MRI as a Noninvasive Method to Study Na⁺ Distribution in Soil and Roots

One of the major challenges in studying root-soil interactions is the lack of easy access to the rhizosphere and measurements in the root zone are often destructive to the roots. In this study, we demonstrate the potential of Magnetic Resonance Imaging (MRI) as a 3D, noninvasive and continuous measurement technique to study the temporal development of the spatial distribution of dissolved sodium (Na⁺) around roots. MRI was used for the first time to image Na⁺ distribution in soil and around roots, and even inside the root and the leaves. A preparatory experiment was done to determine detectability of sodium ions by MRI in partially saturated soil material. According to expectation, MRI signal is not dependent on saturation and signal intensity is proportional to the Na⁺ bulk concentrations. Next, we imaged the evaporation driven enrichment of Na⁺ in the top layer of a soil column under constant lower boundary conditions. The final step was imaging Na⁺ enrichment in tomato root systems. The plants were irrigated with saline water (EC =4.5 dSm^-1) and each plant was scanned twice: first scan was done using ¹H^-coil for imaging the root system architecture (RSA); immediately after, the plant was scanned with ²³Na sensitive coil for mapping Na⁺ distribution in the soil. Scan times were about 1.5 hour for RSA with an in-plane resolution of 0.16 mm and slice thickness of 0.6 mm. For Na⁺ we achieved an isotropic resolution of 1 mm at a scan time of about 8 hours. The obtained images showed the Na⁺ tends to accumulate more in the root zone compared with the bulk soil. MRI allows us to study processes at the root-soil interface without disturbing the root system.