Glycolate synthesis in a C3 C4 and intermediate photosynthetic plant type

Excised leaves of a C₃-photosynthetic type, Hordeum vulgare,a C₄-type, Panicum miliaceum, and an intermediate-type, Panicummilioides, were allowed to take up through their cut ends a1 mM solution of butyl hydroxybutynoate (BHB), an irreversibleinactivator of glycolate oxidase. After 30 to 60 min in BHB,extractable glycolate oxidase activity could not be detectedin the distal quarter of the leaf blades. Following this pretreatment,recovery of ¹⁴C-glycolate from ¹⁴CO₂ incorporated in a 10 minperiod was nearly maximal for each of the three plant types.Labeled glycolate was 51% of the total ¹⁴CO₂ incorporated forthe C₃-species, 36% for the intermediate-species, and 27% forthe C₄-species Increased labeling of glycolate was compensatedfor primarily by decreased labeling of the neutral and basicfractions for the C₃ and intermediate-type species. In the C₄-type,label decreased primarily in the neutral and insoluble fractions,but increased in the basic fraction. A lower rate of glycolatesynthesis is indicative of a lower rate of photorespirationand consistent with a lower O₂/CO₂ ratio present in the bundle-sheathcells of C₄-plants. We conclude that both decreased glycolatesynthesis and the refixation of photorespiratory-released CO₂are important in maintaining a lower rate of photorespirationin C₄-plants compared to C₃ plants. Intermediate glycolate synthesisin Panicum milioldes is consistent with its intermediate levelof O₂ inhibition of photosynthesis and intermediate rate ofphotorespiration. (Received May 6, 1978; )