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Physiology of Photoperiodism

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  1. Introduction.
  2. Discovery and variation
    1. The response of plants to the daily duration of light.
    2. The length of the day and night.
  3. Transmissible signals.
    1. A common feature of photoperiodism.
    2. The signal passing between leaves and response sites.
  4. Pathways to flowering in Arabidopsis.
    1. The predominance of the different pathways.
    2. Expression of the floral-integrator genes.
  5. Floral promotion pathways.
  6. Conclusion.

Photoperiodism can be defined as the response to changes in day length that enables plants (or any other living organism) to adapt to seasonal changes in their environment. Except at the equator, the passage of the year is marked by a continuous but highly reproducible variation in the length of the day. In order to locate the time of year accurately, a timekeeping mechanism operates with precision as part of the plant's photoperiodic sensing mechanism in a way that is insensitive to less predictable variations in the environment such as temperature. Photoperiod alone is not an unambiguous signal as any particular day-length occurs twice in an annual cycle. Progressive changes in day length, which are at their greatest around the equinoxes in spring and autumn, do, however, provide a certain environmental signal for the passage of the seasons. The seasonal range and rate of change of day length is lower in the tropics than at higher latitudes and photoperiodic mechanisms need to be sufficiently precise and flexible to operate across the entire range of day lengths.

[...] Transmissible signals A common feature of photoperiodism appears to be that day length perception is a separate process from the response to photoperiod. When either the leaves or the shoot tips of photo periodically sensitive plants are exposed to different day lengths, flowering depends on the day length given to the leaves and not to the apex (Knott, 1934). In several instances, leaves from plants, which have been given a day length treatment that initiates flowering have been grafted on to plants that have not been exposed to permissive day lengths, with the result being flowering in the receptor plants. [...]

[...] Floral promotion pathways Of the floral-promotion pathways, the photoperiodic pathway are probably the best understood. Key elements include a gene named CONSTANS, or CO, which encodes a nuclear protein with two zinc fingers at the amino terminus and a conserved carboxyl-terminal domain, known as the CCT domain for the three plant proteins in which it was identified COL, TIMING OF CAB1 (TOC1)). CO is regulated both transcriptional and post-transcriptional. Transcription of the CO mRNA is controlled by the circadian clock and it is also up regulated by the nuclear protein GIGANTEA (GI). [...]

[...] However, the level of complexity in these interactions is much greater than conceived in classical physiology (Fig Homologues of the central genetic elements of the model in the LDP Arabidopsis have been shown to affect photoperiodic regulation in SDPs such as rice and Pharbitis, implying a common photoperiodic mechanism for all plants. The role of a florigenic signal has also been confirmed and although its exact nature is still unknown, there are prospects of it being discovered in the foreseeable future. [...]

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