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In my alternate history (1800s), I have troop hiking through tropical sub-Saharan Africa. For plot reasons, I have man-killing mosquitos, but I also have a plant that exudes CO2 and other attractants. These flowering plants are placed in the room at night to attract the mosquitos away from people.

Evolution-wise, however, 'consuming' CO2 and giving off O2 is so deeply ingrained in a plant's DNA, it is hardly believable to be the other way around.

I'd easily hand-wave it away as some special scent, but the CO2 is critical to the plot. Can a plant evolve to give off CO2? Or rather, how difficult would that be to explain.

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    $\begingroup$ Man-killing mosquitoes? So, normal mosquitoes? $\endgroup$
    – Mike Kellogg
    Apr 7, 2016 at 21:05
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    $\begingroup$ Mushrooms (or any fungus) give off CO2 and consume Oxygen. If you can't do it with a plant, maybe some exotic mushroom could fill the gap. In a tropical env, I'd bet you'd find loads. $\endgroup$
    – coblr
    Apr 8, 2016 at 0:00
  • $\begingroup$ It would be more interesting to speculate about a plant permanently giving off more CO2 than producing O2. It would have to eat other organic material for that... so a carnivorous plant like sundew maybe. $\endgroup$
    – NoDataDumpNoContribution
    Nov 16, 2016 at 9:08

6 Answers 6

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Plants give off CO2 at night, when they start to respirate using the 'fuel' (glucose) synthesised by photosynthesis in the day. Check out the Calvin-Benson cycle

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    $\begingroup$ I remember this being called the Krebs cycle.... Possibly getting my biology mixed up. $\endgroup$
    – Matt
    Apr 7, 2016 at 15:09
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    $\begingroup$ @Matt is right and you are wrong; it is clear from your own link that Calvin-Benson does not produce CO2 (check that convert carbon dioxide and other compounds into glucose). Calvin-Benson is a part of photosyntesis that uses the energy captured from the sun (as ATP) to store it building glucose molecules. Krebs is the opposite, gets glucose and oxidizes it to extract energy as ATP molecules, and leaves H2O and CO2 as products. Also, Krebs is not restricted to photosyntentic organisms, at least animals (and probably all other organisms) use it for respiration. $\endgroup$
    – SJuan76
    Apr 7, 2016 at 15:50
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    $\begingroup$ The CO2 released when photosynthesis stops is significant in submerged plant life - that may be something for @costrom to take into account when creating the plants. Not only can the chemical changes damage the plants, they can also damage the other organisms in the water. And in some cases completely change the bacteria (aerobic v anaerobic). $\endgroup$
    – Raystafarian
    Apr 7, 2016 at 16:35
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    $\begingroup$ There are also plants (such as the snow plant: en.wikipedia.org/wiki/Sarcodes ) that have lost their chlorophyll, and exist as parasites on other plants. Thus they'd only give off CO2. $\endgroup$
    – jamesqf
    Apr 7, 2016 at 18:42
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    $\begingroup$ Actually plans always respire, emitting $CO_2$. However, during the day their $CO_2$ input exceeds their $CO_2$ output so the net effect is $CO_2$ consumption. $\endgroup$
    – Jim2B
    Apr 7, 2016 at 20:40
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All living things respire, and when they respire they break down 'fuel' to produce energy, which gives off CO2. A plant makes its source of fuel through collecting water, CO2 and light, the by-products of which are oxygen (output/waste) and glucose (the fuel). At night the plants can't collect light and instead they increase respiration, creating the energy needed for the night and the rest of the next day (stored in neat little packages called ATP).

Because of this, it seems you have no problem. Since you use the plants at night, they will naturally be giving off CO2, you only need to make a plant that respires heavily at night.

Of course, unless you want to hand-wave, you'll need a reason for this. Perhaps the simplest solution would be to have a fast-growing plant, maybe herbaceous. The rapid growth demands more energy, which means more CO2 intake during the day and therefore more CO2 output during the night.

