Photoperiodism: What is photoperiodism? This is one question which usually comes to students’ minds when studying the concept. The concept is not a hard one but it is one which needs proper focus to understand properly. Photoperiodism occurs in plants and animals and it is a periodic phenomenon. Students will get a proper understanding of the concept wehn dwelling into the chapter deep.

This article has provided all the important topics and details that are associated with photoperiodism. After reading this article, students will be able to confidently define photoperiodism, types of photoperiodism and so on. Keep reading this article to master the concept of photoperiodism.

What Is Photoperiodism?

“Effect or requirement of the relative length of day and night on flowering is called Photoperiodism.” (Source: Arihant Biology Handbook)

The term Photoperiod has been derived from the word ‘Photo’ means ‘light’ and ‘period’ means ‘length of time’. Photoperiodism is the typical movement in plants under the influence of the availability of light to induce flowering. Plants respond to light, both intensity and quality, to convert from vegetative to flowering.

Discovery of Photoperiodism

Garner and Allard first studied Photoperiodism. They took a tobacco mutant, i.e., ‘Maryland mammoth’, for their experiment. They observed that this tobacco mutant flowered at different times at different places. After controlling other factors like temperature, nutrition, etc., they concluded that it was the length of the day which affected flowering. Most of the plants would initiate flowering only when subjected to light for lesser or more than a certain period called ‘critical photoperiod.’

Photoperiodism Diagram

Fig: Diagram of Photoperiodism

Photoperiodism Plant Types

Depending on the light requirement, plants can be divided into the following categories:

1. Short Day Plants
2. Long Day Plants
3. Day Neutral Plants
4. Intermediate Plants
5. Short – Long Day Plants
6. Long – Short Day Plants

Fig: Classification of Plants based on Photoperiodism

1. Short Day Plants (SDP)

(a) These plants flower only under day lengths shorter than the critical period. 
(b) They require a short daylight period of \(8-10\) hours and a continuous dark period of about \(14-16\) hours to flower.
(c) They do not flower when the day is long and the night is short.
(d) They do not flower if the dark period is briefly interrupted by red light.
(e) They are also known as long night plants.
(f) Examples: Xanthium strumarium (Cocklebur), Nicotiana tabacum, Glycine max (Soyabean), Coffea arabica (Coffee), Chrysanthemum, etc.

2. Long Day Plants (LDP)

(a) These plants flower only under day length more than the critical period.
(b) They require longer daylight, usually \(14-16\) hours, for subsequent flowering.
(c) They require a relatively short period of darkness.
(d) The light period is critical in LDP plants.
(e) They require prolongation of the light period for a brief exposure to light during the dark periods that bring about flowering.
(f) They are also known as short night plants.
(g) Examples: Beta vulgaris (Beet), Raphanus sativas (Reddish), Spinacia oleracea (Spinach), Daucus carota (Carrot), Allium cepa (Onion), Triticum aestivum (Wheat), Avena sativa (Oat), Zea mays (Maize), Saccharum officinarum (Sugarcane).

3. Day Neutral Plants (DNP)

(a) These plants do not require any specific day or night period for flowering.
(b) They can blossom throughout the year.
(c) Examples: Helianthus annuus (Sunflower), Lycopersicon esculentum (Tomato), Cucumis sativus (Cucumber), Gossypium hirsutum (Cotton), Pisum sativum (Pea), Capsicum annuum (Chilli).

4. Intermediate Plants (IP)

(a) These plants flower within a definite range of light hours.
(b) They cannot flower above and below this range.
(c) Example: Wild Kidney Bean.

5. Short–Long Day Plants (S-LDP)

(a) They are generally long-day plants.
(b) They require short photoperiods for floral initiation and long photoperiods for blossoming.
(c) They usually flower between spring and summer.
(d) Examples: Triticum vulgare, Secale cereale (Rye), Trifolium repens, etc.

6. Long-Short Day Plants (L-SDP)

(a) They are generally short-day plants.
(b) They require long days for floral initiation and short days for blossoming.
(c) They usually flower between summer and autumn.
(d) Examples: Cestrum nocturnum, Bryophyllum, Kalanchoe.

