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

Abstract

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.