Magnetic resonance studies of adaptation to freezing and injury in plants

Freezing is likely the severest environmental stress for plants. Plant distribution and agricultural production are often limited by the minimum winter temperatures. Temperate woody perennials are exposed to recurrent freeze-thaw cycles during winter. How do their organs/tissues cope with freezing of tissue water? Cold hardy plants have evolved various strategies (freezing behaviours) to avoid lethal intracellular freezing. These freezing behaviours are tissue and species specific and require various mechanisms to regulate the behaviours (phase and movement) of tissue water. Yet, their diversity, dynamics and mechanisms remain poorly understood as freezing events in complex thick plant organs are not visible and difficult to analyse. Magnetic resonance provides a suite of tools, including magnetic resonance imaging, relaxation and diffusometry, for studying the underlying processes in exquisite detail both at the plant organ/tissue level and also for understanding the molecular level interactions involved in the ice nucleation process. Importantly, magnetic resonance can do this non-invasively which is especially important when studying such delicate thermodynamic systems involving supercooled water. This chapter gives an overview of the relevant physiological backgrounds and water physical properties including the molecular dynamics of water (i.e., self-diffusion and reorientational motion) and relationships to plant structure. An overview of the salient points of the magnetic resonance 'toolbox' is given to enable non-magnetic resonance experts to understand the literature. Finally, numerous applications of the magnetic resonance techniques to freezing and injury in plants are surveyed and consideration given to possible future studies, such as imaging localisation of particular functional compounds and visualising adaptive strategies to arid environments.