Changes in soil water content resulting from Ricinus root uptake monitored by magnetic resonance Imaging

Magnetic resonance imaging (MRI) was used to study the soil water content changes caused by root water uptake. A 4-wk-old Ricinus communis carmencita with a highly developed root system was planted in a cylindrical container filled with a model soil ( 99.5% fine sand, 0.5% clay), fully water saturated. The bottom and surface of the container were sealed so that water loss by factors other than transpiration via the leaves could be neglected. The water content of the soil was monitored for 3 wk using the MRI sequence SPRITE at an isotropic spatial resolution of 6.3 mm. In contrast to conventionally used MRI sequences, the T-2* relaxation was monitored, temporally resolved, and extrapolated to zero. This procedure is a better measure of water content than a single signal at a given time point since it eliminates varying MRI relaxation times during soil desiccation. A linear correlation between the MRI-determined and gravimetrically measured total water content proves the correctness of the monitoring and data evaluation procedure. Simultaneously, the root architecture was also imaged at 0.6 mm isotropic resolution by the MRI sequence constructive interference in steady state (CISS), which yielded a good contrast between soil and roots. The coregistration of both types of imaging ( water content and root architecture) indicates that greater changes in water content took place in the bottom region and near the surface, where the highest root densities were found.