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). [...]
[...] This interaction between FLC and the photoperiodic pathways provides a mechanism for plants such as winter annual or biennials to delay flowering until winter has passed (and vernalization has reduced the levels of FLC) and the warm weather of late spring or summer has arrived (when LDs will activate the photoperiodic- promotion pathway). Conclusion Genetic and molecular studies, largely with the model plant A. thaliana, over the last decade have gone a long way to confirm and explain the essential elements of photoperiodism as it applies to flowering in plants. The physiological conclusions that photoperiodic mechanisms involve multiple interactions between photoreceptors and an underlying circadian rhythm in light sensitivity through an external-coincidence model have been borne out. [...]
[...] Also, when leaves taken from an induced plant are grafted on to a receptor plant, promotion of flowering or other responses occurs only when a graft union has developed. It was proposed by Chailakhyan (1936) more than half a century ago that the signal passing between leaves and response sites is a specific flowering hormone: florigen. This idea was based on a series of experiments showing that grafting of leaves from one donor species to a separate receptor species could cause flowering. [...]
[...] and soybean, in which for a particular variety, flowering tended to occur at the same time in the field, irrespective of planting date. After experimentally eliminating temperature and light intensity as causal factors they concluded that the tobacco and soybean plants would only flower if the duration of the daylight period was sufficiently short. They introduced the terms photoperiod and photoperiodism and classified plants into the photoperiodic groups in use today. Short-day plants (SDPs) are those that flower or in which flowering is accelerated by days which are shorter than a critical day length. [...]
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