Endodormancy is the second, and true stage of dormancy. During endodormancy the tree is in such a deep state of rest that it cannot respond to favourable environmental factors even if it wanted to. This probably protects it from responding to brief, unseasonal warm spells in the middle of winter.

This deep state of dormancy also makes study of the deep state of dormancy quite difficult! More is known on how green annual plants respond to temperature than how deciduous woody perennials do.

But what is known is that continuous chilling exposure throughout endodormancy (’chilling hours’) leads to the formation of proteins belonging to the glycoside hydrolase family 17 (GH17) group of enzymes. These enzymes gradually break down the callose (formed during onset of dormancy induction) in shoot meristems. This unblocks the plasmodesmata and restores cell-to-cell communications in the meristems. Restoration of communications helps buds maintain an inactive state and remain resistant to freezing.

Callose deposits in the phloem remain undisturbed, and this together with air bubbles in the xylem (caused by reduced transpiration and a freeze-thaw cycle as temperatures get colder) prevents any sap flow through the tree whatsoever.

Continual production of the phytohormone (plant hormone) abscisic acid during dormancy appears to maintain bud dormancy, and reducing absisic acid levels appears to release a tree from dormancy.

Another phytohormone, gibberellic acid, appears to act in reverse. It is at low levels during dormancy, as it enters catabolic (breakdown) pathways that maintain bud dormancy, while increasing levels of the hormone help bring about dormancy release.

Expression of the DORMANCY-ASSOCIATED MADS-BOX (DAM) genes are high during endodormancy but become repressed as chilling exposure increases. These genes appear to be epigenetically regulated in response to temperature changes, and factor in bud dormancy release and dormancy release in general.

The carbohydrates stored within the plant during dormancy induction become important in preventing freezing damage to tissues throughout endodormancy. A cell full of water will burst when frozen; a cell full of sugars will not. (Sugar added to water lowers the freezing point by preventing water molecules forming hydrogen bonds and solidifying and expanding — the water has to become even colder before these bonds can form.)

Abscisic acid causes a rise in dehydrins, a family of plant proteins produced in response to cold and drought stress. These further prevent injury by regulating the concentrations of salts and sugars in cells. (This regulation is known as osmotic regulation or osmoregulation.) Dehydrins also maintain a very low rate of plant metabolism.

As temperatures slowly rise again, and chilling exposure is reduced, carbohydrate metabolism begins to increase. This leads to an increase in free radicals and a condition known as ‘oxidative stress’. Free radicals are highly unstable and very reactive molecules (often of oxygen, ^•O₂^-) produced as waste products of metabolic reactions in all living organisms, including plants. These are not removed from an organism, but rather ‘hang around’ until they encounter a molecule to react with. Their highly reactive nature makes them very destructive, and in humans are responsible for faulty DNA repair, tissue damage, degenerative diseases, wrinkling, and aging in general. (Unfortunately this free radical battle is a very natural and unavoidable part of life — oxygen gives us life while it slowly oxidises us to death.)

Oxidative stress is a normal state of all organisms, and comes from an imbalance of free radicals and the means to counter them. (A diet full of antioxidants is one way we control free radicals, and reduce our oxidative stress effects, for example.)

As metabolism increases in a tree, so does free radical production, and oxidative stress. This increase of free radicals is possibly another mechanism which signals a tree to leave dormancy. Similarly, as callose degrades and unblocks the plasmodesmata in meristem tissue, oxygen levels can rise to kickstart cell metabolism, leading to increased free radicals and another dormancy release signal.

Next week we’ll examine the processes in a tree after it receives these dormancy release signals and enters the third and final stage of dormancy: ecodormancy!