Vernalization: How cold affects flowering ❄️

Hello friends! In today's post, we look at the implications of vernalization, an evolutionary adaptation that means some plants, for the most part

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Some plants need the cold of winter to be able to flower in spring (Photo: Peter Stenzel)

List of contents 🔻

  • Why do some plants need vernalization?
  • What plants require vernalization?
  • How does vernalization work?
  • Vernalization for faster flowering
  • photoperiod dependence
  • Vernalization

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If, like me, you are a passionate grower, you will be very aware of how the cold can affect your plants, whether we can see the slow growth of plants in an indoor grow tent without heating during the winter or the last outdoor plants that They do not finish ripening before the first frost and end up succumbing to the cold. But despite the negative effects it can have on cannabis, cold is essential for the success of many plants and plays a fundamental role in their correct development and in key functions such as germination, growth and flowering.

You have probably heard of stratification, in which the seeds of some plant species will go dormant until they have been exposed to a prolonged period of low temperatures. This is an evolutionary adaptation to ensure that the seeds do not germinate until the cold winter is over, thus greatly increasing their chances of survival. This is the case for many wild-type cannabis strains (better known as landrace genetics), which will have much higher germination rates after a period of cold storage.

Vernalization is another example of the vital role cold plays in plant development, and it is this process that we will look at today. It can be defined as the acquisition of the competence to flower as a consequence of prolonged exposure to cold. This means that plant species that require vernalization must go through a period of cold weather - up to several months at temperatures between 1ºC and 7ºC - before they can begin to flower and finish their life cycle. Without cold weather to trigger them, these plants would remain in vegetative growth indefinitely.


Why do some plants need vernalization? 📌

Vernalization is an evolutionary adaptation that allows flowering plants to gain an advantage over their competition when the growing season begins in the spring by germinating and beginning their development in the fall before the winter chill sets in. But this impending cold weather poses a problem for young plants as they begin life. If the plants continue to grow and begin to flower during the winter, the flowers or fruits are most likely destroyed by the harsh winter conditions: rain, snow and sub-zero temperatures, thus nullifying the reproductive efforts of the plants throughout the year. season.


The buds of many trees cannot open without a long period of cold in winter.

Similarly, and for the same reasons, many trees and perennials require a period of cool weather to shake off winter dormancy. In other words, for the buds that developed in autumn to open and form leaves, they must be exposed to prolonged cold. Short spells of cool weather will not be enough to give flowering competition or free trees from bud dormancy, otherwise these processes could be triggered by fluctuating weather in autumn, with disastrous consequences. To avoid this, these plants have evolved solely to respond to the long periods of cold that are experienced during the winter, causing the reproductive process to be inhibited until spring.



What plants require vernalization? 📌

Generally, plants that require vernalization to induce the ability to flower are biennials and winter-hardy annuals from temperate zones.


This overwintering cabbage will flower in spring if not harvested beforehand (Photo: Yasunari Nakamura)

Biennials are plants that have a two-year life cycle, using the first year to build a healthy structure, storing energy in their roots to survive the winter, and then devoting the second year to reproduction, i.e. flowering and growing. fructification. There are many varieties of biennial vegetables, however they are mostly grown as annuals and are usually only grown as biennials when gardeners need to save seeds for future seasons. Popular biennials include beets, Brussels sprouts, cabbage, carrots, cauliflower, celery, Swiss chard, endive, kale, rutabaga, turnip greens, leeks, onions, parsley, and parsnips.

Winter hardy annuals start in the fall and the plants will overwinter before beginning to bloom in the spring. Examples of popular winter hardy annual vegetables are lima beans, peas, and spinach.

In the agricultural world, winter-hardy annual cereal crops play an important role in annual grain production, filling a gap in the rotation that other crops cannot and taking advantage of winter snow and rain to grow, which means they tend to give much higher yields than summer cereal crops. Winter strain cereal crops include rye, barley, wheat and triticale, all of which are also perfect for use as green manure cover crops to protect and enrich the soil over winter as they can be harvested and incorporated into the soil long before sowing the seed.


How does vernalization work? 📌

Studies in Arabidopsis thaliana (considered by scientists as a model plant for research) have been able to identify two genes whose expression makes vernalization necessary. Plants that require cold in order to flower have active copies of these genes, known respectively as Frigida and FLC (Flowering Locus C). FLC is a repressor that is activated in autumn and inhibits the expression of key genes in the flowering mechanism. Frigida, on the other hand, activates and raises the level of FLC within the plant RNA. An extended period of cold weather has the effect of silencing FLC expression, leading to competition to flourish.


Wild mustard plants that flower in early spring while the trees are still bare (Photo: Carol M. Highsmith)

In grass species that require vernalization, such as barley or winter wheat, the mechanism works slightly differently and flowering induction is inhibited by a gene called VRN2. Exposure to long periods of cold weather increases the expression of VRN1, another gene that works to repress VRN2, allowing induction of flowering.

Vernalization for faster flowering

The vernalization technique can also be used to trick plants into producing a faster harvest than would otherwise be possible. This is the case of winter wheat, which, unlike spring wheat, requires 6-10 weeks of vernalization in order to flower and produce grain.

It is traditionally sown in the fall so that young, hardy plants can overwinter and begin production in spring for harvest in early summer, which requires three seasons to go from seed to harvest. In an effort to improve crop productivity, agricultural researchers have found that by exposing sprouted grains to carefully calibrated temperatures and a specific photoperiod, the time between germination and harvest can be dramatically shortened. Using this technique, farmers were able to achieve up to five grain harvests per year where previously only two were possible.


Photoperiod dependence

In some plant species, vernalization alone is not enough to allow flowering, and the correct photoperiod is required. This means that a photoperiod sensitive or photodependent plant that has gone through vernalization but has been subjected to the wrong day length will remain in vegetative growth until it is exposed to the correct photoperiod to induce flowering. Thus, a photoperiod plant with a vernalization requirement will not flower unless it has been exposed to a long cold period, even under the correct photoperiod. Likewise, the same plant, having gone through vernalization, will in turn refuse to flower if the photoperiod is wrong.


Winter-type cereal seedlings in the vernalization process (Photo: Credit: Bibiana Espinosa/CIMMYT)


Just as cold weather causes vernalization and gives plants the ability to flower, heat can be used to reverse this process as long as its effects are counterproductive to crops. Devernalization is the reversion of the plant to its original non-flowering state through exposure to high temperatures. This is a technique used for young onion plants, known as sets, which are stored at very low temperatures to reduce spoilage and are then sold as an easy alternative to onion seeds. This cold storage equates to their vernalization, meaning they will begin to flower immediately after planting, something to avoid if gardeners want to harvest good sized onions. To do this, the seedlings are exposed to temperatures of just under 30ºC for 2-3 weeks before sowing, returning them to a non-flowering state so that they can remain in vegetative growth and produce beautiful bulbs for the kitchen.

That's it for vernalization, we hope you enjoyed reading it as much as we enjoyed writing it. As always, we'd love to hear your thoughts and answer any questions in the comments section below.

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Jin-Kyung Cha, Kathryn O’Connor, et al. (2021). A new protocol for speed vernalisation of winter cereals.
Friedrich Laibach (1951). About summer and winter annual breeds of Arabidopsis thaliana (L.) Heynh. A contribution to the etiology of flowering.
Napp-Zinn, K. (1987). Vernalization: Environmental and genetic regulation.

Happy farming!

Post by: Kaiggue (post made by me)
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