A pumpkin is not airtight. Like most living things, plants breathe in O2 and breathe out CO2 (in addition to photosynthesis which does the opposite). So the air inside a pumpkin is most likely atmospheric air with less O2 and more CO2, maybe other gasses that participate in the pumpkin metabolism.
> Like most living things, plants breathe in O2 and breathe out CO2 (in addition to photosynthesis which does the opposite).
At what "level" does this happen? Is it similar to an animal with a diaphragm centralizing the intake of oxygen and lungs centralizing distribution to every part of the animal, or more along the lines of cells that are exposed to the air taking in oxygen locally and using it for their own needs?
Plants absorb oxygen, moisture, etc through their stoma found in the stems, roots, and leaves directly into their cells. From there its processed into ATP via the mitochondria just like we would.
Many life forms on earth are anaerobic, powered by chemical reactions that do not involve O2. There are even species for which O2 is toxic. It most likely includes the very first lifeforms.
There are many definitions of life, some of them broad enough to include things like compute programs, there is a debate about viruses, but I don't know of any definition that requires breathing O2.
