BIOLOGICAL CLOCKS -PLANTS
=Photoperiodism= Flowering is a good example of an annual rhythm controlled by exogenous (external) factors. Manly plants only flower at a particular time of the year. This response to the changing day-length (photo-period) is known as photoperiodism. Plants can respond to changing day length in three different possible ways: * '''Long Day Plants''' (flower spring / summer) * '''Short Day Plants''' (flower autumn / winter) * '''Day-Neutral Plants''' (flower irrespective of season) ''Note that while many plants recognise seasonal changes by the changing photoperiod (day length), this is not always the sole determining factor. Many plants will not flower until they have been exposed to colder winter temperatures (this ensures that they do not flower just before winter begins). This process is known as '''vernalisation'''.'' =Critical Day Length= [image:http://i.imgur.com/QJayhZs.png?1] '''Long Day Plants''' only flower when the photoperiod (day) exceeds a critical day length. '''Short Day Plants''' only flower when the photoperiod is less than the critical day length. However, it has actually been discovered that it's the '''night length''', not the day length that controls flowering. The period of uninterrupted darkness (critical night length) is what actually controls flowering. Therefore... • Long Day Plants are actually Short Night Plants • Short Day Plants are actually Long Night Plants [image:http://i.imgur.com/SL0QlyK.png?1] A flash of light during the night may ‘trick’ a plant into responding as though it were a long day. The diagram left shows how a short day plant will only flower of the period of uninterrupted darkness exceeds the critical night length. Conversely, the long day plant only flowers when the period of uninterrupted darkness is less than the critical night length. [image:http://i.imgur.com/3QQMxub.png?1] Researchers soon also discovered that that interrupting the night with a flash of '''red''' light had the same effect as normal white light. However, it was also found that '''far-red''' light (not to be confused with infrared) had the same effect as an extended period of uninterrupted darkness. This is how flower growers can control when their plants flower (and make sure they all flower on Valentines Day!). If growers want long day plants to flower, they simply interrupts the night with white or red light. If they want short day plants to flower, they place the flowers under far-red light. If a grower mistakenly interrupted a long night with white or red light, the long night could effectively be restored simply by placing the plants under a far-red light source.
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