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W. W. Garner and H. A. Allards work on Maryland Mammoth, a variety of tobacco and Glycine max established the fact that it was the length of the day, which controlled flowering in these plants.

Garner and Allard suggested the term photoperiodism to designate the response of organisms to the relative length of the day and night and photo period to designate the favourable length of day for each plant.

According to their photoperiods, they classified plants into three groups:

a) Short day plants

photoperiodism observed in long day short day plants

Photoperiodism - Long-day, short-day and day-neutral plants

b) Long day plants

c) day neutral plants

day length requirements for flowering in three categories of plants

The Day-length Requirements for Flowering in Three Categories of Plants


Short day plants (SDP)

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They flower when the day length is less than a certain critical length say 12 hrs. If these plants are exposed to day lengths in excess of this critical point, they continue growing vegetatively.

Examples: Nicotiana tabacum, soyabean, strawberry, oryza sativa, chenopodium, cocklebur and chrysanthemum.

Long Day Plants (LDP)

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Long day plants flower when the day length is greater than a certain critical length, say twelve hours.

Examples: Pisum sativum, Triticum aesativum, Beta vulgaris, Onion, carrot and spinach.

Day Neutral Plants

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These plants flower in all possible photoperiods, ranging from a few hours to 24 hours of uninterrupted light exposure. They are also called photo neutrals or intermediate plants.

Examples: Tomatoes, cucumbers, sunflower, dandelions and cotton.

The terms long day/short day plants are actually misnomers. Research has proved that when the long night period was interrupted by a brief exposure to light, the plants failed to flower. So the scientists concluded that what is essential for flowering is long and uninterrupted dark period rather than a short day length. A brief interruption of the dark period with light nullified the effect of long night.

So short day plants were called long night plants and long day plants were called short night plants.

effect of night interruption on flowering in a short day plant
Effect of Night (dark) Interruption on Flowering in a Short-day Plant

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