Introduction

Why do certain flowers bloom in a specific season? It happens due to seasonal changes or change in length of day and night. Some plants require long nights to grow, and some need long days to bloom. This is photoperiodism.

Photoperiodism in plants is a usual process. Depending on the variation of day and night lengths required for flowering, botanists divide the plants into different categories. 

Definition

‘‘Photo’ means light, and ‘period’ means the length of time. Thus, ‘photoperiodism’ means the time of the day during which the plants receive light. It is the response of plants to the periods of day and night. Plants use this time measurement mechanism to grow, develop and reproduce.

Discovery

Garner and Allard first studied photoperiodism in plants in 1920. They took a tobacco mutant to observe its flowering pattern for their experiment. The scientists observed that it flowered at different places at different times.

After controlling temperature, nutrition, and other factors, they concluded that the plant flowered only when subjected to light for a lesser time than darkness. In simple words, the length of the day affected flowering.

Photoperiodism in plants is a fascinating process. The shoots modify themselves into flowering apices before flowering. However, the shoot apices of the plants cannot perceive photoperiods by themselves. Then how do plants sense changes in day length? Let’s take a look. 

Role of phytochromes

Flowering plants use photoreceptor proteins present in their leaves to detect periods of light and darkness. The photoreceptor proteins or phytochromes are leaf pigments that initiate photoperiodism in plants.

Phytochromes exist in two forms – active and inactive.

• Active form predominates during the day.
• Inactive form predominates during the night.
• Additionally, the active form of phytochromes reverts to an inactive form in the absence of light. It implies that the active form acts as a switch that turns-on flowering.

Types of Plants on the basis of Photoperiodism

We understand that flowering plants use phytochromes to sense the length of exposure to light. Plants require light for a specific duration (critical photoperiod) to bloom. The active form of phytochromes determines the critical photoperiod of different plants. And, depending on the amount of light required for flowering, plants are classified into the following categories:

• Short day plants (SDP)

• Short day plants require less exposure to light. They bloom when the days are shorter (8-10 hours).
• Photoperiodism in short day plants requires a continuous dark period (14-16 hours) for flowering. SDP won’t bloom if red light (daylight) briefly interrupts the dark period.
• These are also known as long night plants.
• For example – soybean, tobacco, coffee, chrysanthemum. 

• Long day plants (LDP)

• LDP flowers when the days are longer (14-16 hours).
• Unlike SDP, flowering in LDP boosts by prolonging the light period or a brief exposure of light during the dark period.
• These are also known as short night plants.
• For example – radish, spinach, Hibiscus, sugarcane. 

• Day neutral plants (DNP

• Day-neutral plants don’t require a critical photoperiod for flowering.
• They are neutral to the length of day/night.
• These plants are perennial.
• For example – roses, tomatoes, cucumbers and more.

1. Intermediate plants (IP)

• Intermediate plants require a definite range of light hours to flower.
• These do not flower above or below the defined range.
• For example – wild kidney beans. 

• Short – long day plants (S-LDP)

• These are usually long day plants.
• S-LDP requires shorter photoperiods for initiating flowering and long photoperiods for blossoming.
• They flower between spring and summer.
• For example – rye. 

• Long – short day plants (L-SDP)

• These are typically short day plants.
• Contrary to S-LDP, these require short days to blossom and long photoperiods for floral initiation.
• L-SDP flower between summer and autumn.
• For example – bryophyllum. 

Importance of photoperiodism in plants

Photoperiodism in plants plays a significant role in hybridisation experiments, cross-breeding, and more.

• Horticulturists use photoperiodism in plants for obtaining a high yield of tubers and rhizomes.
• Photoperiodism helps in determining the flowering season of different plants.
• It encourages cross-breeding between flowers that grow in different seasons.
• Any flower can be grown throughout the year under greenhouse conditions by giving suitable photoperiod.

In a nutshell, flowering depends on how the plant responds to light. So far, we have read about photoperiodism in plants. There is yet another way plants react to light, as explained below.

Phototropism

It is a directional response of plants to light that allows them to grow either towards or away from the light source.

• Positive phototropism involves plant growth towards the light source. Shoots or parts of the plant above the ground usually display positive phototropism.
• On the contrary, the roots of the plant exhibit negative phototropism. They tend to grow away from the source of light.

Both photoperiodism and phototropism in plants orchestrate seasonal activities – growth, development, reproduction, migration, dormancy. All these contribute to reproductive success and survival.

Photoperiodism in animals

Though photoperiodism affects the colour of fur, sexual behaviours of animals, it is not a dominating phenomenon in the animal world. Mostly, flowering plants (angiosperms) show photoperiodism.

Conclusion

Growth is one of the most noticeable phenomena in plants. It is affected by various factors such as temperature, humidity, light, nutrition, and more. Some plants flower only when exposed to the critical photoperiod. There are two ways plants respond to light – photoperiodism and phototropism.

Depending on the duration of light the plants require for flowering, they are classified into different categories. Phototropism in plants allows botanists/horticulturists to cross-breed plants that grow during different seasons.