NMR micro-imaging for visualizing freezing behaviour in plant tissues

Abstract

High resolution nuclear magnetic resonance imaging (i.e., NMR microscopy) was used to study the freezing behaviors in wintering flower buds of azalea (Rhododendron japonicum), leaf and flower buds of full-moon maple (Acer japonicum) and leaf buds of Momi fir (Abies firma) and lilac (Syringa vulgaris). The images acquired predominantly reflected the density of mobile (i.e., non-ice) protons mainly from unfrozen water. By comparing these images taken at various subfreezing temperatures, we could successfully visualize typical extra-organ freezing in the florets of R. japonicum and shoot primordia of maple and fir and deep supercooling in the maple xylem and azalea pith. Extracellular freezing was also clearly seen in the bud scales and bark tissues of these species and primordial leaves of lilac. To correctly interpret the images, all the factors that affect image intensity at differing temperatures are considered but the water to ice transition is the most dominant. Other problems that may be encountered in the interpretation of the images are also considered. The results of our studies show that NMR microscopy allows dynamic and species specific freezing processes of various plant tissues in a single specimen to be noninvasively visualized. Further, the information obtained provides deeper insight into the diversity and mechanisms involved in freezing behaviors.