VISUALIZATION OF FREEZING BEHAVIORS IN LEAF AND FLOWER BUDS OF FULL-MOON MAPLE BY NUCLEAR-MAGNETIC-RESONANCE MICROSCOPY

H-1-Nuclear magnetic resonance (NMR) microscopy was used to study the freezing behavior of wintering buds of full-moon maple (Acer japonicum Thunb.). The images obtained predominantly reflected the density of m obile (i.e. non-ice) protons from unfrozen water. A comparison of NMR images taken at different subfreezing temperatures revealed which tiss ues produced high-and low-temperature exotherms in differential therma l analyses. In leaf and flower buds of A. japonicum, the scales and st em bark tissues were already frozen by -7 degrees C, but the primordia l inflorescence and terminal primordial shoots remained supercooled at -14 degrees C, and the lateral primordial shoots were unfrozen even a t -21 degrees C. The freezing of these supercooled tissues was associa ted with their loss of viability. The size of the supercooled primordi al shoots and inflorescences was gradually reduced with decreasing tem perature, indicating extraorgan freezing in these tissues. During this process the formation of dark regions beneath the primordia and subse quent gradual darkening in the basal part of supercooled primordia wer e visible. As the lateral shoot primordia were cooled, the unfrozen ar ea was considerably reduced. Since the lateral primordia remained viab le down to -40 degrees C, with no detectable low-temperature exotherms , they probably underwent type I extraorgan freezing. Deep supercoolin g in the xylem was clearly imaged. NMR microscopy is a powerful tool f or noninvasively visualizing harmonized freezing behaviors in complex plant organs.