ABSTRACTS-TENTH ANNUAL MEETING 5 3 I

73. Alterations in Chloroplast Membranes during Cold Acclimation and Freezing. II. Altera- tions during Cold Acclimation. M. P. GAR- ~ER* AND P. L. STEPONKUS (Dept. of Floriculture and Ornamental Horticulture, Cornell University, Ithaca, New York 14850).

Freeze-fracture electron microscopy of accli- mated and nonacclimated chloroplast thylakoids revealed a significant alteration in the particle distribution of the outer fracture face. Fractured outer faces of acclimated thylakoids possessed ap- roximately one-half the number of particles per unit area as compared to nonacclimated thyla- koids. Acclimated thylakoids prefixed in gultaralde- hyde show the same particle distribution as un- fixed acclimated thylakoids. Sonication of accli- mated thylakoids resulted in a particle distribution similar to nonacclimated thylakoids. Sonication of nonacclimated thylakoids did not alter the parti- cle distribution significantly. Total lipid content of acclimated and nonacclimated thylakoids was not significantly different. The significance of the above in relation to cold acclimation will be dis- cussed in detail. (Supported in part by NSF Grant GB-23,354. )

74. A Possible Role of Cell Membranes in Cryo- protection of Comus Spp. R. J. WILLIAMS AND P. RhMASASTRY * (American Red Cross Blood Research Laboratory, 9312 Old Georgetown Road, Bethesda, Maryland 20014).

When cells from hardy dogwoods are plasmo- lyzed, they do not continue to shrink as predicted but reach a volume plateau at an osmotic pressure lower than that necessary to injure them. We have interpreted this as a storage of potential energy resisting osmotic water loss. When the cell is os- motically injured this energy, about 1.1 kcal/ mole, becomes the work required to destroy the cell. No intracellular component we have examined can account for this plateau, and we propose that the resistance to shrinkage lies in the membrane. This principle requires that the membrane show a high interfacial energy and contain lipids of suf- ficiently high molecular weight that they act (in Langmuir's terminology) as a two-dimensional liquid. Micrographs of plasinolyzed cells show a large surface energy, with a contact angle of about 155 degrees. Thermal analysis of extracted hardy cell lipid shows a transition at +17 ° C. Analysis of fatty acids in chloroform-methanol extracts show that while nonhardy dogwood contains 18C and shorter chain saturated fatty acids, hardy dog- wood contains fatty acids up to ~C. This tendency

is accentuated in more hardy cultivars. While this pattern is predicted by our proposal, it is contrary to that usually observed in most plant and animal tissues. Extracted lipids, spread as a monomolecu- lar layer on a Langmuir trongh, showed collapse at a pressure of 34 dynes/cm. Using Gibbs equa- tion to estimate molecular area, this is equivalent to 600 gcal/mol. A bimolecular film or membrane would show collapse at twice this value, close to the energy required to destroy dogwood cells. (Supported in part by National Institutes of Health Grant GM 17959.)

75. The Utility of the Motile Alga Dunaliella in Experimental Cryobiology. T. W. SPE~a~L AND S. M. SIEGEL (Department of Botany, University of Hawaii, 3190 Maile Way, Honolulu, Hawaii 96822).

The accounts of physiological research on the active metabolism and behavior of autotrophic eukaryotes at subzero temperatures are relatively meager. Possibly, this is because few organisms remain unfrozen at temperatures of 0 ° to ± 5 ° C. Dunaliella is a halophilic, unicellular alga which retains its motility at - 1 5 ° C in saturated NaC1 and CaCt2. Mechanisms of cryo- and halotolerance differ between species of the genus. Dunaliella saline passively maintains an intracellular electro- lyte concentration slightly higher than the sup- porting medium, therefore avoiding freeze and osmotic stress. Dunaliella tertiolecta excludes elec- trolytes, but can build up high glycerol concen- trations. Lowering of external osmotic pressures result in elaboration of the cryoprotectant. Dunali- ella is easily cultured and motility of the flagellate readily observed. Since all species lack cell walls, changes in cell shape are a good indicator of os- motic state. Examples of its use in cellular studies in low temperature and saline environments will be described. (Supported by NASA Grant NGL 12-001-042. )

76. Effect of Freezing on Cell Ultrastructure in Citrus Tissues. ROGER YOUNC AND GEORGE YEL~NOSKY (Southern Region, U.S. Horti- cultural Research Laboratory, 2120 Camden Road, Orlando, Florida 32803).

Citrus leaf tissues, exposed to - 4 . 4 to - 6 . 7 ° C, with and without heavy frost, were sectioned and examined by light and electron microscopy. Ob- servations of freeze-injured cells indicated disrup- tion of plasmolemma, tonoplast, and outer chloro- plast membranes and general disorganization of cell contents. The presence of frost on the leaf surface resulted in additional severe epidermal and palisade cell distortions in many instances. Some