Illuminating Water and Nutrient Uptake in Plants with Neutron Radiography: Comparing Deuterium Oxide and Gadolinium Tracers

Abstract

Neutron radiograph imaging has shown immense potential to improve our understanding of internal plant water dynamics. Due to \(^{1}\)H’s high scattering of neutrons, neutron radiograph images can distinguish water from other materials in a plant to visualize water distribution across the soil or plant matter. In order to visualize temporal dynamics in a largely steady-state system, deuterium oxide (heavy water) is often used as a contrasting agent since \(^{2}\)H has a lower attenuation of neutrons than \(^{1}\)H. This experiment seeks to compare plant water and nutrient uptake patterns using D\(_{2}\)O and a new gadolinium metal-based tracer through a black poplar sapling’s (Populus nigra) stem, petioles, and main veins. Gd compounds are commonly used as intravenous tracers for MRI brain imaging because of Gd’s unique nuclear behavior. Its nuclear behavior also enables Gd to attenuate neutrons, rendering it a useful contrasting agent in neutron radiography. This experiment found that Gd compounds in solution moved through the plant less uniformly than the D\(_{2}\)O tracer and thereby the bulk water flow, suggesting aggregation and accumulation of Gd particles in certain plant regions. Although this study demonstrates that a chelated Gd compound is not a suitable tracer for bulk water flow, it has potential to be a proxy for nutrient uptake through a plant—particularly for chelated heavy metal nutrients such as Fe and Mn—and should be further investigated to determine if its behavior matches that of plant micronutrients or macronutrients. Visualizing nutrient flow through the plant may inform our understanding of crop uptake of key nutrients or ecological responses to different soil nutrients.