Oh wow you just set an alarm off in my brain.

I was reading some article the other day about greenhouse growing of spinach and strawberries and some other things. In all cases they were able to significantly reduce the amount of light required by increasing the concentration of CO2 in the air. This is because the last part of photosynthesis, which takes CO2 from the air and converts it to sugars will also take in Oxygen and use it for photorespiration. Photorespiration allows the plant to sustain itself but doesn't really let it grow. In normal air a molecule of oxygen is used for every molecule of CO2 but the rate of CO2 can be increased by increasing its concentration in the air. This is important because as of now, it is more expensive to provide supplemental lighting (both the initial and electrical) than it is to fill a growing chamber with additional CO2.