Carbon Dioxide Compensation Point

The carbon dioxide compensation point is the specific atmospheric CO2 concentration, measured at a given (non-limiting) light intensity, at which a plant's rate of photosynthetic CO2 fixation exactly equals its combined rate of CO2 release from ordinary respiration and photorespiration together - so that there is, at that instant, zero net exchange of CO2 or oxygen between the plant and the surrounding air. For typical C3 plants, this balance point falls in the range of roughly 50 to 100 ppm atmospheric CO2. Because C4 plants suppress photorespiration almost entirely through the CO2-concentrating mechanism described in section 13.14, their compensation point sits far lower than this - close to zero - meaning a C4 plant can keep photosynthesising with a net positive carbon gain at CO2 concentrations so low that a C3 plant would already have reached its own compensation point and stopped gaining any net carbon at all. This difference in compensation point is one of the cl …