Supercooling in overwintering azalea flower buds: additional freezing parameters.

Results of calorimetric, nuclear magnetic resonance, and low temperature light microscopic studies on supercooled azalea (Rhododendron kosterianum, Schneid.) floral primordia are reported. Heat release during freezing of the supercooled floral primordia is in the range predicted for supercooled pure water. Spin-lattice and spin-spin relaxation times measured by pulsed nuclear magnetic resonance spectroscopy decreased after freezing, suggesting that a redistribution of tissue water is associated with injury to the floral primordium. The calorimetric and low temperature microscopy studies showed no detectable ice formation in floral primordia until the major freezing event at low temperature. No resistance to ice growth is found to exist in the primordium tissues, indicating that a freezing barrier or thermodynamic equilibrium exists between the unfrozen primordium and other flower bud parts which contain ice at subfreezing temperatures.