Effect of Oxygen and Carbon Dioxide on Photorespiratory Flux Determined from Glycine Accumulation in a Mutant ofArabidopsis thaliana

Abstract

Exposure to atmospheric conditions which promote photorespiration strongly inhibits photosynthesis in a mutant of Arabidopsis lacking mitochondrial serine transhydroxymethylase activity, and glycine accumulates as a stable end-product of photorespiratory carbon and nitrogen flow. By providing exogenous serine and ammonia to leaves of the mutant, wild-type photosynthesis rates can be temporarily maintained in the absence of photorespiratory CO₂ evolution. In these circumstances, the rate of glycine accumulation provides a direct measure of photorespiratory flux which is not complicated by the efflux and refixation of photorespired CO₂, the dilution of radioactive label by endogenous metabolic pools, or non-specific effects of metabolic inhibitors. At the standard atmospheric concentration of CO₂, the rate of glycine accumulation in the mutant was proportional to the oxygen concentration, amounting to 53% of the rate of gross CO₂-fixation at 21% O₂. At normal levels of O₂, glycine accumulation was maximal at about 475 μl CO₂1⁻¹ and was reduced at higher or lower CO₂ concentrations, being almost abolished at 3000μ1 CO₂1⁻¹. These observations are discussed in the context of a model of photorespiration based on the properties of ribulose 1, 5-bisphosphate carboxylase/oxygenase, and in relation to the results of previous attempts to measure photorespiration. Preliminary evidence from ¹⁴CO₂-labelling experiments which suggests a non-photorespiratory pathway of serine synthesis is also presented.