Plant Growth and Development


Have you ever wondered why certain flowers bloom only in certain seasons? Why do we get certain fruits in one particular season and not the others? This is because seasonal changes involve changes in the length of day and night and some plants need a certain amount of light to flower. This is photoperiodism. Photoperiodism applies not only to plants but animals too! Let’s try and understand this concept in more detail.

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‘Photo’ means ‘light’ and ‘period’ means ‘length of time’. Therefore, by definition, photoperiodism is the reaction of plants and animals to the length of day and night.

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Photoperiodism in Plants:

Most flowering plants have the ability to sense changes in season (i.e. the length of day and night) and flower at the right time. To do this, they make use of photoreceptor (light-sensitive) proteins called ‘phytochrome’.

Plants need exposure to light for a ‘critical duration’. This duration is different for different plants. Based on this critical duration, plants can fall into the following three categories:

Long Day Plants (LDP)

hibiscus photoperiodism

  • These plants flower when the days are longer.
  • They require more than the critical duration of light to flower (usually 14-16 hours).
  • The light period is very critical in LDP plants. Prolongation of the light period or a brief exposure to light during the dark period boosts flowering in these plants.
  • One usually does not find LDP plants in places where the length of a day is too short.
  • They are also called ‘Short Night Plants’.
  • Examples – spinach, radish, hibiscus etc.

Short Day Plants (SDP)

  • These plants flower when the days are shorter.
  • They need less than the critical duration of light (about 8-10 hours) and a continuous dark period (about 14-16 hours) to flower.
  • The dark period is very critical for SDP plants and has to be continuous. These plants will not flower if the dark period is briefly interrupted by light.
  • SDP plants are usually not found in places where the length of a day is too long.
  • They are also called ‘Long Night Plants’.
  • Examples – soybean, tobacco, chrysanthemum etc.

Day Neutral Plants (DNP)

  • These plants do not follow this restriction of critical duration.
  • In other words, they are ‘neutral’ to the length of day or night.
  • Examples – tomatoes, pea plants, rose etc.

Did you know that scientists use the concept of photoperiodism to classify plants and to identify their location? As mentioned earlier, photoperiodism exists in animals too.

Photoperiodism in Animals

Depending on the length of the day, animals also show behavioural and biological changes. Day length affects their fur colour, migration, hibernation and also sexual behaviour. For example, the singing frequency of the canary bird depends on the length of the day.

Solved Example for you

Question: What are the plants that flower during seasons with long days called?

  1. Long Night Plants
  2. Short Day Plants
  3. Long Day Plants
  4. Short Night Plants

Solution: Answer is ‘”c” and “d’. Long days also means shorter nights. Therefore, these plants are called Long Day Plants or Short Night Plants.

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