# Plant growth and photoperiod



## scottward (May 12, 2008)

Hi,

Theoretically, assuming a constant supply of CO2, lighting and nutrients in sufficient quantity, will plants continue to grow at a steady rate?

I have been thinking of a 'hypothetical aquarium' concept whereby I connect an imaginary tank to an imaginary computer that can precisely control all aquarium parameters (light, CO2 level, temperature, nutrients etc).

If we were to fill this 'hypothetical aquarium' of mine with a fast growing plant, let's say some Ambulia, and, using the computer, 'lock in' a balanced CO2, lighting and nutrient selection (that would be precisely maintained in an 'auto-pilot' style fashion via the computer), would the plant grow continiously? Or would the plant grow a bit, stop for a while, grow a bit more etc etc??? If there was nothing ever inhibiting growth, I can't see any reason why a plant would not just continously grow? Do plants need to 'sleep', so to speak? And, if so, would the plant get to a point where it would refuse to grow any more and hence give algae it's chance to bloom?

I have noticed that in my real aquarium, where I use a 12 hour photo period, the plant leaves start to close up towards the end of the photoperiod. I'm interpreting this, maybe incorrectly, as the plants saying 'I've had enough for now'. If this is true, I would guess that if the photo period was any longer, algae would start to get it's chance?

Scott.


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## NoSvOrAx (Nov 11, 2006)

You got it. They have to switch metabolic functions. Thats why when the lights are off the plants actually intake oxygen and expell co2 as they burn the sugars they made during the day.


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## scottward (May 12, 2008)

Ok, so you're saying that the plants do indeed *need* a 'sleep' period?


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## Brendan Redler (Jun 1, 2008)

scottward said:


> Ok, so you're saying that the plants do indeed *need* a 'sleep' period?


To grow the best, yes. I would imagine that plants in 100% light conditions would fair poorly.

Do a search on google for "photosynthesis+light+dark" and you can check it out. You might not understand all of the "bio-major" kind of stuff, but you'll pick up on enough of it.

Essentially it's this: plants make sugars and starch during the day (the so-called "light reactions"), and consume them at night (the so-called "dark reactions"). In actuality, the light reactions NEED light because that's the photosynthetic process we all know and love, but the dark reactions do _not_ require light and so operate in the light as well.

Summary: Plants will grow in 100% light conditions. Plants do best with a mix of light/dark conditions instead.


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## essabee (Oct 11, 2006)

I love plants and try to understand them. Plants are governed by their circadian clock. During the photo period, plants are engaged in photosynthesis, an action about which there are several misconceptions. 

Yes the plants do in fact use chlorophyll, a number of enzymes, water and carbon-dioxide to create simple sugars and liberate oxygen; but using the light, the minerals, and other elements present; the plant also synthesizes fats, proteins, starch and many other substances which include acids, alkaloids, enzymes, etc. etc. It’s quite a manufacturing unit over there. It also lives and respires all through the photo period and also in the darkness, releasing CO2.

By switching off the light (or keeping them on over 24 hours) you will not be able to break the circadian rhythm. You might cause it harm by providing it with stress, or helping its antagonists. Plants are active all through the 24 hours, its series of tasks of assimilation and growth takes much more time than the photo period. Giving them a 100% photo period will not give you any extra growth than removing the growth limiting factors with a normal photo period of 10-14 hours for tropical plants.


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## dwalstad (Apr 14, 2006)

It may depend on the plant. One scientific paper clearly showed that duckweed grew about 30-50% better when it was given continuous light than when it had a 18 hr light photoperiod and 6 hr of darkness. And night-time sugar metabolism is the way that some aquatic plant species (CAM plants) conserve CO2; it doesn't necessarily mean that aquatic plants need a nighttime period.

However, many plant species do have circadian rhythms and hormones that respond to light cues. Folding up the leaves for the Ambulia (and many water lilies) is one response. Whether these aquatic plants actually need a sleep period, I don't know. I think animals need sleep to maintain brain function, but do plants have a brain?


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## scottward (May 12, 2008)

It seems I have invoked an interesting discussion here - thanks everybody for the input.

I was hoping my question would have a simple answer, but, understandably, some things in life just aren't that simple.

While I think of it, I know that a certain type of marine algae (Caleurpa - sorry, not sure how to spell it), is kept under 24-7 lighting in the refugium of a marine aquarium. I guess this is still a plant, even thought it's actually an algae, I believe. So, I guess, there is one example of a plant that can funtion perfectly well with continuous lighting and does not need a 'sleep'.

Ditto for the duckweed. I like the results of scientific experiments under controlled conditions, I know that I can rely on this information as fact.

Is it possible that whether plants require a 'sleep' (i.e. extended darkness period) or not could depend on where they are located, in the wild, with respect to the equator? For a particular species of plant that is used to very long days, it may, just like the duckweed, grow even better under continuous illumination. There may also be a species of plant that is further from the equator and is used to a more extended darkness period? In the same way that human beings grow to be barrell-chested in high altitude environments where oxygen is harder for the human body to obtain (I think that's a fact?), could it simply be that some plants are biologically engineered for a certain light/dark period?

In any case, I'm going to stick with a 12 hours on/12 hours off for my planted aquarium lighting.

I too am just interested in plants and what makes them tick.

Without a doubt, plants are FAR more challenging than the fish! ;-)

Scott.


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## yum (Feb 11, 2008)

from what my saltwater friend tells me, the caleurpa is kept in 100% light to avoid the "life cycle" in which it dies and releases it's reproductive spore; at the same time it dumps all of it's stored nutrients and some other bad stuff and negatively impacts everything around it. by not giving it a dark cycle it freezes it in that moment of it's life cycle. interesting stuff.

it makes me think of what sort of alleopathy the freshwater plants exhibit.


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## Diana K (Dec 20, 2007)

I am not sure if tropical plants do this, but temperate plants respond not just to the fact of night and day, but also to day length. 
In the winter days are short. Any plants that are not dormant (and probably even the dormant ones) are aware of the short days that are also cold. 
As the days lengthen in the spring, even if is it still freezing cold the plants start to come out of dormancy, and grow. (Think of tapping Sugar Maples for the sap rising in the late winter) 
Some plants grow leaves in the spring, then start growing flowers when the day lengthens. Often this is triggered by day length vs. night length. 
Plants that go dormant in the fall are triggered by day/night length. 

In the light plants make certain growth regulating material. At night some of this material is broken down. 
If the material we are considering tells the plant 'make flowers' then here is what happens as Spring turns into Summer:
Relatively short days does not make a lot of 'make flowers' hormone, and the relatively longer nights breaks down whatever was formed. 
Days get longer as Summer comes, so the plant has more time to make more 'make flowers' substance, and the shorter nights do not break down as much of it. At some point (varies for each plant) the plant will start to flower.

The opposite may be true for other plants that flower in the Fall: They produce a 'Don't Flower Yet' substance in the light, and it is broken down in the dark. Think of what happens as long Summer days turn into short Fall days (and longer nights)


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## essabee (Oct 11, 2006)

dwalstad said:


> do plants have a brain?


If it is the tissue you speak of, I concede that plants don't have it, but if it is intelligence you mean, the intelligence to understand and judge the environment surrounding it, planning the growth pattern to suit it, plants demonstrate it.

I will leave this topic here for I fear the way man's arrogance make use of his brain.


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