Photoperiodism may be defined as the reaction of plants and animals to the length of the day and the night. Some plants need to be exposed to sunlight for a particular duration of time to induce flowering. It is the reason behind the seasonal blooming of flowers.

For example, tulips flower in spring and lilies flower in winter.

Certain animals also depend on the day length for a variety of their characteristics.

Photoperiodism in Plants

Photoperiodism in Plants

Most angiosperms (flowering plants) use photoperiodism to determine when to flower. To do that they use one of the photoreceptor proteins present in their bodies, such as cryptochrome or phytochrome.

There is a well defined critical duration. This critical duration is different for different plants. Based on this duration, plants can be categorized into three categories:

  • The plants which require the more exposure to light than its duration are called long day plants (LDP). For example – Radish, Spinach, Sugar beet etc.
  • Those plants which require a less exposure to light than its duration are called short-day plants (SDP).For example-  Sunflower, Rice, Soybeans, Tobacco, etc.
  • Plants in which the flowering occur irrespective of the day lengths are called as day-neutral plants (DNP). For example- Pea plants, Sunflower, Tomatoes,  etc.

Scientists worldwide use photoperiodism for the classification and location analysis of plants. Plants which are long day plants, such as chrysanthemums, hibiscus, petunias, and spinach, can never be found where the length of the day is less. Similarly, short-day plants, such as cotton, spinach, rice, and sugarcane are not typically found where the length of the night is less.

Research and investigation have shown that darkness is more important to short-day plants than light day plants. That is to say that long day plants are more likely to flower even if their period in the light is interrupted by darkness for a certain amount of time than short-day plants if their period in darkness is interrupted by sudden bright light.

Day-neutral plants, on the other hand, do not let light or darkness affect the timing of their flowering. Instead, their flowering is based on other factors such as age or some external stimulus. Rose, tomato, and cucumber may be cited as examples.

Photoperiodism in animals

Many animals, especially those living at higher latitudes, use photoperiodism to adjust themselves to the seasonally behavioral and developmental strategies. The phenomena are not as drastic and evident in animals as much as it is in plants but certain animals do respond to the time of the year for certain behaviors.

For example, seasonal breeding in many animals, birds sing more frequently when the days are longer, the diapause in the insects, etc.

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Winter varieties are usually planted in: