Metabolism of glycolic acid-1-14C in barley leaves with or without added CO2

Abstract

Detached primary leaves of barley (Hordeum vulgare L. var. Dayton) were supplied with glycolate-1 -14C in light or darkness with or without CO2, and the radioactivity of soluble metabolites and evolved CO2 was measured at intervals. The results of these experiments suggest that exogenously supplied glycolate is metabolized by two distinct pathways in illuminated leaves. It may be converted to sucrose outside the chloroplasts, presumably via the glycolate pathway. Alternatively glycolate carbon, through some product of its metabolism, enters the chloroplasts and becomes incorporated into intermediates of the Calvin cycle, from which photorespired CO2 is derived. Endogenous glycolate, originating from Calvin cycle intermediates, may leave the chloroplasts and become a substrate for sucrose synthesis. Exchange of carbon therefore appears to take place between the Calvin cycle inside and the glycolate pathway outside the chloroplasts. This exchange gives a net flow of carbon out of the chloroplasts during photosynthetic CO2 fixation. However, at low external CO2 concentration the flow of carbon from the glycolate pathway into the Calvin cycle in chloroplasts is greatly enhanced, providing substrate for CO2 production. Exogenous glycolate apparently does not directly enter the site of CO2 production in light. However, it is converted to CO2 in darkness, without the participation of the Calvin cycle.