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The fact that some plants start to close up about an hour before the lights turn off should NOT be interpreted as evidence that they have stopped photosynthesizing or that they will only photosynthesize for a period that is one hour shorter than the photoperiod they are getting.

Lets say we have some plants on a 12 hour photoperiod. They start closing up after 11 hours. If you reduce the photoperiod to 11 hours, you will find that they will start closing up after 10 hours. If you increase the photoperiod to 16 hours, you will find that they will start closing at 15 hours.
 

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Is this just a theory or are you saying you have done this and seen that this was the effect?
No it is more than a theory of mine. I have seen that plants adjust to a 16 hour photoperiod and begin to close up about an hour before. Besides, it is a well known phenonemon. Plants are known to have various circadean (about a day) rhythms. If a plant is out of synchrony with the day-night cycle it will take a few days up to a week to get back in synchrony. If a plant is kept under constant conditions, 24 hours a day, constant light or constant darkness, the rhythm of opening and closing leaves will continue, but will gradually get out of synchronization.
 
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