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Vernalization has multiple interactions with photoperiodism. Thus, a cold requirement for flowering appears to be associated with a specific photoperiod. The most well quoted instance is that of need for vernalization and LD which leads to flowering in late spring or early summer. However, a cold requirement may also be accompanied by a SD photoperiodic response.
Vernalization is also known to substitute or modify a photoperiodic response. Thus, in Spinaciavernalization may reduce the critical day length about four hours. Similarly SD treatment could also partly or entirely substitute for cold. Thus, such plants could be classified as SLDP or as cold needing LDP. However, SD must or low- temperature must precede the LD exposure. However, the reverse is not effective.
Once a plant is vernalized, it may be de-vernalized by immediate exposure to high temperature (e.g. 30°C or above) and in some cases, revernalized with another exposure to low temperature. De-vernalization does not occur if the plant is allowed a period of 4 to 5 days at 20°C. During this time vernalization is stabilized.
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G. Melchers has demonstrated transmission of a vernalization stimulus across a graft union in Hyoscyamus. It a plant part such as stem or leaf of a vernalized Hyoscyamus is grafted to an unvernalized one, the latter will flower. In another experiment an unvernalized Hyoscyamus plant was grafted to a non-cold requiring variety of tobacco plant (Maryland Mammoth) the Hyoscyamus plant flowered.
Since the stimulus was transmitted from tobacco plant on to photo non-inductive cycles as well as inductive cycles, it could not be florigen. Since the tobacco plant is not a cold requiring plant, the stimulus or substance produced by vernalization must be present in the absence of cold treatment.
Melchers named this substance vernalin. The example of vernalization induction from donor to receptor are few. Moreover, vernalin has not been extracted so far. Thus evidence for the existence of vernalin, in a mobile form is rare.
As is evident from Fig. 22-13 a suitable photoperiod is highly essential in order that vernalin is converted into florigen. There is also possibility to assume that vernalin regulates the biosynthesis of florigen from some other precursor (s). Obviously, in vernalization, length of day is very essential for flower formation.
Lang has shown that application of GA could replace vernalin. However, GA is not vernalin. Gibberellins simulate properties of vernalin, and can even substitute for cold treatment in rosette plants such as Hyoscyamus. The Hyoscyamus plants applied with GA, flower in the absence of a low temperature.
Majority of the plant, physiologists do not equate GA with vernalin because when GA’s are applied to a cold-requiring rosette plants, they enhance stem elongation and not flowering. Indirectly, through stimulation of stem growth GA might be inducing changes within the plant which is turn lead to the production of vernalin and then flower formation. GA has also been shown not as the general flowering hormone since SD plants do not show any response to it.
