K. Pihakaskimaunsbach et M. Kukkonen, ULTRASTRUCTURAL-CHANGES INDUCED BY SUBZERO TEMPERATURES IN THE PLASMA-MEMBRANE OF PROTOPLASTS FROM WINTER RYE, Physiologia Plantarum, 100(2), 1997, pp. 333-340
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.