Photoperiodism - Knowledgebase Question

Marathon, IA
Avatar for rojohn
Question by rojohn
December 13, 1997
When growing seedlings under lights and they are in a vegetative stage prior to setting them outdoors I have read that 14-16 hours of light are all they should receive as they need several hours of darkness as a "rest period". I have also read that keeping the lights on for 24 hours will make them grow more rapidly without being detrimental. Is there anything to this?

Answer from NGA
December 13, 1997
Photoperiodism is the response of plants to changing lengths of day and night. Plants actually measure the duration of darkness, rather than the hours of daylight. Plants differ in their day length requirements, because they each have a certain photoperiod requirement to help them accomplish the process of flowering. Of course, only mature plants will be able to flower and it takes time for leaves and stems to become mature enough that their systems can be stimulated by the hormones within.

So, the short answer to your question is simply this - if you force a plant to grow at an accelerated rate by keeping the lights on constantly, the plant will become large but not necessarily mature. In the case of seedlings, I've grown them under constantlight and haven't had any problems--I imagine it's because they're inside for a relatively short time.

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