Carbon dioxide is one of the important factor in photosynthesis. It helps photosynthesis to take place and it will increase its rate provided the Oxygen supply level at night remains constant.It would probably keep increasing up to 100 percent assuming the light was on. The plant probably needs O2 at night. They generally don't make greenhouses more than about 1000 ppm because the improvement isn't worth the cost of pumping in all that extra CO2. The amount of light that the plant absorbs limits the benefit so it won't increase in proportion to the amount of CO2 past a certain point. It wouldn't be much improvement past about 5000 ppm from what I have heard before.
