On average they gain 32.6%, 40.9% and 46.8% additional dryweight mass, respectively, when grown in air containing an extra 300, 600, and 900 parts per million of CO2.
They would be converted into glucose (of which cellulose, which makes up much of the dryweight of a plant, is a polymer) and the glucose would be shipped around much as oil is today, for eventual conversion into biofuels and bioplastics.