What is dormancy and why is it so crucial?
Story and Photos by Garry V. McDonald
This is about the time of year I start getting inquiries from local media about why leaves turn colors in the fall. What they really want to know is the exact week of peak color to inform the leaf-peepers. I usually respond that the plants are preparing to enter dormancy and peak color depends on prevailing weather conditions and is often unpredictable.
But what exactly is dormancy and why is it crucial to plants? Like explaining why leaves change color, the answer is not straightforward and “depends,” which is not the answer most people want to hear. I’ll attempt to explain in layman’s terms an interesting facet of a plant’s life.
It is often assumed that deciduous plants, or many grasses, are “dead” during the winter or when flowering bulbs disappear altogether. Far from it – the plants are really just “resting” and very much alive. This state of inactive growth or rest is called dormancy.
Plants undergo different types of dormancy, but all are adaptations for survival during adverse growing conditions – either cold winters or hot, dry summers. Fossil records suggest that the earliest land plants arose in areas with warm, wet tropical conditions. As eons passed, continents drifted, climates changed, and plants migrated; many plants had to adapt to cooler and eventually freezing weather. On the other hand, as ancient ancestors of cacti and Euphorbia experienced hotter and drier conditions in what is now Central America and sub-Saharan Africa, these plants adapted mechanisms to survive conditions unfavorable for growth. In the case of cold temperatures, plants that shed their leaves and increased sap sugar content (which acts as antifreeze in both deciduous and conifers) were able to survive, whereas their tropical cousins died out. Likewise, those plants that were able to adapt to hot and dry conditions through various morphological or physiological modifications were able to survive months if not years with little or no water. While all this seems simple enough, the mechanisms and biochemistry that control dormancy are intricate and closely in tune with nature and the natural environment, with dormancy a requirement for most plants to complete their life cycle, even if evergreen.
Plants experience two types of dormancy, which can best be summed up as either internally or externally imposed. Internal dormancy, or what is called in the trade physiological dormancy, is caused by chemical changes that occur within the plant as a response to several factors – mainly environmental. External dormancy, known as quiescence, is usually exclusively controlled by environmental factors such as rainfall or temperature. Seeds have other types of dormancy, but essentially controlled by the same factors. All types of dormancy are controlled by substances made by the plant called plant growth regulators. These are also referred to as plant hormones. Plant growth regulators are grouped as either growth promoters or growth inhibitors. Both are crucial in controlling dormancy.
The best way to understand internal dormancy in trees is to follow the season of a plant such as sugar maple (Acer saccharum) since they display brilliant fall color most years. During the growing season, sugar maples are actively photosynthesizing – producing sugars and other substances for growth. When the days begin to shorten and the nights cool down in early autumn, this is a signal for the sugar maple tree to start “hunkering down” to survive freezing winter temperatures. The tree can “sense” changes in day length by a process called photoperiodism. This change in day length causes those internally produced plant growth regulators mentioned earlier to trigger changes in the tree. Chlorophyll, the green pigment in leaves, ceases to be produced and sugars and other nutrients in the leaves are broken down and transferred to stems and the root system to act as food reserves and, in the case of sugars and resins, a type of antifreeze. As chlorophyll breaks down, other plant pigments such as carotenoids, yellow and orange, contained in the leaf are unmasked to display their colors. Anthocyanin, the red and purple pigment, production actually increases as days shorten and cooler temperatures prevail and are also unmasked as the green pigment chlorophyll breaks down. It is the elusive combination of bright sunny days and cool nights along with not too much or too little rainfall that determines fall color. But what does all this have to do with dormancy? One theory about why plants exert energy when they should be conserving it to produce the red and purple pigments, as well as the yellow and orange, is that insects and other predators are “blind” to red and will not feed on red foliage, allowing for maximum food storage. As fall deepens, nutrients and minerals migrate out of the leaves and are re-distributed into the stems and roots; the leaves fall off, as they are now a liability to the tree. At this point, the tree has entered dormancy, all snuggled in for the winter.
So what happens when it is time for the plant to wake up? As days lengthen and days, and especially nights, warm up, this signals the plant to rise and shine. Those plant growth regulators that induced dormancy decrease and those that promote growth increase. The food that was stored during the fall is mobilized and moved toward the growing tips. Most folks would call this the “sap rising” and it is the time to tap sugar maples for syrup making. The food supplies energy to the plant for the leaves to develop and the process of photosynthesis begins again.
Most plants have a chilling hour requirement, meaning that they must receive a certain number of hours of temperatures ranging from 32 to 45 F to break dormancy. This chilling requirement is crucial for fruit growers. If fruit, and many flowering, trees do not receive enough chilling hours they fail to flower properly or are delayed leafing out for weeks if not months.
Prolonged heat or drought will also cause plants to enter into a type of dormancy. As a survival mechanism, adapted plants will drop their leaves or otherwise go dormant. This is why many trees, such as sycamore (Platanus spp.) and river birch (Betula nigra), are almost bare by the end of August during hot and dry summers. Unless those conditions are too prolonged, they will survive until the next growing season. Bermudagrass does the same thing when drought hits, but recovers with cooler and wetter weather.