Photoperiodism

• It is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals.
• Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods.
• Here it should be emphasized that photoperiodic effects relate directly to the timing of both the light and dark periods.

Types of Photoperiodic Plants .

a) Long-day plants

A long-day plant flowers when the day length exceeds their critical photoperiod. These plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer. Some long-day obligate plants are:

• Carnation (Dianthus)
• Henbane (Hyoscyamus)
• Oat (Avena)

Some long-day facultative plants are:

• Pea (Pisum sativum)
• Barley (Hordeum vulgare)
• Lettuce (Lactuca sativa)
• Wheat (Triticum aestivum, spring wheat cultivars)

—prepared by: Subodh Khanal

• Turnip (Brassica rapa)

b) Short day plant

Short-day plants flower when the day lengths are less than their critical photoperiod. They cannot flower under long days or if a pulse of artificial light is shone on the plant for several minutes during the middle of the night; they require a consolidated period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering.

Some short-day obligate plants are:

• Chrysanthemum
• Coffee
• Poinsettia

c) Day-neutral plants

Day-neutral plants, such as cucumbers, roses and tomatoes, do not initiate flowering based on photoperiodism at all; they flower regardless of the night length. They may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalisation (a period of low temperature), rather than in response to photoperiod.

Other classification

Short long day plants: these are long day plants but must be exposed to short day in early periods of growth for subsequent flowering. E.g.rye, wheat

Long short-day plants: these are short day plants but must be exposed to long day during early period for subsequent flowering e.g. Bryophyllum, night jasmine.

Intermediate plants (sterophotoperiods) : these plants can only flower within a definite range of light hours above and below where no blossoming occurs. E.g. some varieties of sugarcane

Amphiphotoperiodic plant: they behave just opposite to the intermediate plants where they remain vegetative in intermediate day length e.g. Mediaelgens.