Many temperate woody perennials, including the jujube tree, enter dormancy during winter so as to survive the very low temperatures and greatly reduced sunlight of that season. During this period all activity and other signs of life cease — leaves fall off, and the tree enters a suspended animation in which no nutrients or water are uptaken by the roots, and no growth occurs.

The Three Stages of Dormancy

The dormant period is really a continuous process, but can be divided into three distinct stages: dormancy induction, endodormancy, and ecodormancy.

Dormancy induction is when a tree begins the process of entering dormancy by slowing growth and shedding leaves. Endodormancy is the true dormant, or rest, phase, and ecodormancy (also known as quiescence) is where a tree is not deeply dormant, but remains in a state of suspension owing to environmental factors such as extreme cold preventing growth.

Dormancy Induction

During dormancy induction, growth slows and the tree draws what nutrients it can from its leaves before shedding them. (This is why the leaves change colour. The green chlorophyll pigment is broken down and its components returned to the tree for use post-dormancy, which reveals the yellow, orange and red pigments that were always there.) Buds are also set.

Jujube trees not only shed their leaves, but the deciduous fruiting branchlets unique to them as well — these too change colour from green to yellow before dropping off.

Both temperature and photoperiod (length of night or a dark period) can trigger induction, but which of these — or in which combination — is the most influential will depend on the species of tree, and even the cultivar.

Fruit trees of the Rosaceae family (examples include almond, apricot, cherry and many more) are not sensitive to photoperiod, but are sensitive to temperature, requiring low nighttime temperatures to induce dormancy.

My personal experience with jujubes (which are in the Rhamnaceae family)  is that photoperiod is the most influential for them, and that the degree of this influence differs amongst the cultivars. For example, here in Wollongong we have one of the mildest temperate winters in the country, and the coldest minimum temperature on record is 6°C. Early-to-mid autumn is typically very mild both day and night. (We’re really copping it now though!) Yet Chico, one of the early-ripeners, was the first to enter dormancy weeks ago, despite lovely warm days at the time, while Silverhill, the absolute latest ripener of all, is still in full leaf at time of writing, though the leaves are just starting to yellow now.

Rosaceae fruit trees also tend to set their flower buds during this induction phase — don’t prune branches with fruiting spurs during winter if you want fruit the following season!

Jujube trees differ from Rosaceae fruit trees in that they produce all their flowers the following spring, but they do set down bud scales on their main buds from which future branches may grow.

Endodormancy

Endodormancy is the true dormant, or rest, stage, in that the tree cannot grow even if subjected to favourable environmental conditions. This is probably a protective mechanism to prevent the tree coming to life during any short period of unseasonally warm days during winter. Trees in this state are able to survive freezing and other conditions that would otherwise kill them during any other stage of the growth cycle. Low temperatures appear to be the main factor enforcing this dormancy — as leaves (absent during dormancy) are the only organs with photoreceptors, photoperiod is believed to have little to no effect during this ‘deep sleep’ phase.

Exposure to enough ‘chilling hours’ below 7°C during this dormant period is crucial if a tree is to produce flowers the following spring. Jujubes may need even less than the accepted data stated on that page as the mild winters here don’t seem to affect flowering and fruiting, and they do flower and fruit in more northern climes.

Ecodormancy

During this period a tree is not in as deep a dormancy as during endodormancy, but still cannot grow owing to external factors such as still-cold temperatures. Temperature does appear to be the main driver behind a tree leaving this state rather than photoperiod, regardless of species.

Trees in ectodormancy become less and less tolerant of freezing conditions the closer they get to bud burst, and trees that set flower buds during induction show bud burst after a minimum amount of heat exposure to those buds. In the case of jujubes, our trees in Wollongong consistently break dormancy a good month before their counterparts in Goulburn NSW, in line with the warmer weather.

But what, specifically, in a tree responds to lower temperatures and/or lower daylight hours to induce dormancy in the first place? And what responses to warmer temperatures and/or increased daylight hours causes it to leave dormancy? We’ll explore the physiological processes behind these in the next post!