Well I have realized a big problem with the experiment. Due to conservation of mass whatever weight is gained is going to be gained from chemical reactions that turn the dirt, CO2 and water into plant. This measures mass so it will be unchanged! hahaha. I don't see how I missed that when looking at the idea. Also due to lost water from evaporation the experiment wold become flawed if it were uncovered. If you tried unplanting it and replanting it every test you would likely kill it or leave behind part of the root system.
nah, you're still good - if you've got your plant and pot, water etc all sealed up in a clear plastic bag, then you've got no mass entering or leaving the system, apart from... sunlight ! Energy/mass equivalence (Einstein and all that), so I think you should see it gain weight over time ? I'm thinking it may even gain/lose weight during the day depending how sunny it is if you can measure it that sensitively.
I'm not sure if sealing it would suffocate the plant though, don't they take in CO2 and expel oxygen, so eventually it runs out of CO2.