PLANT PHYSIOLOGY , Vol 115, Issue 4 1515-1524, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Visualization of Freezing Behaviors in Leaf and Flower Buds of Full-Moon Maple by Nuclear Magnetic Resonance Microscopy
M. Ishikawa, W. S. Price, H. Ide and Y. Arata
Department of Genetic Resources, National Institute of Agrobiological Resources, Kannondai, Tsukuba, Ibaraki 305, Japan (M.I.)
1H-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 mobile
(i.e. non-ice) protons from unfrozen water. A comparison of NMR images
taken at different subfreezing temperatures revealed which tissues produced
high- and low-temperature exotherms in differential thermal analyses. In
leaf and lower buds of A. japonicum, the scales and stem bark tissues were
already frozen by -7[deg]C, but the primordial inflorescence and terminal
primordial shoots remained supercooled at -14[deg]C, and the lateral
primordial shoots were unfrozen even at -21[deg]C. The freezing of these
supercooled tissues was associated with their loss of viability. The size
of the supercooled primordial shoots and inflorescences was gradually
reduced with decreasing temperature, indicating extraorgan freezing in
these tissues. During this process the formation of dark regions beneath
the primordia and subsequent gradual darkening in the basal part of
supercooled primordia were visible. As the lateral shoot primordia were
cooled, the unfrozen area was considerably reduced. Since the lateral
primordia remained viable down to -40[deg]C, with no detectable
low-temperature exotherms, they probably underwent type I extraorgan
freezing. Deep supercooling in the xylem was clearly imaged. NMR microscopy
is a powerful tool for noninvasively visualizing harmonized freezing
behaviors in complex plant organs.
