get some ph 7 water and throw dry ice into it let disolve you will then have a solution with a ton of carbonic acid(co2 in solution) without any additives ph will be low probably. i use to have a freshwater plant aquarium sometimes i would give this to the aquarium at room temp the plants like it only do this with lights on though if there are fish you cant they will suffocate. anyways just thinking out load never used on terrestrial plants. besides how usefull is co2 at the root level im not sure i guess you could foliar spray?
it is true co2 begins to leave a solution as soon as it is no longer under pressure but it takes a little time. think of a beer going flat thats the carbonic acid converting back to co2 and leaving in the form of the bubbles you see in your glass. but it does take time depending on temp and movement of the solution. when sprayed it would probably dissipate in a few seconds. one way to test would be to spray soda water in your mouth if you still taste carbonation that tells you it is still in solution(if it doesnt taste flat there is still co2 in solution) and the plants would receive this i dont know to what benefit