Respiration in the light measured by 12CO2 emission in 13CO2 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 (CO2)-C-12 from illuminated leaves exposed to air containing (CO2)-C-13 measures photorespiration and mitochondrial respiration in C-3 leaves. This method was used to measure the mitochondrial respiration in illuminated maize leaves. The (CO2)-C-12 emission was steady after 30 s, a time sufficient to label the CO2 leakage from bundle sheath cells with (CO2)-C-13, but not the mitochondrial respiration in the light. The emission was low (0.1-0.4 ppm or 0.2-0.4 mu mol m(-2) s(-1)) in a wide range of leaf temperatures and light intensities, but increased at light intensities below 200 mu mol m(-2) s(-1) and at temperatures above 42 degrees 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 (CO2)-C-12 in the light was inversely correlated with photosynthesis. This suggested that most of the respiratory CO2 was refixed by photosynthesis. The amount of refixed intercellular (CO2)-C-12 was calculated from gas-exchange parameters. It was 60 to 90% of the total (CO2)-C-12 in leaves illuminated and exposed to temperatures below 42 degreesC. In leaves with reduced photosynthesis because of exposure to higher temperatures or low light, the (CO2)-C-12 refixation decreased. The sum of refixed and emitted (CO2)-C-12 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 (CO2)-C-12 was lower than mitochondrial respiration in the dark, suggesting that the mitochondrial respiration may be inhibited in the light.