Respiration Operates during Photosynthesis

Mitochondria are involved in the metabolism of photo-synthesizing leaves. The glycine generated by photorespiration is oxidized to serine in the mitochondrion (see Chapter 8). At the same time, mitochondria in photosynthesizing tissue also carry out respiration via the citric acid cycle (often called dark respiration because it does not require light). Relative to the maximum rate of photosynthesis, dark respiration rates measured in green tissues are far slower, generally by a factor ranging from 6- to 20-fold. Given that rates of photorespiration can often reach 20 to 40% of the gross photosynthetic rate, citric acid cycle-mediated mitochondrial respiration operates at rates also well below the rate of photorespiration.

A question that has not been adequately answered is how much mitochondrial respiration (apart from the involvement of mitochondria in the photorespiratory carbon oxidation cycle) operates simultaneously with photosynthesis in illuminated green tissues. The activity of pyru-vate dehydrogenase, one of the ports of entry into the citric acid cycle, decreases in the light to 25% of the dark activity (Budde and Randall 1990). The overall rate of respiration decreases in the light, but the extent of the decrease remains uncertain at present. It is clear, however, that the mitochondrion is a major supplier of ATP to the cytosol even in illuminated leaves (Kromer 1995).

Another role of mitochondrial respiration during photosynthesis is to supply carbon metabolites for biosynthetic reactions—for example, by formation of 2-oxoglutarate needed for nitrogen assimilation. Leaf mitochondria typically have high capacities of nonphosphorylating pathways in the electron transport chain. By oxidizing NADH with lower ATP yield, mitochondria can maintain a higher 2-oxoglutarate production by the respiratory pathways without being restricted by the cytosolic demand for ATP (see Figures 11.7C and 11.12) (Hoefnagel et al. 1998; Noctor and Foyer 1998).

Additional evidence for the involvement of mitochon-drial respiration in photosynthesizing leaves has been obtained in studies with mitochondrial mutants defective in respiratory complexes, showing that leaf development and photosynthesis are negatively affected (Vedel et al. 1999).

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