The response of a plant to the relative lengths of light and dark periods is known as photoperiodism. In plants, most significant photoperiodic response is the initiation of flowering. It has been first observed in Maryland Mammoth variety of tobacco (Nicotiana tabacum).
From the observation of Garner and Allard all the plants do not require the same length of light and dark periods for flowering. Plants require specific period of light and darkness for flowering. It is known as critical period.
1. The plants requiring longer exposure to light than their critical period are known as long day plants eg. wheat and oats.
2. The plants requiring light for a shorter period than their critical period are known as short day plants eg. tobacco and Chrysanthemum.
3. The plants in which flowering is unaffected by the photoperiod are known as day neutral plants eg. sunflower and maize.
In 1959, Butler et al. were able to discover a photoreceptor flower inducing pigment in plants which they name phytochromes. It is believed to be widely present in all green plants. Chemically, phytochrome is a biliprotein and exists in two forms. One form absorbs red with the wave length of 660 nm called Pr and the other form absorbs far red with the wave length of 730 nm called Pfr. The two forms of phytochrome are interconvertible as shown below:
Based on the absorption spectra, Pr is also called P 660 and Pfr is P 730. In short day plants, Pr promotes flowering while Pfr suppresses it , while it is viceversa in long day plants.
The term vernalization was first introduced by a Russian scientist T.D. Lysenko in 1920. Many species, especially biennials and perennials are induced to flower at low temperature range of 1oC to 10oC. This is known as vernalization.
The response to the cold temperature stimulus is not uniform in all plants. Plants, which are vernalized, are called inductive types. Those nonvernalized are called noninductive types.
The following are the steps to be taken to induce vernalization. Seeds are allowed to germinate and subjected to cold treatment for varying period of time depending on the species. Germinated seeds after this treatment are allowed to dry for sometime and then sown.
Reversal of the effect of vernalization is called Devernalization. Subjecting the plants to higher temperature after a cold treatment brings about devernalization.
Russian scientists have used vernalization to shorten the time of crop maturity by hastening the flowering processes which are brought about by cold treatment.
Crops can be produced earlier by vernalization. They can be cultivated in places where they naturally do not grow. Vernalization helps to accelerate the plant breeding.