What is Vernalization?
Light and temperature are two primary external factors which directly influences the plant growth and development; inducing changes at cellular, molecular levels; with distinctive mechanisms: Photoperiodism, Vernalization, Seed dormancy, Climate acclimation, bolting etc.,
All these mechanisms and responses are in accordance with light and temperature. Light and temperature; external determinants; are inevitable in determining the transition of vegetative phase to reproductive phase ensuring the progeny.
Where photoperiodism induces flowering and classify plants based on their photoreception as Short-Day Plants, Long – Day Plants, Day neutral plants, long short-day plants and Short long day plants.
However, temperature also induces the transition and Seed dormancy, Vernalization are evident of the fact. One of the temperature induced growth or mechanism is the vernalization, a quantitative process where the temperature is modified to obtained better yield from plant.
Vernalization is a process of quantitative yield, where plants undergo cold treatment or chilling period during winter, without hindering the vegetative phase of the plants which continues throughout the winter.
The cold treatment prevents the development of premature reproductive phase in early winter preventing poor growth of the plant which eventually might fail during the course of development.
Not all plants require such a treatment for producing reproductive phase, many species are capable of flowering without cold treatment and inducing cold treatment might increase their productivity thereby accelerating the flowering capacity of the plant.
Vernalization is more specific characteristic feature in temperate plants where the sunlight is insufficient for optimal growth.
The plants in this region have developed many modifications to withstand the climatic variations of the region and to yield better.
The more prevalent plants of the region are Biennials and Winter annuals where they grow well in 2 growing season and annuals have one growing season respectively.
History of Vernalization
The phenomenon was first observed at temperate regions of the world. Early agriculturists of the region started sowing cereal seeds that are winter annual type at the end of winter.
This prevented the early development of the plant accompanied by stunted growth and prevents poor yield. The plants made germinating in this period; so that the vegetative phase progress slowly along the reduced growth and on optimal conditions; can support flowering.
Systematic research on vernalization begun by 1858 by Klippart were he found that cold temperature is the determining factor producing qualitative flowering in the following photoperiodic season.
In 1918, Gassner worked extensively on the concept of vernalization and identified plants sensitized for such growth. They are: Biennials and Winter annuals. He also highlighted the importance of germinated seedling which are capable of receiving “chilling treatment” signals and promote growth.
Lysenko in 1928, a Russian genetist who clearly proposed the water imbibition in winter seedling is the key feature which increases the sensitivity to initiate germination and the vernalizing capacity of the plant.
The name vernalization was initially termed as ” Jarovization” were JAR means spring or fire god in Russian. Hence, spring plants are called JAROVOE and he termed the process as Jarovization.
Jarovization is a phenomenon where winter plants behave as a spring plant to produce healthy crops. Later, Lysenko translated Jarovization as “vernalization” derived from Latin.
Concept laid out by Lysenko was scientifically disapproved because he considered vernalization as an inherited acquired characteristic passed on from parent to offspring.
But this is the main lag for the Soviet biology as a whole. Later, an accurate study was taken up by 2 groups at different region of the world with two plants.
Gregory and Purvis studied Petkus Rye a winter annual in London. Melchers and Lang, Tubigen studied on Henbane a biennial plant separately.
These two studies had led a great and detailed understanding of vernalization and its application in agriculture.
Characteristic Features of Vernalization
1. Effects of vernalization is visible after the process had taken place, where the efficiency is seen in reproductive phase of the plant in flowering.
2. It does not induce flowering but retains the capacity under unfavourable condition and to flower during favourable condition.
3. It is a supportive feature for plants during winter, sustains the vegetative growth and produces reproductive efficiency for the plants. Vernalization and breaking of dormancy are two varied phenomenon involving cold temperature.
The main difference during dormancy is the leaf primordia and other organs stops their growth with very minimal or no metabolic activity.
Dormancy is oriented towards the growth phase. But vernalization is substituting cold exposure to prevent unfavourable expression of reproductive phase and at the same time the growth phase remains unaffected.
4. It is species specific and observed only during selective climatic time period. Temperate plants exhibit the phenomenon more clearly than other plants.
Not all plants require cold treatment for productive yield. Few plants on exposure to cold treatment may cause death to the plant.
5. Vernalization is a preparatory process, where essential growth factors and hormones are manufactured and stored. On favourable condition, provided an appropriate photoperiodic stimulation will induce flowering.
6. Vegetative phase retention by vernalization is limited and species specific. The phenomenon employs short time retention of vegetative phase.
A vernalized plant must be exposed to appropriate condition within a time period. When appropriate conditions are unavailable within the limit the plant may not grow well and die earlier.
7. It occurs naturally and can be induced artificially under laboratory conditions. Vernalizing temperature range lies between 1°C – 15°C for most of the plants.
Natural vernalization takes place but not under all circumstances as it will be influenced by various other factors. But under artificial condition factors can be maintained constantly to induce efficient growth in vernalized plant.
8. The vernalized state can be reversed by subjecting it to higher temperature, Low irradiance, darkness immediately after vernalization; will remove the effects of vernalization; hence called as the phenomenon of devernalization.
9. The phenomenon provides competence for the plant to transform from vegetative phase to reproductive phase.
10. Vernalization is followed by photoperiodic period for flowering; is a mandatory process.
11. The response to vernalization is inherited to the dividing cells during vernalization. The dividing cells are present in the flowering parts of the plant, the cell division and DNA replication provides mitotic stability and mitotic inheritance of the response.
