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12CO2 emission from different metabolic pathways measured in illuminated and darkened C3 and C4 leaves at low, atmospheric and elevated CO2 concentration

12CO2 emission from different metabolic pathways measured in illuminated and darkened C3 and C4 leaves at low, atmospheric and elevated CO2 concentration


Abstract The detection of 12 CO 2 emission from leaves in air containing 13 CO 2 allows simple and fast determination of the CO 2 emitted by different sources, which are separated on the basis of their labelling velocity. This technique was exploited to investigate the controversial effect of CO 2 concentration on mitochondrial respiration. The 12 CO 2 emission was measured in illuminated and darkened leaves of one C 4 plant and three C 3 plants maintained at low (30–50 ppm), atmospheric (350–400 ppm) and elevated (700–800 ppm) CO 2 concentration. In C 3 leaves, the 12 CO 2 emission in the light ( R d ) was low at ambient CO 2 and was further quenched in elevated CO 2 , when it was often only 20–30% of the 12 CO 2 emission in the dark, interpreted as the mitochondrial respiration in the dark ( R n ). R n was also reduced in elevated CO 2 . At low CO 2 , R d was often 70–80% of R n , and a burst of 12 CO 2 was observed on darkening leaves of Mentha sativa and Phragmites australis after exposure for 4 min to 13 CO 2 in the light. The burst was partially removed at low oxygen and was never observed in C 4 leaves, suggesting that it may be caused by incomplete labelling of the photorespiratory pool at low CO 2 . This pool may be low in sclerophyllous leaves, as in Quercus ilex where no burst was observed. R d was inversely associated with photosynthesis, suggesting that the R d / R n ratio reflects the refixation of respiratory CO 2 by photosynthesizing leaves rather than the inhibition of mitochondrial respiration in the light, and that CO 2 produced by mitochondrial respiration in the light is mostly emitted at low CO 2 , and mostly refixed at elevated CO 2 . . In the leaves of the C 4 species Zea mays , the 12 CO 2 emission in the light also remained low at low CO 2 , suggesting efficient CO 2 refixation associated with sustained photosynthesis in non‐photorespiratory conditions. However, R n was inhibited in CO 2 ‐free air, and the velocity of 12 CO 2 emission after darkening was inversely associated with the CO 2 concentration. The emission may be modulated by the presence of post‐illumination CO 2 uptake deriving from temporary imbalance between C 3 and C 4 metabolism. These experiments suggest that this uptake lasts longer at low CO 2 and that the imbalance is persistent once it has been generated by exposure to low CO 2 .
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