The term vernalization comes from the Latin word “vernus” meaning “spring”. “Like spring” means to do. It is the initiation of the flowering process of the plant by exposing it to cold winter or similar conditions for a long time. In order for plants to produce flowers and seeds, they go through a process called vernalization. they must pass. When you think of flowers, spring immediately comes to mind. Spring is when plants grow flowers that will develop into seeds to grow more plants. When spring comes, when the threat of winter frosts has passed, the plants take action and shoots begin to form and flowers begin to bloom. Many plants do this every year. But some plants, called biennial (biennial) plants, only produce flowers every two years. These plants grow every year but produce only roots and leaves, not flowers in the first year. As a result, they bloom only once every two years, so they spend a winter between their initial growth and flowering stage.
Vernalization occurs in two ways, optional and mandatory. Vernalization includes the following steps:
Humidification of seeds: First the seeds are soaked in water and then allowed to germinate in the processing room below 10-12 degrees Celsius.
Cooling process: The germinated seeds are exposed to low temperature (3-5 degrees Celsius) for the desired time.
Drying and sowing: The seeds are then dried and planted in an area with an adequate supply of water and oxygen.
There are two main hypotheses to describe the vernalization mechanism:
1-Phasic Developmental Hypothesis
According to this hypothesis, there is an organization of stages in the development of the plant. Each stage is affected by environmental factors such as temperature and light. In turn, it has two main stages:
a-Thermo stage: It depends on the temperature. It is the primary stage in which lightly germinated seeds are exposed to a low temperature of 0-5 degrees Celsius in the presence of oxygen and moisture. At this stage, structures such as stems, leaves, and roots develop, known as the vegetative stage. The seed loses its dormant state and begins to germinate.
b-Photo stage: It is the secondary stage where the seedlings are subjected to the correct photoperiod or high temperature up to 40 degrees Celsius. It refers to the reproductive stage where the seedlings form reproductive structures such as flowers and fruits.
2-Hormonal Retention Hypothesis
According to this hypothesis, the vernalization method has two possibilities.
In long-day plants: The cold application process encourages the development of a flower hormone called vernalin. Anthesin, the flowering hormone in long-day plants, converts the hormone vernalin to gibberellic acid, a growth-regulating hormone that induces flowering in plants. Studies have shown that application of gibberellin replaces cold treatment for vernalization in some biennial plants.
In short day plants: Such plants lack anthesin, a flowering hormone that converts vernalin to gibberellic acid, thereby inducing flowering.
Vernalization is thought to be regulated by an epigenetic mechanism. This mechanism differs from genetic mutations in the DNA sequence, which are inherited but irreversible. As a result of research, it has been determined that plants can turn off the flowering suppressor gene through an “epigenetic silencing” process when exposed to the long winter cold. Known as the FLC (Flowering Locus), this gene acts as a brake to flowering and is silenced in the cold to induce spring flowering. It has been found that this silencing occurs gradually to progressively accelerate the flowering period, but reflects a change in individual gene level as an increasing rate of gene turns off as winter progresses. In laboratory experiments, vernalization was carried out at constant temperatures in growth chambers set between 0 and 15 °C. But true winter temperatures are not constant, with daily fluctuations outside during the day and night often exceeding the difference in seasonal average temperatures. Experiments to date show that plants can extract a reliable signal from varying temperature profiles. From different studies it has been found that vernalization is affected by conditions as hot as cold and over a much wider temperature range than previously thought.
Vernalized Plant Specimens
The vernalization response is enhanced in plants adapted to regions with harsh winters and relatively short growing seasons. This adaptation ensures that the plants bloom at the right time. If they bloom in the fall, the seeds germinate before winter begins, or they don’t have enough time to germinate and sprout and new plants die. This is why these plants germinate in the fall and are exposed to low temperatures during the winter. Later, when the weather gets warmer, they bloom and ensure that seed development takes place in suitable conditions. In fact, some plants do not bloom unless exposed to cold.
