I used to think that animals - at at least, mammals - needed to sleep because their muscles and organs needed to relax (although I never knew what physiological changes occurred during sleep or why they were necessary). Now I think it is generally believed that we sleep because the brain needs some sort of "rest" - although again, I do not know why. Do plants require regular periods of darkness? If I put my tomato seedlings under lights 24/7, what will happen? And why? The wealth of knowledge among our group convinces me that some of you know the answers...
Circadian rhythm exists in plants as well as in animals. It derives from genetic instructions---if you left your plants in total darkness or light some of the behavior would still occur. These include leaf movement, fragrance discharge, opening and closing of stomata. However, the genetic messages or commands are facilitated by optimum growing conditions; i.e., the light and dark cycle in which the plant has evolved. Flowering is one event affected by light cycles, to which anyone who has grown, for example, cattleyas (long-day...? short-day...?) can attest. ---Why do leaves fall from deciduous trees when they do? Moisture levels, or lack thereof, play a part, but the big trigger is the abbreviation of sunlight. Back when I was a kid, I was told that plants took in CO2 during the day and emitted O2 at night. My personal opinion is that sleep is necessary for the brain to sort and examine whatever is milling about in the subconscious mind. Plants do not "sleep" as humans do, not having a human consciousness---but they do need to fulfill the needs of their genetically determined Circadian rhythms to reach their fullest and healthiest potential. So, I say: yes, plants do require periods of darkness.
You can grow enormous cabbages in Dawson City, supposedly because of the extended daylight hours. Note: that's daylight , not sunlight. Mid-summer there is no night. Carl
