Photorespiration during C4 photosynthesis

1. This paper describes aspects of the synthesis and metabolism of glycolate as factors controlling the apparent absence of CO2 evolution in the light (photorespiration) in the leaves of plants which photosynthesise via the C4 dicarboxylic acid pathway (C4 plants). The dicotyledon, Atriplex spongiosa and the monocotyledon Sorghum bicolor were chosen as examples of C4 plants. Atriplex hastata, which fixes CO2via reactions of the Calvin cycle (C3 plant) was used as a reference species. 2. Tracer experiments and enzymic data suggest glycolate synthesis may be quantitatively similar in all three species and is associated with Calvin cycle photosynthesis in bundle sheath cells of the C4 species. 3. Although photosynthetic enzymes showed comparable activity in the bundle sheath of C4 leaves and in Atriplex hastata, the activity of most glycolate pathway enzymes was much reduced in C4 leaves. 4. Glycolate pathway enzymes associated with the peroxisome and serine transmethylase are largely confined to the bundle sheath cells in Atriplex spongiosa leaves. Tetrahydrofolate enzymes of C1 activation, and glycerate kinase, are principally associated with the mesophyll cells. 5. In C4 species, the CO2 release of photorespiration may be controlled in two ways. In Atriplex spongiosa CO2 release, and refixation via phosphoenolpyruvate carboxylase may occur in mesophyll cells. In Sorghum bicolor the activity of the glycolate pathway is so low that little CO2 release may occur.