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Many angiosperms flower at about the same time every year. This occurs even though they may have started growing at different times. Their flowering is a response to the changing length of day and night as the season progresses. The phenomenon is called photoperiodism. It helps promote cross pollination.

In 1920 two employees of the U. S. Department of Agriculture, W. W. Garner and H. A. Allard, discovered a mutation in tobacco — a variety called Maryland Mammoth — that prevented the plant from flowering in the summer as normal tobacco plants do. Maryland Mammoth would not bloom until late December.

Experimenting with artificial lighting in winter and artificial darkening in summer, they found that Maryland Mammoth was affected by photoperiod. Because it would flower only when exposed to short periods of light, they called it a short-day plant. Some other short-day plants are Some plants such as flower only after exposure to long days and hence are called long-day plants.

Still other plants, e.g. the tomato, are day neutral; that is, flowering is not regulated by photoperiod.

Photoperiodism also explains why some plant species can be grown only in a certain latitude.

Photoperiodism in a Short-Day Plant

Experiments with the cocklebur have shown that the term short-day is something of a misnomer; what the cocklebur needs is a sufficiently long night.

These response are mediated by phytochrome.


The Hourglass Model

The behavior of phytochrome provided the first model — called the hourglass model — of the mechanism of photoperiodism in short-day plants.

The Circadian Rhythm Model

Recent work — mostly in the long-day plant, Arabidopsis — supports a different model of photoperiodism. This work suggests that the photoperiodic response is governed by the interaction of daylight with innate circadian rhythms of the plant.

Long-Day Plants

Arabidopsis is a long-day plant and has provided many clues about the mechanism involved in this photoperiodic response.

Any response to photoperiod requires a method of keeping time; that is, a clock. Plants, like so many other organisms, have an innate circadian rhythm that regulates the expression of many genes.

Among these in Arabidopsis is CONSTANS (CO), a gene that encodes a zinc-finger transcription factor whose levels of mRNA rise and fall with a circadian rhythm.

Translation of CONSTANS mRNA produces the transcription factor that turns on a number of genes, including FLOWERING LOCUS T (FT), a gene needed to start the conversion of apical buds into flower buds. [More]

CONSTANS messenger RNA (mRNA) However,

Now with the CONSTANS protein accumulating, it is available to turn on the gene transcription (e.g., FT) needed for the induction of the flowering.

In short days, with darkness falling before the rise in CONSTANS mRNA, there is not enough CONSTANS protein synthesized to induce flowering.

So flowering in Arabidopsis seems to require the interaction of

Short-Day Plants

The roles of circadian rhythms and light in short-day plants are not yet as well understood. Studies with rice, a short-day plant, suggests that the mechanism described for Arabidopsis may work there as well but with CONSTANS acting as a suppressor of FLOWERING LOCUS T and thus as an inhibitor of flowering under long days.


Photoperiodism not only controls flowering in some trees but also stops vegetative growth and promotes the setting of winter buds as the days grow shorter in the autumn. In aspens (Populus sp.) this control is mediated by CONSTANS and FLOWERING LOCUS T.

The phytochrome-mediated response to photoperiod occurs in the leaves. Flowering occurs in the apical meristems. A signal, called florigen, connects the two. Link to a discussion of florigen and the flowering response.

Phytochromes also mediate etiolation, the shade-avoidance response [Link to a discussion], and are involved in seed germination.

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17 August 2015