Thermal effect of CO(2) on apoplastic ice in rye and oat during freezing.

Meristematic tissues from rye (Secale cereale) and oat (Avena sativa) were studied in an isothermal calorimeter at -3 degrees C. When the frozen tissue was placed in the calorimeter, the pressure increased within 4 d to 25 and 9 kPa above ambient pressure in the sample vessels containing crowns of rye and oat, respectively. Concurrently, the thermal output went down to -194 microW in rye over the 4-d period; this negative thermal activity could be accounted for by ice melting in the plants. When the pressure was released, the output from the calorimeter went from -194 to 229 microW within 1 h, suggesting that water had frozen in the plants. We propose that CO(2) from respiration had dissolved in the water in the plants and caused melting of ice (heat absorption) due to the colligative properties of solutions. When the pressure was released, the CO(2) came out of solution and the water froze (heat evolution). These thermal observations were duplicated in a simplified, non-biological system using a glycol/water mixture that was partially frozen at -3 degrees C.

Arabinoxylans from rye and wheat seed that interact with ice.

Arabinoxylans that interfere with growth of ice crystals have been purified from rye (Secale cereale L., Rosen) and two varieties of wheat (Triticum aestivum L., Genesee and Hillsdale) seed. The most active polysaccharide from each seed type was homogeneous in the sense that all the molecules were in the same size range, they contained the same sugar residues, and they reacted similarly in chemical characterization experiments. Structural studies showed that the polysaccharides consist of a xylan chain to which are attached side-chains that contain a single, terminal arabinose residue. The polysaccharides differ with respect to the number of arabinose residues. The xylose:arabinose ratios in the most active fractions from rye, Genesee wheat, and Hillsdale wheat are 1.26, 1.54, and 2.08, respectively. Gel-permeation column chromatography showed that the most active polysaccharide from each seed type has a molecular weight greater than 2 x 10(6) and that the rye polysaccharide is slightly larger than the Hillsdale wheat polysaccharide. The rye polysaccharide is a better inhibitor of ice-crystal growth than is the Hillsdale wheat polysaccharide.

Ice adhesions in relation to freeze stress.

In freezing, competitive interaction between ice and hydrophilic plant substances causes an energy of adhesion to develop through the interstitial liquid. The thermodynamic basis for the adhesion energy is discussed, with estimates of the energies involved. In this research, effects of adhesion energy were observed microscopically in conjunction with energies of crystallization and frost desiccation. The complex character of ice in intact crown tissue of winter barley (Hordeum vulgare L.) and the problems of sectioning frozen tissue without producing artifacts led to an alternative study of single barley cells in a mesh of ice and cell wall polymers. Adhesions between ice, cell wall polymers, and the plasmalemma form a complexly interacting system in which the pattern of crystallization is a major factor in determination of stress and injury.

Energies of freezing and frost desiccation.

A stable cellulose paper system was studied to relate water distribution data, as obtained previously from plant tissues, to the analysis of freezing energy. Water distribution data for the cellulose system were obtained by several techniques and were coordinated with calorimetric data. The effect of the cellulose system on the latent heat of freezing was evaluated to estimate activation energies as functions of the amount of associated liquid water. Similar activation energies of water phase transitions in critical plant tissue systems may be heritable characteristics that affect freezing stress. Adhesion energy, that develops between ice and hydrophilic polymer systems as they compete for liquid water in a complex interface, was suggested as one possible source of freezing stress. This does not occur in frost desiccation.

Comparative freezing patterns in stems of cherry and azalea.

Ice formation in stems, as determined by means of an electrophoretic mobility technique, occurs much more rapidly in azalea than in sour cherry. The difference is more marked in the bark than in the wood. Disrupting the structure of the tissues completely eliminates differences in freezing patterns, although gross anatomical differences do not appear to account for differences in species response. Microscopic examination of frozen stems indicated that little redistribution of water occurred during freezing in azalea, and the tissues were disrupted as these crystals developed. In cherry, on the other hand, water diffused to nucleating centers where crystal growth was not opposed, giving rise to "glaciers."