Plants (or seeds) can be artificially exposed to low temperatures to promote flowering or increase seed production. In the vernalization process, flowering is facilitated by a cold treatment provided to a fully moistened seed or a growing plant.
Because it requires vernalization, garlic is planted in the fall, mainly during the winter. If the temperature is not as low as necessary for a certain period of time, garlic will not form a tuber, winter wheat will not bloom and the next season will not hold grain. Peaches, apples, and many other fruit trees require a minimum of cold periods in the winter each year to produce good yields.
The vernalization process is used extensively in food crops (wheat, barley, rye, etc.), biennial crops (cabbage, carrot, sugar beet, etc.), and perennial crops (such as chrysanthemum). Some food plants have a spring or winter variety, the spring variety is planted in the spring and the winter variety is planted in the fall. Therefore, flowers produce grains or seeds towards the end of this season. However, the winter variety is planted in the fall, germinated in the winter, grown in the spring and harvested the following summer. By partially germinating the seed and then cooling it down to 0°C until spring, it is possible to cause winter wheat to yield in the same year. Biennial plants take two years to bloom. In the first year, the plant undergoes vegetative growth and grows stems, leaves, and roots, while in the second year it goes into dormancy during the winter, thereby producing flowers and fruit, and eventually dies the next summer or spring.
The response in tulips is slightly different from the classical vernalization response. The cold allows the flower stalk to grow, rather than initiating flower development in tulips. The growth of the stem raises the flower bud formed on hot summer days from the ground so that the flower can be seen.
Temperature has a significant effect on germination, growth and metabolic activities. Vernalization is the exposure of the seedling to low temperature during the winter months. Devernalization is the reversal of the vernalization process as a result of exposing the vernalized buds or seeds to higher temperatures (about 40 degrees Celsius). Cold treatment should not be immediately followed by high temperature treatment.
Factors Affecting Vernalization
Factors affecting vernalization or vernalization requirements are:
Low temperature: A general process for growing healthy plants using the vernalization technique primarily requires a low temperature ranging from 0-10 degrees Celsius. After that, the seedlings need to be exposed to a high temperature of 40 degrees Celsius. The temperature of the cooling process depends on the plant species.
Time period: The vernalization period also depends on the plant species and can vary from a few hours, days, weeks or even months.
Actively dividing cells: The vernalization process takes place in germinating seeds that contain an active embryo. Thus, the process does not occur in dry seeds. It requires seeds moistened with active meristematic cells. Metabolically active apical meristems are temperature sensing sites to initiate flowering. Young leaves are more susceptible to the vernalization or chilling process. The shoot tip of the embryos of mature stems or seeds receives a low temperature warning. As a result, the stimulus of this process is only shoot tips, root tip, developing leaves, embryo tips, etc. detected by meristematic cells.
Water and Oxygen: Proper protoplasmic hydration is a prerequisite for the seedling to recognize a stimulus. Vernalization needs sufficient oxygen as it is an aerobic process that regulates metabolic energy inside plant cells.
The age of the plants and the place of vernalization also affect the process.
The Importance of Vernalization
In fact, every plant needs a certain number of hours to be cooled before it can grow. Very cold winters can be harmful to trees and even deadly over time. Basically, the plants have adapted to ensure that they will survive the winter and be able to reproduce when spring comes back. Vernalization, an important process in the life of plants, provides ample time for plants to mature and also prevents premature ripening during the growing season. It reduces the vegetative phase and accelerates the reproductive period. It enables biennial (biennial) plants to behave like annual plants, early flowering and early fruit set. Vernalization makes plants more adaptable by allowing them to grow in areas where they would not normally grow. It is applicable not only for the cultivation of temperate plants, but also for some tropical plants, such as wheat, rice, millet. Thanks to the vernalization, the grain wrinkles of Triticale (a hybrid of wheat and rye) can be eliminated. Increases the yield of plants, resistance to cold and fungal diseases, helps to improve the crop.
Writer: Musharref Ozdas