Cold hardening is the physiological and biochemical process by which an organism prepares for cold weather.
Plants
Plants in temperate and polar regions adapt to winter and sub zero temperatures by relocating
from leaves and shoots to
.
Freezing temperatures induce dehydrative stress on plants, as water absorption in the root and water transport in the plant decreases.
Water in and between cells in the plant freezes and expands, causing tissue damage. Cold hardening is a process in which a plant undergoes physiological changes to avoid, or mitigate cellular injuries caused by sub-zero temperatures.
Non-acclimatized individuals can survive −5 °C, while an acclimatized individual in the same species can survive −30 °C. Plants that originated in the tropics, like
tomato or
maize, don't go through cold hardening and are unable to survive freezing temperatures.
The plant starts the adaptation by exposure to cold yet still not freezing temperatures. The process can be divided into three steps. First the plant perceives low temperature, then converts the signal to activate or repress
Gene expression of appropriate
. Finally, it uses these genes to combat the stress, caused by sub-zero temperatures, affecting its living cells. Many of the genes and responses to low temperature stress are shared with other
, like drought or salinity.
[[Image:Plant cell structure-en.svg|thumb|200px|Schematic of typical plant cell]] When temperature drops, the [[membrane|Cell membrane]] fluidity, [[RNA]] and [[DNA]] stability, and [[enzyme]] activity change. These, in turn, affect transcription, translation, [[intermediate metabolism|Metabolism]], and [[photosynthesis]], leading to an energy imbalance. This energy imbalance is thought to be one of the ways the plant detects low temperature. Experiments on ''[[arabidopsis]]'' show that the plant detects the change in temperature, rather than the absolute temperature.
Cold increases cell membrane permeability
Chilling injury occurs at 0–10 degrees Celsius, as a result of membrane damage, metabolic changes, and toxic buildup. Symptoms include wilting, water soaking, necrosis, chlorosis, ion leakage, and decreased growth. Freezing injury may occur at temperatures below 0 degrees Celsius. Symptoms of extracellular freezing include structural damage, dehydration, and necrosis. If intracellular freezing occurs, it will lead to death. Freezing injury is a result of lost permeability, plasmolysis, and post-thaw cell bursting.
When spring comes, or during a mild spell in winter, plants de-harden, and if the temperature is warm for long enough – their growth resumes.
Insects
Cold hardening has also been observed in
such as the fruit fly and
diamondback moth. These insects use rapid cold hardening to protect against
cold shock during overwintering periods.
insects remain active through the winter while non-overwintering insects
Insect migration or die. Rapid cold hardening can occur during short periods of undesirable temperatures. The buildup of
Cryoprotectant compounds such as
glycerol is one mechanism of cold hardening in insects.
Glycerol interacts with other
cell components in order to decrease the insect's permeability to the cold.
When an insect is exposed to cold temperatures, glycerol rapidly accumulates. Glycerol is a
non-ionic Kosmotropic forming powerful
with
. The hydrogen bonds in the glycerol compound compete with the weaker bonds between the water molecules, interrupting ice crystal formation.
This reaction between glycerol and water has been used as an
antifreeze in the past. Proteins also play a large role in cold hardening. Glycogen phosphorylase (GlyP) is a key enzyme that increases in comparison to a control group not experiencing cold hardening.
Once warmer temperatures are observed, the process of
acclimation begins, and the increase in the concentrations of glycerol and other cryoprotective compounds is reversed. There is a rapid cold hardening capacity found within certain insects that suggests not all insects can survive a long period of overwintering. Non-
Diapause insects can sustain brief temperature shocks but often have a limit to what they can handle before the body can no longer produce enough cryoprotective components.
In addition to improving insects' survival during cold temperatures, cold hardening also improves the
organism's performance.
Rapid cold hardening (RCH), one of the fastest cold temperature responses recorded,
allows an insect to quickly adapt to severe weather change without compromising function.
Drosophila melanogaster (the common fruit fly) is a frequently experimented insect involving cold hardening. An example of RCH enhancing organisms' performance comes from courting and mating within the fruit fly. Fruit flies mate more frequently once RCH has commenced, compared to a control insect group not experiencing RCH.
Most insects experiencing extended cold periods are observed to modify
membrane lipids. Desaturation of
is the most commonly seen modification to the
cell membrane.
When fruit flies were observed under a stressful climate, the survival rate increased in comparison to the fly prior to cold hardening.
In addition to the common fruit fly, the cold-hardening response of
Diamondback moth (the diamondback moth) also has been widely studied. While this insect also shows an increase in glycerol and similar cryoprotective compounds, it also shows an increase in
. These compounds are specifically linked to cryoprotective compounds. The polyol compound is freeze-susceptible and
Freeze tolerance.
Polyols simply act as a barrier within the insect body by preventing
intracellular freezing by restricting the
extracellular freezing likely to happen in overwintering periods.
During the larval stage of the diamondback moth, the significance of glycerol was tested again for validity. The lab injected the larvae with added glycerol and in turn proved that glycerol is a major factor in survival rate when cold hardening. The cold tolerance is directly proportional to the buildup of glycerol during cold hardening.
Cold hardening of insects improves the survival rate of the species and improves function. Once environmental temperature begins to warm up above freezing, the cold hardening process is reversed and the concentrations of glycerol and cryoprotective compounds decrease within the body. This also reverts the function of the insect to pre-cold hardening activity.
See also
