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VERNALIZATION The low temperature treatment given to the seed or plant for stimulating flowering in them is called vernalization. Some seeds required low temperature treatment to break dormancy. Similarly shoot bud dormancy in some plants like Acer and Betula is overcome by low temperature. But the low temperature requirement for processes other than flowering is not taken as vernalization. Seed is given low temperature treatment during vernalization. Its effect persists and flowering takes place much later. Thus the effect of vernalization is inductive in nature. Therefore, the plant need not be exposed to low temperature at the time of flowering. Discovery of vernalization J. Gustav Gassner (1928) was a German plant physiologist. He was working with rye (Secak cereale) found that temperature is critical during early stages of seed germination. He showed flowering and fruiting depends on temperatures during this critical stage. .Experiment of Gassner. He planted winter rye in spring. The seeds germinate and produce vegetative plants. But they did not flower until late in the growing season. The winter variety requires low temperature. Gassrier found that winter Petkus rye could be given low temperature requirement artificially. He placed imbibed seeds in chilling temperatures between 2 to 5 °C for 5 to 6 weeks. He planted the chilled seeds in the spring. They grew and flowered like ordinary Petkus rye. Thus he discovered the phenomenon of vernalization. Vernalization in Annual and Biennial Forms(Qualitative and Quantitative Vernalization) There are two types of plants: 1.	Annual forms (Quantitative vernalization) The plants which survive one growing season are called annual plants. Its examples are rye, pea and gram. They are quantitative in nature. These plants can flower without vernalization. But vernalization develops the flowering earlier in these plants. For example Petkus rye flowers in 15 weeks without vernalization. But it takes only 7 weeks when it is given low temperature treatment. 2.	Biennial forms (Qualitative vernalization) The plants which survive two growing seasons are called biennial plants. Its example is Hyoscyamus.. It has qualitative vernalization requirement. Their first growing season is before winter. They are sown in summer. They develop vegetative body in first growing season. They store food in their roots. Then winter comes. Vegetative plant dies. After winter the under ground root give rise to new plant body. It is their second growing season. The plant develops flowers and seeds. It is harvested in spring. These plants fail to flower without vernalization. They remain vegetative indefinitely without vernalization.

SITE OF VERNALIZATION I. Stem tip: The localized cooling treatment was given to stem tip. It receives the vernalization effect. The stem tip is chilled. The rest of the plant may be kept at high temperature. It vernalizes all the plants. 2. Seed: Seeds are also vernalized before sowing. The embryo in seed receives the vernalization stimulus. Then seed is sown. It produces flowers–.. 3.	Root tips and leaves: The root tips and leaves of Lunaira biennia contain dividing cells. Therefore, its root tip and leaves can be venalized. Thus, dividing cells are necessary for vernalization.

FACTORS AFFECTING VERNALIZATION 1.	Water and Oxygen Seed need to imbibe water. Therefore, dry seed cannot be vernalized. Similarly oxygen is also necessary for vernalization. Oxygen is used during respiration. A germinating seed need energy. This energy is provided by respiration. The inhibition of respiration blocks the process of vernalization. 2.	Temperature The seeds are exposed to low temperature during vernalization. This temperature ranges from I — 7°C. Temperature lower than 4°C is not effective. Similarly, temperature above 12°C does not cause vernalization. The duration of vernalization varies in different species. It varies from 10 to 100 days. The period of 8. week is quite effective for vernalization in most of the species. 3.	Devernalization The exposing of vernalized seed to high temperature lost the effect of vernalization. It is called Devernalization. It occurs in case of sub¬optimal vernalization. The temperature above 30 -35°C is effective for devernalization. However, the optimum vernalization cannot be devernalized. 4.	Food substances Food substances are required for respiration. Isolated embryos cannot be vernalized. 5.	Effect of light Light has no effect on the process of vernalization. Thus seed of a plant can be vernalized in the presence or absence of light. But many plants like carrot require both vernalization and photoperiodic stimulus. They must be first vernalized. Then are given photoperiodic stimulus for flowering. 6.	Effect of hormones Many species like carrot and Hyoscyamus have obligate vernalization requirement. They are rosette in habit during vegetative stage. These .plants show rapid flowering after vernalization. It also causes elongation or bolting in plant. The requirement of vernalization can be replaced by application of gibberellic acid. It causes both bolting and flowering.

Mechanism of vernalization Following views tried to explain the mechanism of vernalization: 1.	Concept of Gregory Gregory and coworker were working on cereals. They believed that vernalization process consists of several partial reactions. Cells within the shoot apex receives low temperature stimulus. It starts metabolic processes. These processes synthesize the flower stimulus. The flower stimulus is then transformed into localized areas within the shoot apex. Thus flowering start in it. 2.	Hormonal Theory of G Melchers: He suggested that low temperature induces the formation of vernalin. Vemalin is a growth substance. It initiates the synthesis of the flower stimulus. The vernalin has not yet been isolated. But some indirect evidence supports the existence of vernalin. G. Melchers performed two experiments: (a)     The grafting experiments of Melchers provide evidence about the existence of vernalin. He grafted a plant part, tear or stem, of a vernalized henbane (Hyoscyamus) to a – non-vernalized henbane plant. He found that the non-vernalised henbane flowered. (b)     Florigen also passes through graft union. Some physiologists suggest that florigen may be vernalization stimulus. The experiments of Melchers and Lang proved it wrong. They grafted an tin-vemalized henbane (Hyoscyainus niger) plant to a Maryland Mammoth tobacco plant. The henbane plant flowered. The stimulus transmitted from tobacco plant to the henbane may be through photo inductive cycle or non-inductive cycles. ‘Flats florigen is not produced in non-inductive cycles. Therefore, it cannot act as vernalin. Chemical nature of vernalin The low temperature treatment produces vernalin. This vernalin induces flowering. However, this hypothetical hormone vernalin cannot he isolated. But some plant physiologists believed that some plant growth hormone gibberellins may act as vernalin.

Advantages/Importance of Vernalisation: (i) Vernalisation can help in shortening the juvenile or vegetative period of plant and induce early flowering. It is applicable to not only temperate plants but also to tropical plants, e.g., wheat, rice, millets, cotton. (ii) It increases yield, resistance to cold and diseases. (iii) ‘Kernel wrinkles’ of Triticale can be removed by vernalisation. (iv) It enables the biennials to behave as annuals. (v) Plants can be grown in such regions where normally they do not grow.