Photoperiodism in plants class 11th

Have you ever wondered why the plants or leaves grow in a certain way. This is because of photoperiodism about which we will talk about ahead in the article. Before understanding photoperiodism there are two terms which we should know the meaning of - Critical day length and critical photoperiod.

Critical day length is the light period which a plant needs for flowering and critical photoperiod is the duration of light which is needed continuously for long day plants but should not exceed a certain point in short day plants in order for it to flower. 

What is photoperiodism in plants ?

The response a plant gives to a particular light period or dark period in order to start the process of flowering is known as photoperiodism. It was Charles Darwin who first experimentally showed that the coleoptile tip bends towards a source of light. 

The term photoperiodism was coined by Gerner and Allard. They first studied and highlighted how a plant responds to a particular period of time when they supplied it with light or darkness. This is called photoperiod. It has been observed that the dark period is important than the light period for flowering. 

Importance of the length of the day in flowering of plants was first seen in Tabacco. Have you wondered there are some fruits which are just seasonal like lychee, oranges, etc., it is because of photoperiodism the plants are seasonal. These plants are classified into 3 categories - Short day plants, long day plants and day neutral plants.

Short day plants:

There is a certain time for short day plants that the day length should not exceed a certain critical value, i.e the value of light or dark period a plant should get. These are also known as long night plants as dark is also needed in 24 hours. 

If the dark period is interrupted even by a slit of light, flowering won't occur. Examples of short day plants are soayabean, potato, sugarcane, cosmos, dahlia, tabacco, etc..

Long day plants: 

These plants require less of the dark period and more of the light period. This photoperiod may vary from 4 to 18 hours. Flowering can occur in continuous light. Examples of long day plants are spinach, radish, lettuce, maize, oats, wheat, etc..

Day neutral plants:

Flowering is not affected in these plants even if the light period is more or less and same goes for the dark period as well. Even if there is a full day of dark period or light period these plants will continue to flower. Examples of day neutral plants are tomato, cucumber, cotton pea, sunflower, etc..

The hypothesis show that a hormonal substance named florigen is responsible for flowering but there is no experimental proof for this hypothesis. It is believed that the active principle is synthesized in the leaves of the plants and it goes to the apical meristem which gives rise to flowering which takes place by converting vegetative meristem to reproductive meristem

Photoperiodism takes place due to synthesis of phytochrome, pigments which are present in the roots, stem, leaves, cotyledons, bud, etc.. It controls photomorphogenesis and development of the plant. Photomorphogenesis is when the plant responds to a light spectrum for growth and development. 

If a certain plant doesn't get the required light period or dark period it can be grafted on another flower which has received proper amount of light period or dark period. By this, it indicates the presence of hormones which are transported between the two plants. 

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