signal to activate or repress expression of appropriate genes. 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 abiotic stresses, like drought or salinity.[2]
transcription, translation, intermediate 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.[2] The rate of temperature drop is directly connected to the magnitude of calcium influx, from the space between cells, into the cell. Calcium channels in the cell membrane detect the temperature drop, and promotes expression of low temperature responsible genes in alfalfa and arabidopsis. The response to the change in calcium elevation depends on the cell type and stress history. Shoot tissue will respond more than root cells, and a cell that already is adapted to cold stress will respond more than one that has not been through cold hardening before. Light doesn't control the onset of cold hardening directly, but shortening of daylight is associated with fall, and starts production of reactive oxygen species and excitation of photosystem 2, which influences low-temp signal transduction mechanisms. Plants with compromised perception of day length have compromised cold acclimation.[2]
Cold increases
intracellular water freezes, the cell will expand, and without cold hardening the cell would rupture. To protect the cell membrane from expansion induced damage, the plant cell changes the proportions of almost all lipids in the cell membrane, and increases the amount of total soluble protein and other cryoprotecting molecules, like sugar and proline.[3]
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.[1]
Insects
Cold hardening has also been observed in
acclimation begins, and the increased glycerol along with other cryoprotective compounds and proteins are also 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-diapausing
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.
The common fruit fly
Inclusive to the cold hardening process being beneficial for insects survival during cold temperatures, it also helps improve the
membrane lipids within the body. Desaturation of fatty acids are the most commonly seen modification to the membrane.[9]
When the fruit fly was observed under the stressful climate the survival rate increased in comparison to the fly prior to cold hardening.
The diamondback moth
In addition to testing on the common fruit fly,
extracellular freezing likely to happen in overwintering periods.[10] 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.[10]
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 glycerol and cryoprotective compounds decrease within the body. This also reverts the function of the insect to pre-cold hardening activity.
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