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    $\begingroup$ Or they where bred by humans to grow mainly at night, possibly induced by something like sugar water being fed to it for hyper-growth. $\endgroup$
    – Yakk
    Apr 8, 2016 at 19:10
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    $\begingroup$ “Fast growing” implies that the plant needs plenty of carbon to build sugars, which in turns implies large consumption of CO2 during the day. I don’t see why this would imply large rejection of CO2, however, it’s not in the plant best interest to shrink at night. $\endgroup$
    – Édouard
    Apr 10, 2016 at 12:37
  • $\begingroup$ A simple answer is that the canopy is very high and thick and the air at ground level is very still. The O_2 simply trickles down in the day and gets consumed through the night. CO_2 falls below and blankets the area. $\endgroup$
    – The Nate
    Apr 10, 2016 at 18:06
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As has been pointed out, plants can give off CO2 at night. If you pick a plant that only blooms at night, you'd have a plant that is using up much more energy at night, thus producing more CO2, and the scent of the flower could add to the insect repellent function. My mother used to grow these orchids in our home (back in my home-country) that would only bloom once every few weeks, and only at midnight and the bloom would be wilted by morning. They were so beautiful, when my mom knew they would be blooming, we'd stay up to see them.

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Follow the side effect. When things evolve, it's because some random side effect proved increasingly beneficial. If this plant of yours found these deadly mosquitoes to be a good source of food, it could have evolved to attract and "fumigate" the mosquitoes, using their dead carcasses as an alternative source of energy, making the plant less dependent on photosynthesis, causing the plant to give off increased levels of CO2, which further served to attract more mosquitoes...

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  • $\begingroup$ but mosquitoes come out in the evening, at dusk, so after that, at night time, the plant would be transforming their bodies into energy, so it would be more like the plant's regular process during the day, not producing CO2, wouldn't it? I am no biologist, so I could be totally off here. $\endgroup$
    – user11864
    Apr 7, 2016 at 19:00
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You might also look into photorespiration, an error in photosynthesis which causes a plant to produce CO2 instead of O2. This only occurs when CO2 supply to the plants is low, which might happen when the stomata are closed to prevent water loss. Research suggests that photorespiration might have to do with nitrogen assimilation as well, meaning a plant in a nitrogen-poor environment might be more likely to photorespire.

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Can a plant evolve to give off CO2?

Look at the definition of the plant. The plant is a creature that grows by $H_2O + CO_2 +light \to C_nH_m + O_2$. The $C_nH_m$ part, called biomass, hydrocarbons, glucose or oil in plain English, is what constituates the body of the plant. The only way you can produce $CO_2$ is to turn the reaction backwards. But, it will destroy the plant since the opposite reaction, $C_nH_m + O_2 \to H_2O + CO_2 +heat$, is known as "burning" or "decay". This is what mushrooms and animals do: they decay the biomass and use the sun energy to move around.

That is, white can become black but it won't be white after that "transformation". Ok? Plant, which produces CO2 denies its "plant feature".

I also have a plant that exudes CO2 and other attractants. These flowering plants are placed in the room at night to attract the mosquitos away from people.

The "plants" that exhale CO2 are called "mushrooms" or "animals". Mosquitoes hunt for the latter, I suppose. You can easily use such animals as such "plants" to attract mosquitoes away from people (I am not sure why animals would be more attractive than human though).

Ok, diverting mascitos from human to animals may be a bad idea since maskitos will proliferate on the blood. You probably could have plants that grow up to certain size and then stop growing. They will accumulate CH during the day sunbath and release all the CO2+H2O they accumulated during the day at night. But you could just cut some plants and burn them at night by feeding to the animals and mushrooms, for instance.

Hope that now my point that plant that releases CO2 is nonsense is clear. You would write about this and literate other people (I guess that majority are as illiterate not to understand this basic fact of life) because it is really important for sustainability we are talking about when build our worlds. Being illiterate, we move into the false direction. That is a problem. We build the better world by having false notion of beauty.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. Valentin, I know that it can be frustrating when you get a lot of criticism, but please don't respond rudely. As Lightness Races in Orbit said, the Be Nice policy must be adhered to at all times. $\endgroup$
    – HDE 226868 ♦
    Apr 8, 2016 at 22:06
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    $\begingroup$ If you gave some justification for why the responses to your answer were incorrect, as you claim they are, that's one thing. But you didn't. You only stated, time and time again, that those who disagreed with you were wrong. Your position would be taken more seriously if you actually supported it. The way to react to criticism - even if you think it's "nonsense" - is to explain why you're correct, not to simply get mad at the commenters and insult them. If you're right, then they'll realize that quickly once you give your rationale, and that will be that. $\endgroup$
    – HDE 226868 ♦
    Apr 9, 2016 at 21:34

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