VISUALIZATION OF FREEZING BEHAVIORS IN FLOWER BUD TISSUES OF COLD-HARDY RHODODENDRON JAPONICUM BY NUCLEAR-MAGNETIC-RESONANCE MICROIMAGING

H-1 nuclear magnetic resonance (NMR) micro-imaging was used to study t he freezing behaviour of wintering flower buds of Rhododendron japonic um (A. Gray) Suringer. A multi-slice multiecho pulse sequence was used to acquire images at different subfreezing temperatures. The images o btained predominantly reflected the density of mobile (i.e. non-ice) p rotons mainly from unfrozen water. By comparing these images taken at various subfreezing temperatures, we could determine which tissues pro duced high temperature exotherms and low temperature exotherms in diff erential thermal analyses. In flower buds of the cold-hardy R. japonic um, typical extra-organ freezing was successfully imaged. The scales r eadily froze at -7 degrees C but some florets remained supercooled eve n at -21 degrees C. The size of the supercooled florets was reduced wi th decreasing temperature which indicated a gradual decrease in floret water content. With decreasing temperature, there was a gradual decre ase in the signal intensity of the flower bud axis including the pedun cle and immature pith tissues, which implies either dehydration or par tial freezing of these tissues. Deep supercooling in the entire mature pith tissues was also clearly visible in these images. Due to its non -invasive nature, NMR micro-imaging is a useful tool for studying free zing behaviours in various plant tissues, especially for imaging organ ised or harmonised freezing in complex organs as well as for clarifyin g the diversity and mechanisms involved in freezing behaviours.