Avoidance of freezing

Once exposed to freezing temperatures, many plants are able to prevent the formation of ice. Freezing avoidance in exposed tissues can be achieved by

  • accumulation of anti-freeze solutes, which depress the freezing point of the cell sap. This is a very inefficient mechanism, because it needs  1 mole of solutes (e.g. sugars) for a 1.8 K depression of the freezing point, with the osmotic pressure increased by 22 bar per mol of solute (which means more than doubling common osmotic pressures). *
  • preventing freezing by "supercooling". By not fully understood mechanisms, some plants can delay ice formation by inhibiting nucleation. Liquid water is maintained in a meta-stable state. Once nucleated, freezing occurs immediately (commonly around -12° C) and kills the tissue. Thus, supercooling is a fatal strategy in areas in which temperatures may drop below -12° C but is adopted by some high Andean giant rosettes.

Note: no higher plant is known to survive natural protoplast freezing. Ice formation, if it does occur, must be outside the protoplast (see below).

* Be sure to distinguish °C (degree Celsius) from K (the Kelvin), the measure of absolute temperatures and of temperature differences at any temperature. The use of K for differences helps to avoid confusion with temperatures and conforms with rules in physics. Because the Kelvin scale is not a relative degree scale, it is wrong to write and say "degree Kelvin".

1 - Espeletia sp. 4150 m, Paso Pico El Aquila, Merida, Venezuela

In addition, these giant rosettes also engage in some avoidance by closing their leaves over the apical bud during the night, thereby preventing radiative frost and also retaining some heat absorbed during the day in the center of this giant structure. Note that inflorescences (left in the picture) also point downward and are capped with white felt. The water in the stem is also protected from freezing by a massive coat of dead leaf bases, which also serve as a fire guard.

1 - Supercooling is of limited help in most cold regions but works in special regions such as the tropical Andes (here in Venezuela). Tall Espeletia species (giant rosettes) growing above 4000 m elevation adopt this "technique" to avoid freezing.
Saxifraga oppositifolia
2 - Saxifraga oppositifolia

Saxifraga oppositifolia is one of the hardiest plants and is found throughout the coldest parts of the arctic and the northern temperate zone high mountains. It can survive in places with very little or no snow cover and flowers very early. Its tiny leaf rosettes are closely attached to the ground and sometimes form compact cushions.

2 - Some fully hardened alpine plants may survive shock freezing at -196° C in liquid nitrogen (e.g. Saxifraga oppositifolia, Silene acaulis), however, this is not a natural situation. Yet, these plants can resist any freezing temperatures which occur in nature.