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