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According to some information, in the ancient past (at least some) animals grew larger due to a higher levels of oxygen in the atmosphere. So for example there is this study regarding dragonflies.

Can higher levels of atmospheric oxygen also lead to larger plants? For example, would it be possible to grow gigantic strawberries in a hermetically sealed greenhouse that maintains a higher atmospheric oxygen concentration?

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  • $\begingroup$ Remember that plants utilize carbon dioxide to grow, and give off oxygen as a byproduct. A higher partial pressure of oxygen in the environment may actually decrease plant growth by inhibiting respiration. $\endgroup$ – MattDMo Jan 13 '15 at 18:23
  • $\begingroup$ @MattDMo - respiration, if anything, is enhanced by increased O2 $\endgroup$ – AliceD Jan 13 '15 at 23:42
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Oxygen is good for animals because our basic metabolism is this:

High energy carbon molecules + O2 → energy + H2O + CO2

Plants do that too at night, but during the day, they mostly do this:

High energy photons + H2O + CO2 → High energy carbon molecules + O2

Rubisco, one of the most important enzymes in photosynthesis, can bind to O2, leading to less efficient photorespiration, instead of photosyntheis. Rubisco is often a bottleneck in photosynthesis, in large part for that reason. So high oxygen isn't helpful to plants. In fact, C4 plants and CAM plants have evolved more complicated physiology to minimize the effect of O2.

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    $\begingroup$ enzymes merely catalyze a reaction, thereby establishing equilibrium more swiftly then would be the case without. Therefore, Rubsico does not carry out a "wrong" chemical reaction, the equilibrium is just pushed to the other side $\endgroup$ – AliceD Jan 13 '15 at 23:42
  • $\begingroup$ The last parenthesis is not uncontroversial (also, bad in what way?) - I think you should back this up with something (or remove since it's not relevant to the question). $\endgroup$ – fileunderwater Jan 13 '15 at 23:49
  • $\begingroup$ @ChrisStronks The one posting the answer. $\endgroup$ – fileunderwater Jan 14 '15 at 8:21
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The already given answers are correct, but I think explaining basic metabolism does not answer the problem of how big organisms can grow.

The argument about metazoan body size and the growing athmospheric O2 levels is that this latter process makes the diffusion of oxygen through animal tissue faster (because of the growing partial pressure of O2). This, in turn, made it possible for metazoans, which at the time had no elaborate body structures and no developed respiratory apparatuses, to actually grow larger, since it was possible for cells further away from the surface of an organism to respirate as well. Or, to put it differently, cell masses could grow bigger without risking anoxy in deeper parts of the tissue.

The problem with plants is not a very easy one to tackle, and I don't think there's a clear answer to this based on current knowledge. One must experiment with this and see what happens.

My guess is that the response of a plant lineage to grown O2 levels would be very dependent on the exact habitat the plant lives in, since for an evolutionary change to happen fast, there must be a clear evolutionary-ecological adaptive value it provides, and that is very dependent on environmental givens. Also, in recent (current) plants, respiration is a solved problem, so I'm not sure a plant would benefit in any way from elevated O2 levels. It would surely affect the working of RuBisCO as mentioned. Maybe an immediate response would be the spread of C4 metabolism within plants.

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