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Might seem like a stupid question at first, but let's think about this for a minute.

Obviously, the living tissues contain some water, but it seems negligible compared to the quantities that plants can seem to require.

Ask practically any child, or non-expert, "What do plants need?" And you'll probably get some variation of the same answer, "Sunlight and water", every now and then, you might even get "Carbon-dioxide".

Light is understandable, right?
Because, photosynthesis:

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Dihydrogen-monoxide (water) and carbon-dioxide (exhalation) are also required for oxygenic photosynthesis:

CO2 + H2O + photons→ [CH2O] + O2

Or more specifically:

6CO2 + 6H2O + photons→ [C6H12O6] + 6O2

But still, I feel like this doesn't really account for the entirety of water spent (even considering evaporation, drainage, etc). Otherwise (it seems), we could flood LED lit, hydroponic greenhouses with pure carbon-dioxide, and watch the plants grow, science-fiction style.

Am I missing something here, or am I just incorrect?

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    I think you are missing something here. Most organisms, including plants, exist of mostly water. From the top of my head 80%. The cell is mostly water (cytoplasm) surrounded by a membrane, most metabolic reaction occur in the cytoplasm (not all, some are membrane bound reactions). It is not only plants that need water, all life on Earth needs water. Even a cactus needs water, but has adapted so it can handle long periods of drought.
    – benn
    Commented Jun 17, 2018 at 9:02
  • You'd kill your plants with pure CO2. They do regular, aerobic, energy metabolism at night. That requires oxygen. Leave are full off pores, stomata, that let in CO2 and let oxygen and water out. Unless you have a CAM plant or C4, it cannot completely close those holes during the day. --en.wikipedia.org/wiki/Crassulacean_acid_metabolism -- en.wikipedia.org/wiki/C4_carbon_fixation
    – Wayfaring Stranger
    Commented Jun 17, 2018 at 14:26
  • Alright, I'm not a crazy person. Obviously, plants need water. I imagine all biological organisms do. I'm just trying to account for all of it.
    – voices
    Commented Jun 18, 2018 at 14:34
  • @WayfaringStranger Okay, so disable the CO2 at night, or hook it up to the light switch/timer. It's hypothetical. That is interesting though. They must produce more O2 during the day, than they consume at night?
    – voices
    Commented Jun 18, 2018 at 15:00
  • @tjt263 I doubt oxygen driven phosphorylation is completely shut down during the day, haven't read the paper on it though. You might get by with some oscillatory gas mix, high CO2 low O2 during day, lower CO2 higher O2 at night. The carbon fixing enzyme, Rubisco fixes both CO2 and oxygen. The Oxygen reaction is fairly useless for the plant, but it's hard to defeat the thermodynamics of the reaction (CAM or C4). So daytime CO2 enrichment would help, but I'd expect pH to get screwed up by 100%. That's bad. en.wikipedia.org/wiki/RuBisCO
    – Wayfaring Stranger
    Commented Jun 18, 2018 at 15:17

2 Answers 2

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You're definitely missing something; your information is incomplete. You've mentioned evaporation (from the soil, presumably) but not transpiration. Plants need water, it's the stuff of life - they need it for the process of transpiration and carbon exchange, they need it to keep plant tissues hydrated via the xylem and phloem, they need it for the transport of nutrients and without it, photosynthesis doesn't take place. Think of the xylem and phloem system as vaguely like our veins and arteries - if you deprived a human being of water and other fluids for a week, dehydration and death would quickly follow. And the same is true of plants; in fact, I would consider water as number one on the list of a plant's requirements, above all else, though obviously, some plants have adapted to store fluid so their requirements are lower (cacti for instance). Expecting plants to carry out all the processes of life without water is like expecting a non electric car to run without petrol (or gas, depending where you are). See here UCSB Science Line - Why do plants need water?, but there are loads of other links online giving more complicated scientific information on this subject.

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  • "Expecting plants to carry out all the processes of life without water is like.." I don't think anyone expects that. But it's just like you said, "I would consider water as number one on the list of a plant's requirements, above all else.." I think most people would agree. I guess, I'd like to determine all the processes that plants undergo that spend water, and how much of it goes to which processes.
    – voices
    Commented Jun 18, 2018 at 15:26
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During photosynthesis plants break down water to get the energy carrying molecule hydrogen. This is also the reason why plants produce oxygen. So long story short plants need water to perform photosynthesis to make food.

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