Enhancing the study of plant root and soil interactions using radiocontrast agents in X-ray computed tomography Imaging

Abstract

X-ray computed tomography (XCT) is a non-destructive 3D imaging technique now commonly employed for imaging plant and soil systems. In particular it is increasingly being utilised to capture plant root/ soil structures, in place of traditional destructive dissection techniques. One of the major challengesin the use of the technology is the similar X-ray attenuation characteristics of plant roots and soil-water which results in a low contrast to noise ratio between these materials – particularly when imaging larger soil samples. Where similar issues of poor contrast occur within biomedical imaging, these issues are overcome with the use of radiocontrast agents (solutions or suspensions containing elements with high X-ray attenuation characteristics). The aim of the research presented in this thesis is to investigate novel strategies for the application of various contrast agents in order to enhance the study of plant and soil structures and to then use contrast agents in innovative applications to capture the movement of solutes through soil and roots. I begin with a review of the current literature, determining the key requirements of contrast agents for imaging biological tissues: minimal toxicity, low reactivity, stability and mobility. From this review of the literature I determined that nanoparticulate contrast agents are best suited to tissue specific accumulation as a functionalised contrast agent. However, the challenge in utilising nanoparticulate contrast agents for this role is the instability of nanoparticles in soil conditions. I also determine that non-ionic iodinated contrast agents are well suited to fluid flow tracing applications owing to their mobility, potential low viscosity and comparatively low reactivity with organic tissues. In the first scientific research chapter, published as Scotson et al., (2019), I present work in which we stabilised gold nanoparticles for use in soil environments using a polymer coating and demonstrate for the first time that high concentrations of nanoparticles are capable of producing contrast against soil in XCT. The second scientific chapter, published as Scotson et al., (2020a), investigates the innovative use of an iodinated contrast agent as an XCT-visible analogue for field applied soluble-agrochemicals in order to trace the movement of solutes through flat or ridge and furrow soil surface geometries and assess the impact of surface ponding. We observe that plant roots can reduce the infiltration depth of solutes, though this effect is localised to likely root-dense regions, but that surface ponding can negate this - especially within furrows. In the final scientific chapter we present a manuscript, published as Scotson et al., (2020b), in which we construct an original and novel experimental system to allow for application of contrast agents directly to in vivo plant roots in soil which is also compatible with synchrotron XCT imaging. Using a combination of synchrotron XCT imaging and synchrotron X-ray fluorescence mapping we are able to capture internal plant solute transport and visualise anatomical features. These represent significant advances in developing new techniques for using contrast agents in plant and soil studies and demonstrates the capacity for contrast agents to trace solute movement in soil and plant systems.

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Identifiers

ORCID for Tiina Roose: orcid.org/0000-0001-8710-1063

ORCID for Martin Browne: orcid.org/0000-0001-5184-050X

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