Background and Aims Portable magnetic resonance imaging (MRI) is a promising tool to study water in plant roots, as it is non-invasive and allows measurements outside the laboratory. We aim, using this method, to investigate root structure-function relationships in ten herbaceous species with diverse root traits. Methods The species were grown in well-watered rhizotrons with the roots separated from the soil. The MR signal from water in the roots was measured using a unilateral MRI to evaluate the water quantity present in the measurement volume, as well as the transverse relaxation time which gives information on water mobility. Ecophysiological and morphological measurements were also performed. Results A positive relationship was observed between the intensity of the root water MR signal and the root water quantity across species. Diurnal variation in the transverse relaxation time was consistent with changes in leaf water potential and soil humidity. Additionally, the water pool fraction linked to the long transverse relaxation time was negatively related to fine root tissue density, a proxy of stele fraction. In addition, our results indicated the presence of root structure-function relationships in water-use, as illustrated by negative relationships between specific root length and root water quantity derived from MR signal, and between fine root tissue density and water mobility derived from T2 measurements. Conclusion This proof-of-concept study demonstrated the capacity of portable MRI to estimate root water quantity and to detect its diurnal fluctuation in various herbaceous species exhibiting diverse water-use strategies.