ULTRASTRUCTURAL-CHANGES INDUCED BY SUBZERO TEMPERATURES IN THE PLASMA-MEMBRANE OF PROTOPLASTS FROM WINTER RYE

Freezing of nonacclimated protoplasts close to lethal temperatures ind uces alterations in the macromolecular organization of the plasma memb rane but the significance of these structural changes in freezing inju ry is still uncertain. We therefore cooled nonacclimated protoplasts i solated from cultivars of winter rye (Secale cereale L.) to two sub-ze ro temperatures using two different cooling rates and analyzed freeze- induced plasma membrane changes by freeze-fracture electron microscopy . When a high cooling rate was used a lipid phase transition was obser ved in 34% of the total membrane fracture faces of the protoplasts, wh ile with a slow cooling rate it occurred only to a very small extent. Smooth, aparticulate lamellae were approximately three times more freq uent at low than at high cooling rate. Lipid phase transition from lam ellar to hexagonal, (Hn) phase occurred at high cooling rate more freq uently at -10 degrees C than at -30 degrees C in three cultivars. The results suggest that the greatly increased proportion of phase transit ion from bilayer to non-bilayer phase is an artifact caused by too fas t a cooling rate of protoplasts. Furthermore, lateral phase separation of the plasma membrane with segregation of intramembrane particles an d the appearance of membrane associated stacks of lipid lamellae, may cause cellular death by retarding the flow of intracellular water towa rds extracellular ice crystals formed during freezing.