Cold hardiness and dormancy in buds
(A disclaimer here is that at times in this text, plants are treated as making decisions through rational thought – or being anthropormorphized. However, they only respond to environmental cues in a mechanical way, although through very intricate mechanisms!)
For plants, the temperatures that occur during winter are too low for growth to continue occurring and too low even for mature leaves to stay around (evergreen species have certain adaptations to keep mature leaves and needles, but no new growth occurs during winter). Annuals will produce seeds, bulbs will hide below ground, but what about trees and other woody perennials that remain exposed to the cold? To understand what they go through, first we have to examine their development.
Many of us might not notice, but during the summer woody perennial plants are already preparing leaves and flowers for the following spring. These baby leaves and flowers (for which the technical terms are leaf and flower primordia) are formed inside structures called buds, which are neatly organized structures with folded leaves and flowers, usually protected by scales.
In growing tips of shoots, we find a meristem. This is a tissue that contains stem-like cells that divide and form all other tissues. During the summer they will become a bud and pre-form many of the plant parts that we see in the early spring. As an example, apples start forming new flowers for the following growing season only 60 days after they bloom.
But how come these buds don’t break and flower in the fall, if temperatures are still warm?
Plants have been around for a long time, and because fall temperatures can vary a lot from year to year, they have learned to take their cue to begin winter preparations from something that always follows the same pattern every year instead: the length of the day (or technically, they perceive the length of the night!). Every year, summer solstice will occur in late June. From there, until the winter solstice in December, daylength will continuously shorten in the northern hemisphere, while the opposite is occurring in the southern hemisphere. The plants will perceive these conditions through their leaves, which will then send signals into the bud to tell it to stay put: winter is coming. The buds then enter a state of dormancy, which we’ll get into below. Alongside dormancy, these buds must also develop the ability to withstand low winter temperatures, which we refer to as cold hardiness. Dormancy and cold hardiness are dynamic, adapting to the environmental conditions the plants experience. The leaves have fulfilled their job of nurturing the plant through a summer of photosynthesis and sending the signals required for the plant to prepare for winter and the next growing season. Now, the plant bids farewell to its leaves in a colorful display as break down as much of their contents as possible to store in stems before they detach and fall to the ground.
The intersection of dormancy and cold hardiness
Dormancy and cold hardiness, although they happen at the same time, appear to be distinct processes. Dormancy always occurs, and buds must overcome dormancy to break and resume growth. But cold hardiness only reaches its maximum if temperatures are low enough. But the two interact, both during acclimation and deacclimation.
Full acclimation to cold requires dormancy to be established. But it also appears that the ability to acclimate decreases with the progression of winter. So, in the case of warmer falls, plants may then take longer to gain their cold hardiness, even when low temperatures eventually set in. Therefore, mild falls leading to cold winters may leave plants prone to low temperature damage, but further research is still required in this area.
The intersection of dormancy and deacclimation is much better understood. Deacclimation does not just respond to how warm temperatures are in the spring, but the same temperature will promote increasing rates of deacclimation after the plant has accumulated more chill. We can use a night of sleep as an analogy for this. Imagine sleeping only two hours: if you’re able to wake up, your efficiency at tasks will be very low. If you sleep 5 hours, you’re going to be able to work better than if you slept 2, but still not at full power. If sleeping 8+ hours, you will now be well rested and will be able to work at full efficiency. The buds will work the same way. If they are only exposed to a little amount of chill, buds lose their cold hardiness very slowly, budbreak will take a very long time and flowering is erratic. As chill continues to accumulate throughout the winter, the plant is prepared to go through deacclimation faster and faster. This means that as spring approaches, the plant is more able to flower quickly in response to warm temperatures. This is also the reason why spring bloom here in Wisconsin happens quickly across many plants: lots of chill are accumulated in our long winters.
While we generally understand how the environment affects dormancy, cold hardiness, and budbreak, there are still a lot of unknowns about the processes and mechanisms – some of these questions have been pointed out through the descriptions here. At the Plant Resilience Lab, we study dormancy, cold hardiness, and budbreak in many species to learn more about how plants respond to the environment so that we can better understand and predict what will happen as the climate shifts. For agriculture, this can mean adapting crops in specific ways so that they are ready for new, warmer climates. For forests, this can mean guiding reforestation efforts with the most resilient populations of plants, or directing assisted migration of forests – a process in which species are moved to certain locations where they are or will be more adapted to.
Further reading:
Take a look at this article Al Kovaleski wrote for Arnoldia exploring similar concepts in grapevines.