I'll have to consider vinegar (acetic acid) as another possible feedstock.

Using a bacteria that accepts electrical feeding, these guys are aiming for a sunlight-to-chemical-energy conversion process 30 times more efficient than leaves:

https://newscenter.lbl.gov/2015/04/16/major-advance-in-artificial-photosynthesis/

Of course, they don't need to use sunlight. S. ovata can be fed from any electrical power source. I'd like to play around with this myself.

This sort of thing being developed is why I think we should be making an effort to bypass the leaves of plants. Leaves are cheap, but not efficient (and far too sensitive to environmental conditions).

Acetic acid is very closely related to acetyl-CoA, so this is another thing, like pyruvate or ethanol, which plants might accept as a source of carbon and energy, to make sugar from.

One thing that interests me about acetic acid, which is largely unrelated to this project, is that you can easily get it to decompose into methane and CO2, with very simple equipment. So the pure stuff (called "glacial acetic acid", since it freezes around room temperature) can be used almost as liquid form of methane. Easier to store and handle than cryogenic LNG, anyway, although as a fuming acid, it's not entirely benign. Production of acetic acid (plus an oxygen store like a peroxide or nitrate) might make a good basis for the energy economy of a Mars mission, especially if it can be used for food production as well as fuel.

So far, my list of things to try is: sucrose, dextrose, corn starch, potato starch, tapioca starch, ethanol, vinegar, and pyruvate. Pyruvate's kind of a last resort for me now. It's available as a salt in supplements, but it seems expensive and likely a hassle to extract.

Back in the world of experiments, my onion plant's submerged tip is losing its color. I think that method 3 is going to be a failure. I was not expecting much from these crude methods, but I'd feel silly if I learned later that something like this was all I needed to do. I wonder about an aeroponic system, where the plant is turned sideways so its roots can be misted with fertilizer while its leaves are misted with sugar. I should at least give that a try.