Following views tried to explain the mechanism of vernalization:
- 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 pynthosize the flower stimulus. The flower stimulus is then transformed into localized areas within the shoot apex. Thus flowering start in it.
- 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.
Evidence of gibberellins as a vernalin
(a) Anton Lang in 1957 applied gibberellins on biennials like henbane. Henbane require. vernaliration stimulus for flowering. Gibberellins induced flowering in it without cold temperature treatment.
(b) Purvis in 1961 induced flowering in winter annuals by treating their seeds with gibberellins.
(c) It was also found that the exposing of vernalization requiring species to low temperature treatment produces a large amount of natural gibberellins. These results indicate that properties of gibberellins are similar to vernalin.
Objections on gibberellins as a vernalin
Gibberellin was applied to vernalization requiring rosette plants. It causes elongation of stem. Then it prodUces floral buds on this shoot. In another experiment plants were provided cold temperature for inducing flowering. The floral buds appeared as soon as the shoot begins to elongate. Thus the effect of application of gibberellin and low temperature treatment are not same. It indicates gibberellin does not produce same affect as vernalin.
- Anthesin hypothesis of Challakhyan
Mikhail Chailakhyan (1968) suggested that two substances involved in flower formation. One is gibberellin or gibberellin-like substance and second is anthesin. Anthesin is hypothetical substance like vernalin. Chailakhyan suggested that Vernalization requiring plants produce vernalin when subjected to low temperatures. The vernalin is then converted to gibberellins in long days. Gibberellin cause flowering in the presence of anthesin. But vernalin is not converted into gibberellin in short days. Therefore, flowering cannot occur in them. This hypothesis could not be proved experimentally.
- Phasic development theory of Lysenko
Phasic development theory was put forward by a Russian plant physiologist Lysenko. According to this theory every plant passes through two phases of development. These phases are thermophase and photophase. Both these phases are controlled by environmental factors.
(a) Thermophase: The early phase of life which requires definite
temperature for development is called thermophase. It is applied to development of seeds and seedlings. Each seed requires definite temperature. This requiremen, varies for different species. For example, biennial winter wheat need low temperature. But the seeds of annual spring varieties require higher temperature. Thermophase must be completed before the photophase. Thus chilled or vernal ized seeds complete the thermaphase.
(b) Photophase: The late phase of life in which plant require definite light requirement for flowering is called photophase. If the chilled seeds are sown, they grow and enter into photophase. This phase require definite light and dark period (photoperiodism) for flowering.