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

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.