Vernalization does not have any types. But based on the requirement to vernalization, plants are divided into Obligate, Facultative plants and certain plants are unresponsive for vernalization.
Obligate Plants: vernalization is essential for flower production (i.e.) transition from vegetative to reproductive phase of the plant without which plant remains in vegetative phase or may die.
Example: henbane – Hyoscyamus niger, sugar beet – Beta vulgaris.
Facultative Plants: Facultative plants does not completely depend on vernalization for flowering but on exposure to vernalization will accelerate and becomes efficient in flowering. The transition from vegetative to reproductive phase does not require the vernalizing phenomenon.
Example: Arabidopsis thaliana, Wheat, Barley.
Vernalization Perception Site
The receptor for vernalization is present in the shoot apex for most of the plant species. In grass family of the temperate zones the receptor sites are present at both leaves and shoot of the plant.
Another specificity of the site of reception is the site must have mitotic division to receive the external environmental cues. The site was identified from various experimental procedures.
Transplantation procedures involving transfer of vernalized apex to non – vernalized plant will induce efficient flowering.
Melchers also found a substance named vernelin in which transverse across different tissues carrying the information of flowering induction and are in coordination with gibberellins or gibberellins will produce vernelin like activity.
Genes such as VRN1, VRN2, VRN3 are essential in determining the vernalization requirements of cereals. Vernalization process involves gene expression regulation of FLOWERING LOCUS C during vernalization.
Mechanism of Vernalization
The mechanism of vernalization is not fully identified. At each step of discovering the process of vernalization, many scientists had laid many hypothesis on their way of discovering. Formally, many theories were given regarding vernalization.
The theories are:
1. Phasic Development Theory
Lysenko, 1934 proposed the theory of phasic development. Vegetative to reproductive transition takes place over a period of time phase after phase involving 2 different exposure of environmental factors.
The exposed factors start after the preceding factor. According to the theory, the plants are initially exposed to the lowest temperature undergoes vernalization.
Plants are then exposed to change in light and temperature which initiates the transition to flowering phase of plant’s life cycle.
Phasic development theory hence has 2 stages: Thermostage and Photostage.
Thermostage depends on the low temperature to accelerate the vernalization. The length of low temperature exposure depends on the plant’s capacity to withstand and maintain the metabolic process.
Photostage requires high temperature to transverse from Vegetative stage to Reproductive stage. The hormone vernelin helps in plants inflorescence. For a winter plant, longer low temperature and appropriate high temperature and long light exposure to produce qualitive flower and seeds.
As a whole, the theory states that vernalization must follow photoperiodic induction for healthy progeny.
2. Hormonal Theory
In 1939, Melcher and Lang proposed this theory and many other authors from their studies revealed a similar mechanism of the theory.
The theory states that the chilling treatment makes the plant to secrete hormones which are responsible to produce reproductive growth in plant body.
The hormone is named as Vernelin and after recent findings and improvement in plant physiology research it is found that Gibberellins has similar action of Vernelin.
To prove the action of vernelin Transportation taking place, Melcher united a vernalized and non – vernalized plant. The flower bloomed in both vernalized and non – vernalized plant.
Chailakhyan also identified the presence of vernelin in plants exposed to vernalization. He also identified the presence of Anthesin in LDP to initiate growth and Absence of Anthesin SDP.
Vernelin and Anthesin produces Gibberellin which induces plant growth. In SDP absence of Anthesin does not induce flowering.
In 1961, Purvis identified that “A” substance is formed from a precursor. On cold temperature A converts to “B”.
B an unstable component during cold environment produces “D” a substance functioning similar to Vernelin or Vernelin itself, on light exposure or photoperiodic induction produces Florigen – a hormone responsible for flowering induces the reproductive phase of the plant.
Factors Affecting Vernalization
1. Age of the Plants: Different types of plants has different receptive capacity of the stimulus. Winter Annuals are capable of producing shoot at the stage of Seedling where it is imbibed with water. on the other hand, Perennials and Biennials require at least 5-week-old to be effective to receive cold stimulus.
2. Temperature: The vernalizing stimulus is effective and receptive under an optimal temperature around 1-6C and the temperature above and below the optimum levels will decrease the receptive capacity of the plants.
3. Site of the Plant: the phenomenon requires continuously dividing mitotic apical meristem to transit the stem or leaf primordia to flower primordia. Hence, the stimulus is receptive at the apices and not induced on lateral side of the plants.
4. Exposure to light: Following vernalization, the plant must be exposed to long light before the effect of vernalization wears off to support the production of efficient reproductive organ.
5. Oxygen: Vernalization takes place under aerobic condition. When devoid of oxygen the plants get devernalized and develops the reproductive phase and the production may or may not be successful in virilization dependent plants.
6. Water: In Winter annuals vernalization has effects on germinating seedling imbibed with water. Dry seeds never respond to vernalization.
Significance of Vernalization
1. Increase the production efficiency of the crop in a year as it reduces the vegetative phase of the plant.
2. Long – Day Plants and Long summer crops raised in tropical regions can be grown in temperate zones by vernalization.
3. Winter stunted growth can be prevented from freezing by introducing vernalized plants in spring.
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