The term ‘photoperiod’ was introduced to designate the favorable length of day for each organism, and photoperiodism to response of an organism to the relative length of day and night. Photoperiodism is defined as the response of a plant to the relative length of light and dark period or the duration of dark period is much more important than duration of light period.
The length of the day and night required for the initiation of flowering is called photoperiod. Depending upon the length of photoperiod, Garner and Allard classified plants into three groups. They are Short day plants, Long day plants and Day neutral plants.
The cold treatment given to the plant buds, seeds or seedlings for promoting early flowering is known as Vernalization or Yarovization. It is defined as acquisition or acceleration of the ability to flower by a chilling treatment.
Photoperiodism vs Vernalization
|It is a response of relative lengths of day and night or light and dark period(photoperiod).||It is acquisition or acceleration of the ability to flower by chilling treatment.|
|It is mediated through florigen.||It is mediated through vernalin which induces the synthesis of florigen.|
|It not only prepares the plant for flowering but also initiates flowering.||It itself does not induce flowering but prepares the plant for flowering.|
|The perception of stimulus is confined to young expanded leaves.||The stimulus is perceived ordinarily by the shoot apical meristem and is apparently dependent upon active metabolism.|
|Photoperiodic induction cannot be reserved or nullified by exposure to unfavourable photoperiods. However the promotion of flowering plant by red light could be reversed by far red radiation.||Effect of low temperature can be reversed or nullified by exposure to high temperature.|
|It can substitute the requirement of long photoperiods in long day plants, but they have no effect on short day plants.||Gibberellins can replace cold treatment for inducing flowering|
Application of Vernalization
- Vernalization shortens the vegetative period of the pants.
- It increases the resistance of the plants to fungal diseases.
- It helps in crop improvement.
- It is a physiological process substituting or compensating the effect of the thermophase.
- Winter varieties of crop plants can be converted into spring varieties by vernalization.
- Flowering can be induced by grafting vernalized shoot apex with a non vernalized shoot apex andthis feature is used in Horticulture.
- In the biennials, vernalization induces early flowering and early fruit setting.