ray transmission through soil often results in poor contrast between root tissues. This reduces the amount of obtainable information about root anatomy and the effects of the soil environment on plant root internal structure. Contrast media is often used in SRXCT imaging to increase the contrast between tissues, enabling greater ease of both visualisation and image processing for internal structures of biological material.

In this work, we demonstrate that by introducing root material exposed to iodinated contrast media we can overcome these limitations and visualise internal root anatomy of in vivo roots intact within soil. To achieve this, we undertook time-resolved SRXCT imaging of juvenile maize plants growing in a specially designed growth system over a period of 24 hours. This system was designed such that only the base of the primary root would be suspended into iodinated contrast media whilst the rest of the root system remained in soil partially saturated with water, and the plant remained intact and alive. This enabled the basal section of primary root to take up iodinated contrast media without dispersal of the contrast media into the soil. Following the time-resolved imaging of the root system, leaf and stem material were then imaged using SRXCT and mapped using synchrotron X-ray florescence (SRXRF). Using this system, we were able to visualise and segment anatomical root features that are otherwise difficult to capture in vivo in soil using non-destructive 3D imaging such as vascular bundles (including phloem, xylem and proto-xylem) and structures within the cortex. We also gained inferences into fluid flow and transport within in vivo roots in soil based on this technique. The SRXCT imaging as well as the SRXRF mapping of stem and leaf material confirmed transport of the iodinated contrast media through plant vasculature and the distribution into leaf venation. This investigation demonstrates the quantity of data on internal root anatomy and fluid transport for in-vivo roots in soil that could be yielded from SRXCT and SRXRF in future.