Tissue-specific nuclear spin-spin relaxation anisotropy and water nano-confinement in celery stalks.

Recent magnetic resonance imaging (MRI) measurements have revealed proton spin-spin relaxation anisotropy in only one component of the celery stalk, even though water accounts for up to 95% of its weight and is present throughout all its tissues. Our analysis shows that this anisotropy arises from nanoconfined water within the collenchyma cell walls. In these regions, restricted molecular motion and hydrogen bonding between water molecules and cell-wall constituents give rise to residual anisotropic dipolar interactions. In contrast, water in the xylem, phloem, and parenchyma occupies larger cavities, acts like a bulk liquid, and shows no angular dependence in its nuclear relaxation behavior. These results suggest that nuclear spin-spin relaxation anisotropy may serve as a unique feature of nanoconfined water in biological tissues, contributing to the study of water behavior in plants, their microscopic structures, and aiding research on plant physiology.