Photoperiodism: Definition, Types, Importance, Examples

Photoperiodism is an organism's physiological response to the length of a night or a dark period. It can be found in plants, animals, and fruiting. Photoperiodism refers to plants' developmental responses to the relative lengths of light and dark cycles. Nature is full of interesting surprises and facts. There are various phenomenons and processes involved in the growth of a plant. At the cellular level, growth is mostly due to a rise in the amount of protoplasm. Because protoplasm increase is difficult to quantify directly, it is frequently expressed as a quantity that is roughly proportionate to it. Plants on the basis of Photoperiodism are grouped into three categories, namely, short-day plants, long-day plants, and day-neutral plants.

Key Terms: Plants, Photoperiodism, Sunlight, Growth, Light, Angiosperms, Flowers, Photoreceptors, Vernalisation

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Photoperiodism in Plants

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Plant Growth and Development deals with various concepts and phenomenons that are responsible for the growth and development of plants. In the year 1880, Charles Darwin and his son Francis discovered and identified Photoperiodism for the first time. Below mentioned are the facts indicating photoperiodism in plants:

• Many flowering plants (angiosperms) detect seasonal changes in night length or photoperiod using photoreceptor proteins like phytochrome or cryptochrome, which they interpret as signals to flower.
• Another type of photoreceptor involved in photoperiodism is cryptochromes. Cryptochromes absorb UV-A and blue light. 
• Both cryptochrome and phytochrome are light-dependent, and the amount of cryptochrome varies according to the day's length. This indicates how both photoreceptors have a role in regulating the length of the day.
• In addition to flowering, photoperiodism in plants includes the growth of stems or roots during specific seasons as well as the loss of leaves. Artificial lights could be used to produce extra-long days. 

Read More: Photomorphogenesis

Photoperiodism in different types of plants is mentioned below:

• Photoperiodism in Long-day Plants: When the night length falls below their crucial photoperiod, long-day plants blossom, i.e, these plants flower when the days are longer as they require more than critical duration of light. As the days become longer, these plants often flower in late spring or early summer. These plants are also called ‘short-night’ plants. Examples are spinach, radish, hibiscus etc.

• Photoperiodism in Short-day Plants: When the night lengths exceed their crucial photoperiod, short-day plants blossom. They cannot flower if the evenings are short or if the plant is exposed to a pulse of artificial light for a few minutes throughout the night. Natural nocturnal light, such as moonlight or lightning, is not bright enough or lasts long enough to stop flowering. These are also called ‘Long Night Plants’. Examples are soybean, tobacco, chrysanthemum etc.

• Photoperiodism in Day-neutral Plants: Cucumbers, roses, tomatoes and Ruderalis (auto-flowering cannabis) are day-neutral plants that do not blossom based on photoperiodism. Instead, they may begin flowering when plants reach a given developmental stage or age, or in response to other environmental stimuli like vernalization (a period of low temperature).

Photoperiodism in Plants

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Photoperiodism in Animals

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Like plants, animals also show photoperiodism. Following are the details: 

• For many animals, knowing the season of the year is as important as knowing the length of the day. Photoperiod causes changes in the color of fur and feathers, migration, hibernation, sexual behavior and even the resizing of sexual organs in addition to temperature variations.
• The frequency with which birds like the canary sing is determined by the photoperiod. The male canary's testes expand in the spring as the photoperiod lengthens (more daylight). More androgens are released as the testes mature, and song frequency rises.
• Some mammals have strong seasonal patterns, although humans' seasonality is thought to be mostly due to evolutionary baggage.

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Things to Remember

• 'Photo' stands for 'light’ while 'period' stands for 'time.' As a result, photoperiodism is defined as the response of plants and animals to the length of day and night.
• Long-day plants are those that need to be exposed to more light (LDP). For instance, radish, spinach, sugar beet and so on.
• Plants that require less light are referred to as short-day plants (SDP). Sunflower, rice, soybeans, tobacco and other similar crops are examples.
• Plants that flower regardless of the duration of the day are known as day-neutral plants (DNP). Roses, tomatoes, cucumber and other day-neutral plants are examples.
• Photoperiodism is used by scientists all around the world to classify and analyse plants. 
• Plants that require a long day, such as chrysanthemums, hibiscus, petunias, and spinach, can never be found in areas where the day is shorter.

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