I think the issue isn't emissions in that case but a "amount of land space necessary for landfills for traditional plastic" vs. "amount of land space necessary for plant growth for bioplastic."
If the bioplastic growth land takes up more space by a certain amount than is used to dispose of regular plastic, that's presents its own set of issues.
If the bioplastic growth land takes up more space by a certain amount than is used to dispose of regular plastic, that's presents its own set of issues.