A broad question, I know. But I’m wondering if it’s better for a plant to have light on it at all times, or is it better to give it a period of darkness every day so it can “sleep”.
Well, plants theoretically don’t expend any energy - although photosynthesis may take up energy, therefore it could be possible that they go into a sleeplike state when night falls. This could be observed by outward appearances in some plants, eg. many flowering plants close their flowers at night.
All living things expend energy, but the function of sleep is not strictly to replenish energy. In animals, energy can be obtained only through digestion of food. I think the function of sleep in animals is to periodically shut down energy-hogging processes such as consciousness and locomotion, so that energy may be used for other things, including digestion but also growth, healing of wounds, etc.
I don’t know, but it’s possible plants have some kind of daily two-stage cycle that shifts their energy allocation in a similar way. But since plants are so different from animals, it probably wouldn’t make sense to call either half of the cycle “waking” or “sleeping.” The analogy just wouldn’t work well.
I think plants grow the best with periods of darkness. I’m not exactly clear as to why, although I think it has to do with their evolutionary history-certain lengths of darkness coorolated with time of the year and when the optimal time for seed dispersment/growth/etc should occur (my WAG at least).
Doesn’t the light/dark cycle trigger some plants to flower?
Au contraire! Plants most definitely expend energy. All that photosynthesis (and subsequent biochemical energy compounds, like starches and sugars) are not just altruistic gestures for our benefit. Plants burn sugar (expend energy and consume oxygen) both day and night, but during the day, the net production of sugar (the primary energy storage product) exceeds consumption, while at night energy production essentially ceases.
I should also note that plants have an additional use for sugar that we do not: cellulose is a polymer (chain) of glucose, the same “blood sugar” we humans use as an important internal fuel). In fact, it’s similar to glycogen, an important energy storage compound in your liver, which fills the gap between the exhaustion of free glucose reserves and the time it takes for lypolysis (fat-burning) to begin. The difference is that the glucoses in glycogen are ά-1,4 bonds (with occassional ά-1,4 side branches) while cellulose uses β-1,4 bonds, which we don’t happen to have an enzyme to break [Neither do cows or termites, but they have bacteria in their guts that do]. What’s with the alphas and beta’s? Well, it’s pretty simple: in cellulose, the β-1,4 bond goes up from one glucose through an oxygen into the bottom of the next glucose, producing a molecule that’s like a staircase; in glycogen, the ά-1,4 bond goes down into the oxygen, then back up tothe next glucose, producing a flatter chain. See - and you thought biochem was complicated!
What happens at night varies with the plant. For example, desert plants might take advantage of the damp cool of the night to grow, bloom or perform other tasks not well suited to the inhospitable dessicating sun of the day. Tropical plants, on the other hand, grow more during the day, because they have ample water, and can take full advantage of evaporation from the leaves to pull water (and dissolved nutrients) from the roots – a general technique used by trees and vascular plants (though desert plants have to be careful not to squander precious water), that’s why they don’t need a pump like out heart.
Control of water flow, concetration and pressure is also the force that allows trees to climb into the sky. Most Dopers know that many kinds of bamboo (a grass) can grow up to a foot a day – that’s mostly just water ‘inflating’ cells of thin cellulose; a similar effect can be seen in lawn grass, which can grow millimeters in the hours after a rain.
Water, of course, is not the only reason some plants are more metabolically active at night, while others prefer the day, but it’s perhaps the easiest to understand.
“Sleep” is a far more complex activity. Plants do not sleep in any proper sense of the word. Though some have reduced metabolic activity --and perhaps some even have very significantly reduced metabolic activity (I don’t know specific examples)-- at some points in the diurnal cycle, it’s not at all comparable to sleep.
In general, I’d guess that it’d be safest to alternate periods of light and dark unless you can find a specific reference to your specific plant. That’s the situation the plant evolved under. Continuous light can throw off biological or annual rhythms, for example. However, having said that, continuous light or dark is used to produce specific effects in specialized botanical applications. I recall some examples in orchids (many of which which grow in the thick shade of jungle canopies, and can be killed by direct sun) and vague mumblings about techniques of indoor culture of certain plants frowned upon by US authorities.
Yes. Poinsettia for instance, won’t flower if the light/dark cycle isn’t what it requires.
Quite a lot of plants are this way.
Could these bacteria be introduced into a human’s gut, allowing him to digest grass and wood chips? Or are they harmful to humans, or unable to survive in the human body?
Whoops! I’m missing a word there in my post - I meant to say that plants don’t expend any energy through exertion, at least in the way we know it - i.e. locomotion or muscle usage. Sorry for the confusion.
As noted, there are certain plant functions that (depending in some cases on genus/species) require a period of darkness.
There have also been studies showing that some ornamentals grow better with continuous light.
You don’t want to go there. Trust me. Without a highly specialized digestive cattle-like digestive system, we’d have to go the route of rabbits…they have grass-digesting bacteria in their upper intestine, but the grass isn’t digested enough on the first pass for the bacteria to go at it. I’m not willing to gain the ability to digest grass if the cost is having to eat my own feces, personally.
Thank you all, particularly KP. Quite enlightening.