12th botany neet school

Photoperiodism and vernalization, Phytochromes and flowering

Photoperiodism and vernalization

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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.

Plants are classified into three classes

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.

Phytochromes and flowering

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.

Techniques of vernalization

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.

Practical application of vernalization

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.

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Other links 

Plant tissue culture – origin and techniques

Plant physiology – photosynthesis and its significance

Site of photosynthesis and Mechanism of photosynthesis

Electron transport system and photophosphorylation types

Dark reaction

C3 and C4 pathways

Photorespiration or C2 cycle

Factors affecting photosynthesis

Test tube and funnel experiment, Ganong’s light screen experiment

Mode of nutrition – Autotrophic, Heterotrophic


Mechanism of Respiration – Glycolysis

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

Mechanism of Respiration – Electron Transport Chain, Energy Yield

Ganong’s respiroscope, Pentose phosphate pathway

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

Plant growth and Measurement of plant growth

Phytohormones Auxins

Phytohormones Gibberellins

Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects


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