The net photosynthetic rate (PN), the sample room CO₂ concentration (CO₂S) and the intercellular CO₂ concentration (Ci) in response to PAR, of C3 (wheat and bean) and C4 (maize and three-colored amaranth) plants were measured. Results showed that photorespiration (Rp) of wheat and bean could not occur at 2 % O₂. At 2 % O₂ and 0 µmol mol-1 CO₂, PN can be used to estimate the rate of mitochondrial respiration in the light (Rd). The Rd decreased with increasing PAR, and ranged between 3.20 and 2.09 µmol CO₂ m⁻² s⁻¹ in wheat. The trend was similar for bean (between 2.95 and 1.70 µmol CO₂ m⁻² s⁻¹), maize (between 2.27 and 0.62 µmol CO₂ m⁻² s⁻¹) and three-colored amaranth (between 1.37 and 0.49 µmol CO₂ m⁻² s⁻¹). The widely observed phenomenon of Rd being lower than Rn can be attributed to refixation, rather than light inhibition. For all plants tested, CO₂ recovery rates increased with increasing light intensity from 32 to 55 % (wheat), 29 to 59 % (bean), 54 to 87 % (maize) and 72 to 90 % (three-colored amaranth) at 50 and 2,000 µmol m⁻² s⁻¹, respectively.