Calorimetric, X-ray diffraction, and 31P nuclear magnetic resonance (NMR) studies of aqueous dispersions of 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) gel phases at low temperatures (-60 to 22 degrees C) show thermal, structural, and dynamic differences when compared to aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) gel phases at corresponding temperatures. Differential scanning calorimetry of DHPC dispersions demonstrates a reversible, low-enthalpy "subtransition" at 4 degrees C in contrast to the conditionally reversible, high-enthalpy subtransition observed at 17 degrees C for annealed DPPC bilayers. X-ray diffraction studies indicate that DHPC dispersions form a lamellar gel phase with dav congruent to 46 A both above and below the "subtransition". It is suggested that the reduced dav observed for DHPC (46 A as compared to 64 A in DPPC) is due to an interdigitated lamellar gel phase which exists at all temperatures below the pretransition at 35 degrees C. 31P NMR spectra of DHPC gel-phase bilayers show an axially symmetric chemical shift anisotropy powder pattern which remains sharp down to -20 degrees C, suggesting the presence of fast axial diffusion. In contrast, 31P spectra of DPPC bilayers indicate this type of motion is frozen out at approximately 0 degrees C.
Observations in a micro-freezing apparatus of isolated tissues of the cortex of hardy and non-hardy plants of Catalpa and Cornus species, and of the epidermis of red cabbage, reveal that there are two modes of freezing of plant cells, intracellular and extracellular.In intracellular freezing, ice crystals form first in the protoplasm and then in the vacuole. In extracellular freezing, ice forms outside the cells from water in the cells. The resulting dehydration of the cell causes its collapse, the opposite walls coming together and squeezing the contents to the periphery. Intracellular freezing is fatal to all cells by visible mechanical disruption of the protoplasm and vacuole. It is facilitated by rapid freezing and occurs less easily and less frequently in hardy tissues and in trees and shrubs than in non-hardy and herbaceous tissues. Extracellular freezing induced through slow cooling is fatal to all unhardy cells in trees and herbs at all temperatures below the freezing point, and to cells of hardy cabbage only at − 10 °C. to − 15 °C., but not to cells of hardy trees and shrubs.Both types of ice formation have been observed in intact plants of red cabbage frozen in a refrigerator.The behavior of hardened plants shows that intracellular freezing tends to be prevented in them by an increased permeability to water. In regard to extracellular freezing, from the behavior of the cells on freezing and in micrurgy, a mechanical injury hypothesis is presented.
Studies of physiological and biochemical changes associated with the seasonal cycle of frost hardiness in the living bark and needles of red pine (Pinus resinosa Ait.) indicate that seasonal changes in soluble protein (borate buffer) and in the rate of incorporation of radioactive leucine into protein are closely correlated with the changes in hardiness. The seasonal cycles of carbohydrates and amino acids do not appear to be closely related to hardiness in red pine. The results support the view that the development of hardiness in woody plants is associated with the augmentation of total cellular protoplasm.
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