Tolerance of freezing in plants

This is the normal way that cold adapted plants cope with freezing temperatures. Since ice formation will always start at the point of lowest solute concentration, freezing will commence at the outer cell wall surface (apoplast), with ice formation gradually filling the intercellular space, the cell wall itself, and the space between the cell wall and the increasingly shrunken protoplast. This mode of ice formation gradually withdraws water from the protoplast, which dehydrates.

True frost resistance is a special form of dehydration (drought) resistance.
It requires a cell membrane with a lipid layer permitting controlled water passage at very low temperature.
It requires compounds in the cytoplasm which stabilise the membrane during deformation in the course of maximum dessication and which suppress ice formation in the protoplast (very high osmotic pressures induced by up to > 90 % water loss).

1 - Intercellular ice formation withdraws water from cells and leads to filling of air spaces, often causing leaves to turn dark (enhanced during thaw). The photograph shows spotty infiltrations in Vaccinium myrtillus, recovering from freezing during the previous night (mid May, near the treeline in the Alps).

1 - Intercelluar ice formation.

Freezing tolerance is not a constant. It varies with climatic conditions (acclimation) and phenological state of plants. The maximum freezing tolerance is a genetic feature.

Average range of freezing tolerance in leaves of alpine plants
Type of plant	Summer (active)	Winter (dormant)
plants requiring snow cover in winter	-3 to -10 °C	-6 to -22 °C
plants which can survive in open places	-3 to -10 °C	-20 to -76(196) °C