© 1998 Heron Publishing—Victoria, Canada
Freezing behaviors in leaf buds of cold-hardy conifers visualized by NMR microscopy
Hiroyuki Ide (1), William S. Price (1), Yoji Arata (1) and Masaya Ishikawa (2, 3)
1. Water Research Institute, Sengen 2-1-6, Tsukuba, Ibaraki, 305-0047 Japan / 2. Department of Genetic Resources, National Institute of Agrobiological Resources, Kannondai 2-1-2, Tsukuba, Ibaraki, 305-0856
Japan / 3. Author to whom correspondence should be addressed / Received August 26, 1997
Summary
1H-Nuclear magnetic resonance (NMR) microscopy was used to study freezing behavior in wintering leaf buds of Momi fir (Abies firma Sieb. et Zucc.) and Japanese red pine (Pinus densiflora Sieb. et Zucc.). The images acquired predominantly reflected the density of mobile (i.e., non-ice) protons mainly from unfrozen
water. By comparing images taken at various subfreezing temperatures, we determined which tissues produced the high and low
temperature exotherms detected by differential thermal analyses. Typical extra-organ freezing was successfully imaged in leaf
buds of A. firma. The bud scales readily froze at –7 °C, but shoot primordia remained supercooled to –14 °C in December buds and to –21 °C
in March buds. The size of supercooled shoot primordia was reduced with decreasing temperature, indicating a gradual decrease
in water content of the shoot primordia. In contrast, the signal from shoot primordia of P. densiflora disappeared between –7 and –14 °C, corresponding to the high temperature exotherm at –8 °C, indicating extracellular freezing
of the shoot primordia. The xylem and bark tissues readily froze at –7 °C in A. firma and between –7 and –14 °C in P. densiflora. We conclude that NMR microscopy can noninvasively provide more spatially specific information about freezing behavior in
leaf buds than traditional methods such as differential thermal analysis. In particular, it allows the organized and harmonized
freezing behaviors in complex organs to be visualized directly thereby revealing the diversity of mechanisms involved in freezing
behaviors.
Keywords:
Abies firma, cold hardiness, extra-organ freezing, fir, magnetic resonance imaging, MRI, microimaging, nuclear magnetic resonance (NMR)
microscopy, pine, Pinus densiflora, shoot primordium, supercooling.
