Vernalization is derived from a Latin word “vernus” which means “of spring”. It means to make “spring-like”. It is the induction of the flowering process of the plant by exposure to the long periods of cold winter or such conditions. Once this process takes place, plants develop the capability of flowering. However, they may necessitate extra seasonal weeks of growing before they flower.
In the process of vernalization, flowering is facilitated by a cold treatment provided to a completely hydrated seed or to a growing plant. As a result of the process of vernalization, the vegetative phase of the plant is restricted, which leads to early flowering. In the absence of cold treatment, those plants which need vernalization exhibit a delayed flowering or stay vegetative.
Types of Vernalization
Vernalization can be of the following types –
- Obligate vernalization
Plants must be exposed to lower temperatures for a specified period of time. Example – Biennial plants (cabbage)
- Facultative vernalization
Upon being exposed to lower temperatures, flowering in plants appears earlier. Example – winter annual triticale.
Mechanism of Vernalization
Through vernalization, there is an advancement in the process of blooming as a result of the delayed period of low temperatures, for instance, that which is attained in winter. To describe the mechanism of vernalization, there are two main hypotheses –
- Phasic development theory
- Hormonal theories
Phasic Development Theory
As per this hypothesis, there is organization of stages in the plant’s improvement. Each stage is under the impact of environmental elements such as light, temperature etc. Here, in turn, there are two main stages –
- Thermostage – depends on temperature, wherein vernalization accelerates thermostat. Thermostage is the vegetative phase requiring low heat, aeration and enough dampness
- Photostage – necessitates high temperature. Here, vernalin assists in producing florigen.
As per this hypothesis, the freezing treatment propels the development of a floral hormone referred to as vernalin. Such a hormone is imparted to various parts of the plant. The vernalin hormone diffuses from the vernalized plants to the unvernalized plants, prompting blooming.
Vernalization in Plants – Site of vernalization
The metabolically active apical meristems are the sites of perception of temperature to initiate flowering. The younger leaves are more susceptible to the process of vernalization. The shoot apex of mature stems or embryo of seeds receives low temperature stimulus. Consequently, the stimulus of this process is perceived by meristematic cells only such as the shoot tips, root apex, developing leaves, embryo tips, etc.
Some food plants have a spring or winter variety, wherein the spring variety is typically planted in the spring. Hence, flowers produce grains towards the end of this season. However, the winter variety is planted in autumn wherein it germinates in winter, grows in the spring and is harvested the following summer. Biennial plants require two years for flowering as they grow stem, leaves and roots in the first year and for the cold months enter into dormancy. In the subsequent months, it requires a period of cold or vernalization for the process of flowering. Gradually, biennial plants flower producing fruits and in the following summer/spring, they die. Some examples are cabbages, carrots, and sugarbeets.
Predominantly, garlic is planted through winter, as it necessitates cold temperature (vernalization). In the event where the temperature is not low enough for a particular duration of time, garlic does not shape the bulbs, winter wheat does not blossom and frame grain in the following season.
Devernalization is the reversion of the process of vernalization as a result of being exposed to higher temperatures. This process is affected by treating vernalized buds or seeds with a high range of temperature. It was in 1957 that Lang et al demonstrated that applying gibberellins coils substitute the cold treatment for vernalization in some biennial plants.
Factors affecting vernalization
The process of vernalization is affected by the following factors –
- Age of the plants
- Site of vernalization
- Provision of suitable low temperature
- Duration of being exposed
Significance of Vernalization
- This process can aid in shortening the vegetative phase of a plant and bring about early flowering, which is applicable to temperate plants and some tropical plants
- Through vernalization, Kernel wrinkles of Triticale can be eliminated
- It elevates the yield, resistance of plants to fungal diseases
- It increases the cold resistance of plants
- Aids in improvement of crop
- Vernalization in biennials can induce early flowering and early fruit setting
- The process of flowering could be induced by grafting which is used in the process of horticulture
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