Respiration in the light measured by 12CO 2 emission in 13CO 2 atmosphere in maize leaves

The mitochondrial respiration during photosynthesis is difficult to measure and is indirectly estimated mainly in C 3 plants. Loreto et al. [(1999) Australian Journal of Plant Physiology 26, 733–736] have shown that the emission of 12 CO 2 from illuminated leaves exposed to air containing 13 CO 2 measures photorespiration and mitochondrial respiration in C 3 leaves. This method was used to measure the mitochondrial respiration in illuminated maize leaves. The 12 CO 2 emission was steady after 30 s, a time sufficient to label the CO 2 leakage from bundle sheath cells with 13 CO 2 , but not the mitochondrial respiration in the light. The emission was low (0.1–0.4 ppm or 0.2–0.4 µmol m –2 s –1 ) in a wide range of leaf temperatures and light intensities, but increased at light intensities below 200 µmol m –2 s –1 and at temperatures above 42°C. At 120 s after labelling, the leaf was darkened and the emission rapidly matched the mitochondrial respiration measured by gas exchange. The emission of 12 CO 2 in the light was inversely correlated with photosynthesis. This suggested that most of the respiratory CO 2 was refixed by photosynthesis. The amount of refixed intercellular 12 CO 2 was calculated from gas-exchange parameters. It was 60 to 90% of the tota l12 CO 2 in leaves illuminated and exposed to temperatures below 42°C. In leaves with reduced photosynthesis because of exposure to higher temperatures or low light, the 12 CO 2 refixation decreased. The sum of refixed and emitted 12 CO 2 was close to the mitochondrial respiration in the dark. This suggested that in these leaves the mitochondrial respiration was not inhibited in the light. In salt- and water-stressed leaves, however, the sum of refixed and emitted 12 CO 2 was lower than mitochondrial respiration in the dark, suggesting that the mitochondrial respiration may be inhibited in the light.