Fig: Photoperiodism Plant Types

Role of Phytochrome in Photoperiodism

Phytochrome is a blue-green pigment that acts as a photoreceptor and helps in photoperiodic induction. Phytochromes exist in two forms: active form \(\left( {{{\rm{P}}_{{\rm{fr}}}}} \right)\) and inactive form \(\left( {{{\rm{P}}_{\rm{r}}}} \right).\) The \({{{\rm{P}}_{\rm{r}}}}\) absorbs red light \(\left( {{\rm{660}}\,{\rm{nm}}} \right),\) and \({{{\rm{P}}_{{\rm{fr}}}}}\) absorbs far-red light \(\left( {{\rm{730}}\,{\rm{nm}}} \right).\) During the daytime, two forms \({{{\rm{P}}_{\rm{r}}}}\) and \({{{\rm{P}}_{\rm{fr}}}}\) interchange till they reach an equilibrium. The \({{{\rm{P}}_{\rm{fr}}}}\) is the biologically active form. It is predominant during the day as more red light is present. The \({{{\rm{P}}_{\rm{fr}}}}\) acts as the switch that turns on flowering or seed germination. The \({{{\rm{P}}_{\rm{fr}}}}\) breaks down into the \({{{\rm{P}}_{\rm{r}}}}\) by absorbing far-red light. The response is determined by the length of the night period. In the absence of light, the \({{\rm{P}}_{\rm{r}}}\) will gradually convert to \({{{\rm{P}}_{\rm{r}}}}.\) So during the night, the \({{{\rm{P}}_{\rm{r}}}}\) form is predominant. The inactive \({{{\rm{P}}_{\rm{r}}}}\) is converted into active \({{\rm{P}}_{{\rm{fr}}}}\) by absorbing red light.

Fig: Role of Phytochrome

Photoperiodism in Animals

Some animals show behavioural and biological changes depending on the length of the day. For example, the length of the day affects their fur colour, migration, hibernation, and sexual behaviour. However, the process of Photoperiodism is not so evident in the case of animals as compared to plants. For example, The diapause in insects, the singing of birds during the day, seasonal breeding in animals, etc.

What Is the Importance of Photoperiodism in Plants?

The importance of Photoperiodism in plants are:
(a) It is of great importance to the commercial flower growers in hybridisation experiments.
(b) The knowledge of photoperiodic effects helps to obtain higher yields of tubers, rhizomes, etc.
(c) It helps a plant to flower throughout the whole year by giving suitable photoperiod. It has helped the plant breeders in effecting crossbreeding in plants which usually develop flowers in different seasons.
(d) It helps a plant to produce flowers in different seasons.

Summary of Photoperiodism in Plants

Photoperiodism is a reaction in which a plant responds to the changes in the length of light and dark periods to complete the process of flowering. It also indicates that a particular flower can bloom in a specific period of the year, which may differ in every plant. Phytochrome is a pigment that helps in photoperiodic induction. Photoperiodism is very essential for plant hybridisation experiments and helps the plant to produce flowers throughout the year in different seasons.

FAQs on Photoperiodism

Let us look at some of the commonly asked questions about Photoperiodism:

Q.1: Define photoperiodism in plants.
Ans: Photoperiodism in plants can be defined as the response of plants and animals to the length of the day and the night.

Q.2: Which plant pigment is responsible for the process of Photoperiodism?
Ans: Phytochrome is the blue-green pigment that is responsible for the process of Photoperiodism in plants.

Q.3: What is Photoperiodism and its function?
Ans: Photoperiodism is the typical movement in plants under the influence of light, to induce flowering. Plants respond to light, both intensity and quality, to convert from vegetative to flowering.

Q.4: How are plants classified based on their Photoperiodism?
Ans: Plants are classified into the following categories, based on their Photoperiodism.

• Short Day Plants
• Long Day Plants
• Day Neutral Plants
• Short – Long Day Plants
• Long – Short Day Plants.

Q.5: Who discovered Photoperiodism?
Ans: Garner and Allard first studied Photoperiodism. They have taken a tobacco mutant, i.e., ‘Maryland mammoth’, for their experiment.

Q.6: Does Photoperiodism happen in Animals?
Ans: Some animals show behavioural and biological changes depending on the length of the day to adjust themselves. For example, the length of the day affects their fur colour, migration, hibernation, and sexual behaviour. However, the process of Photoperiodism is not so evident in the case of animals as compared to plants. For example, The diapause in the insects, the Singing of birds during the length of the day, seasonal breeding in animals, etc.

Q.7: Give some examples of Long Day Plants.
Ans: Some examples of long day plants are Beta vulgaris (Beet), Raphanus sativas (Reddish), Spinacia oleracea (Spinach), Triticum aestivum (Wheat), Avena sativa (Oat).

Q.8: What is Critical Photoperiod?
Ans: Most of the plants would initiate flowering only when subjected to light for less or more than a certain period called critical photoperiod.