Photosynthetic refixing of CO2 is responsible for the apparent disparity between mitochondrial respiration in the light and in the dark

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.