The Flux of Carbon through the Glycolate Pathway during Photosynthesis by Wheat Leaves

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Abstract
In wheat leaf segments carbon can enter the glycolate pathway, via the photosynthetic cycle, either as newly assimilated from CO2 in the atmosphere, from photorespired CO2 that has been refixed, or from recycling of intermediates within the pathway. Furthermore, several intermediates exist in multiple metabolic pools. All these factors make it difficult to determine directly the flux of carbon along the glycolate pathway during photosynthesis. Following 15 min photosynthesis in 14CO2 the active metabolic pools of glycine and serine approached saturation with 14C. If the gas mixture is then changed to one without 14CO2, in conditions where refixation and recycling are minimized, the loss of 14C from glycine gives an estimate of the flux of carbon through the glycolate pathway. Such measurements showed that in 150 vpm CO2 in air the carbon flow along the glycolate pathway exceeded the assimilation rate; with 325 vpm CO2 both rates were approximately equal. Such fluxes are more than adequate to account for CO2 production by photo respiration assuming the reaction in which glycine is converted to serine is the only one in which CO2 is evolved.