It is always a wonder to see plants wake up from their winter dormancy and begin to produce flowers. How do they know that winter is over and that it is time to get ready for spring? Is it the change in temperature or the increase in day length? For plants that are considered perennial, the answer lies in a process known as vernalization. All plants that inhabit temperate climates and bloom year after year fall into this category which not only includes the herbaceous plants that are commonly called perennials, but trees, shrubs, vines, and flowering bulbs as well. Vernalization is the cool down period for plants that always occurs in the winter months and is necessary for the plants to flower the next spring. But how do plants know that it is truly winter time and not just a sudden cool down?
The process that plants undergo to reach vernalization is controlled by internal and external factors. These factors are known as epigenetics. Epigenetics are genes that are switched on and off by factors present in the outside environment of the organism. All organisms are controlled in one way or another by epigenetics, not just plants.
For a plant to know when to begin going dormant, environmental factors such as decreasing temperatures, decreased daylength, and either an increase or a decrease in rainfall will signal genes to stop cells from actively growing and begin the processes necessary for vernalization to begin. This is why plants that were actively growing and perhaps fruiting will stop those actions when autumn arrives. Deciduous plants’ leaves will turn red and gold in the fall and new buds will appear on the nodes of each branch where the old leaves have dropped.
All plants require a specific amount of time for vernalization. As winter progresses to the end of the season, the plant’s genes wait for new signals from the environment to bring the plant out of dormancy. If those signals are present, genes used to promote flowering will start up and the plant will begin the spring season. Without the correct amount of time for vernalization, a plant’s epigenetics would not receive the correct signals and the plant would be completely out of season.
Vernalization can be artificially induced in plants that are meant to bloom out of season such as in a green house and for the floral industry. The vernalization process has been well documented in a number of plant species and the specific genes that are signaled by epigenetics have been located in such plants like Arabidopsis. As long as the genes receive the correct signals and the amount of time spent dormant has been satisfied, a plant can be successfully tricked into going dormant and then blooming at the wrong time of year.
Plants can also be tricked by unseasonably warm winter temperatures caused by El Nino or La Nina. One documented case involved flowering cherry trees in the Northern Californian city of Santa Cruz. In the last week of January, 2010, the Santa Cruz area experianced unusually warm temperatures and sunny skies. This led to the impromptu blooming of the flowering cherry trees which normally bloom in April. The epigenetics of these trees recieved all the signals of an early spring and went along with the process of blooming.
Although this process sounds fairly simple and straightforward, there is actually a whole lot more to it than just decreases and increases in temperature and daylight. Many other factors go into the process of vernalization and the appropriate time to flower and are much more complicated to understand. Understanding the process of vernalization and how epigenetics play a role is just one small piece of the puzzle that scientists are still just beginning to understand.