Vernalization defines as a process of growing flowers and fruits through a phase of cold treatment. It reduces the time period of the juvenile vegetative growth phase in the plants. The active meristematic cells of the shoot apex, root apex, embryo tips etc. participate in the production of stimulus refers as “Vernalin”. The term vernalization has been originated from the Latin word “Vernalis” which means “of the spring”. Before reproduction, it allows vegetative maturity.
The process of vernalization can be generally employed in food plants (wheat, barley, rye etc.), biennial plants (cabbage, sugar beet etc.) and perennial plants like a chrysanthemum.
Definition of Vernalization
Vernalization can define as the process where the seedling is subjected to low temperature (5-10 degrees Celsius) during winter and later develops flowers and fruits when exposed to a high temperature (about 40 degrees Celsius). Merely, it can also refer as the process of going dormant in the cold temperature. The process of subjecting seedling from the cold temperature to the high temperature is called devernalization. The temperature and the time period of the chilling process depend upon the plant species.
A plant depends upon exposure to cold temperature (both qualitatively and quantitatively) to develop reproductive structures like flowers and fruits. The temperature has a major effect on germination, growth and metabolic activities. The plants must need exposure to cold treatment for germination and flowering in plants, whether they are growing at a high or low temperature.
Vernalization can occur in two ways, namely facultative and obligate. Facultative vernalization results in early flowering once exposed to the low temperature, while obligate vernalization requires exposure of the seedling to the desired time period to induce flowering.
Many contributors gave their different theories and approaches to describe the phenomena of vernalization.
|1857||John Hancock Klippart||Demonstrated the effect of winter temperature on germination|
|1918||Gustav Gassner||Published an article to differentiate specific requirements of winter from the summer plants|
|1928||Lysenko||Introduced the chilling process on cereal seeds and given the term vernalization|
|1960||Chourad||Defined vernalization as a process of accelerating flowering in plants by the chilling treatment|
The factors affecting vernalization includes:
Low temperature: The vernalization technique requires a low temperature ranging between 0-10 degrees Celsius. After this, the seedling is followed by a high temperature of 40 degrees Celsius. It is the general process that must be followed to grow a healthy plant.
Time period: The time period of vernalization depends upon the plant type, and can vary from a few days to weeks or even several months.
Actively dividing cells: The process of vernalization occurs in the germinating seed, which comprises an active embryo. Thus, the process cannot occur in the dry seeds, and the seed must be moistened with active meristematic cells are required.
Water and Oxygen: Proper protoplasmic hydration is a prerequisite need for the seedling to recognize a stimulus. Vernalization needs the presence of adequate oxygen as it is an aerobic process which regulates metabolic energy inside a plant cell.
The process of vernalization involves the following steps:
- Seed moistening: First, soak the seeds in water, and then allow it to germinate in the processing chamber at 10-12 degrees Celsius.
- Chilling treatment: Expose the germinated seeds to the low temperature (3-5 degrees Celsius) for the desired period of time.
- Drying and sowing: Seeds are then dried and sown in a field at proper water and oxygen supply.
There are two hypotheses to explain the theory of vernalization.
The hypothesis of phasic development
According to Lysenko, vernalization method completes in two stages:
Thermostage: It can define as the primary stage, where the slightly germinated seeds are exposed to a low temperature of 0-5 degrees Celsius in the presence of oxygen and moisture. During this stage, structures like root, stem and leaves develop and also refers as a vegetative phase. Here the seed loses its dormancy and starts to germinate.
Photostage: It is the secondary stage, where the seedling after vernalization is subjected to the phase of correct photoperiod or high temperature upto 40 degrees Celsius. It can define as the reproductive stage, where the seedlings grow the reproductive structures like flowers and fruits.
The hypothesis of hormonal involvement
According to Chailakhyan, vernalization method has two possibilities:
In long-day plants: A flowering hormone “Anthesins” is present that convert vernalin hormone into the growth-regulating hormone gibberellic acid, which finally induces flowering in a plant.
In short-day plants: These lacks a flowering hormone “Anthesins” that convert vernalin into gibberellic acid, and do not induce flowering.
Examples: Food plants (wheat, rye, etc.) are having both spring and winter varieties. The spring and winter varieties are sown in the spring season and autumn season, respectively.
Biennial plants (carrot, sugar beet, etc.) take two years to flower. In the first year, a plant undergoes vegetative growth, while in the second year the plant becomes dormant during the winter, then produce flowers and fruits, and at last, becomes died in the next summer.
- Vernalization gives adequate time to the plants to get mature and prevents pre maturing during their growing season.
- It reduces the vegetative phase and hastens the reproductive period.
- Vernalization increases the growth yield.
- It makes the plant more adaptable by allowing them to grow in such regions where they usually don’t grow.
- It also removes wrinkles on kernels of Triticale.
- This process is not only applicable to the temperate plants but also some tropical plants like wheat, rice, millets etc.
Therefore, we can conclude that vernalization is a method of inciting early flowering in a plant by the pretreatment of seeds at a low temperature. Reversal of the vernalization process is an effect known as “Devernalization” which can define as the interruption in the period of low temperature into a high temperature.