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Photoperiod  or  Daylength   [ Biology - Horticulture ]

Dictionary of botanic terminology
index of names

  • PHOTOPERIOD: The number of consecutive hours an organism is daily exposed to light in a 24-hour period. The Day/night cycle of light/dark.
  • PHOTOPERIODISM: The responses of plants to the relative length of the light and dark periods.

The daylength or photoperiod is the duration of daily exposure to light and dark periods in a 24-hour period and is used to describe the relationship between phenology and episodes of light. For example, a 12-hour photoperiod consists of 12 hours of light and 12 hours of darkness, whereas an 8-hour photoperiod consists of 8 hours of light and 16 hours of darkness. The duration of daylight is defined as the interval between sunrise and sunset. Under natural conditions, the duration of daylight varies over the earth's surface, and is dependent on season and latitude.

Daylength (natural or artificially manipulated) exerts profound effects on the growth and flowering of many plant species, and manipulation of daylength is essential for several greenhouse crops. Plants need to be able to detect the difference between one season and another in order to flower at the correct time of year. To determine whether it is the correct season for flowering or not, the plant has evolved a way to determine the length of day and to respond correctly.

There are at least two different response systems found in flowering plants. Some plants flower when the daylength exceeds a specific critical photoperiod...these are called long-day plants. Other species flower when the daylength is shorter than their specific critical photoperiod, these are called short-day plants.

Methods of photoperiod control. Photoperiod control is necessary for some greenhouse plants (e,g, Holiday cactus, and poinsettia.) There are two types of photoperiod control systems used in commercial greenhouses:

  1. Using incandescent, fluorescent, or high-intensity discharge  lamps during naturally short days  to create artificial long days.
  2. Using opaque black cloth (or similar material) during naturally long days in order to create artificial short days.





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