Geoscience Reference
In-Depth Information
the number of palisade cell layers, and an increase in cell cyto-
plasm associated with a decrease in leaf water content.
plants survive
freezing
temperatures by
limiting ice
formation
During rapid freezing, the protoplast, including the vacuole, may
super-cool, that is, the cellular water remains liquid because of
its solute content even at temperatures several degrees below its
theoretical freezing point. Super-cooling is common to many
species of the hardwood forests. Cells can super-cool to only
about −40°C, the temperature at which ice forms spontane-
ously. Spontaneous ice formation sets the low-temperature limit
at which many alpine and sub-Arctic species that undergo deep
super-cooling can survive. It may also explain why the altitude
of the timberline in mountain ranges is at or near the −40°C
minimum isotherm. Several specialised plant proteins, termed
antifreeze proteins, limit the growth of ice crystals through a
mechanism independent of lowering of the freezing point of
water. Synthesis of these antifreeze proteins is induced by cold
temperatures. The proteins bind to the surfaces of ice crystals
to prevent or slow further crystal growth.
Cold-resistant
plants tend to
have
membranes
with more
unsaturated
fatty acids
As temperatures drop, membranes may go through a phase
transition from a flexible liquid-crystalline structure to a solid
gel structure. The phase transition temperature varies with spe-
cies (tropical species: 10-12°C; apples: 3-10°C) and the actual
lipid composition of the membranes. Chill-resistant plants tend
to have membranes with more unsaturated fatty acids. Chill-
sensitive plants, on the other hand, have a high percentage of
saturated fatty acid chains, and membranes with this composi-
tion tend to solidify into a semi-crystalline state at a temperature
well above 0°C. Prolonged exposure to extreme temperatures
may result in an altered composition of membrane lipids, a
form of acclimation. Certain transmembrane enzymes can alter
lipid saturation, by introducing one or more double bonds into
fatty acids. This modification lowers the temperature at which
the membrane lipids begin a gradual phase change from fluid
to semi-crystalline form and allows membranes to remain fluid
at lower temperatures, thus protecting the plant against damage
from chilling.
Cold-acclimated
plants secrete
antifreeze
proteins
Plant can tolerate freezing because of their ability to control the
freezing event itself. As long as the freezing of water is confined
to the apoplast, that is the cell wall and the extracellular space,
the plant will survive. Alternatively, if freezing occurs intracel-
lularly, the plant will die. Cold acclimation in many plants is
associated with the secretion of antifreeze proteins from the
