Effect of Oxygen and Carbon Dioxide on Photorespiratory Flux Determined from Glycine Accumulation in a Mutant of<i>Arabidopsis thaliana</i>

Exposure to atmospheric conditions which promote photorespiration strongly inhibits photosynthesis in a mutant of &lt;it&gt;Arabidopsis&lt;/it&gt; 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 CO&lt;inf&gt;2&lt;/inf&gt; 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 CO&lt;inf&gt;2&lt;/inf&gt;, the dilution of radioactive label by endogenous metabolic pools, or non-specific effects of metabolic inhibitors. At the standard atmospheric concentration of CO&lt;inf&gt;2&lt;/inf&gt;, the rate of glycine accumulation in the mutant was proportional to the oxygen concentration, amounting to 53&amp;percnt; of the rate of gross CO&lt;inf&gt;2&lt;/inf&gt;-fixation at 21&amp;percnt; O&lt;inf&gt;2&lt;/inf&gt;. At normal levels of O&lt;inf&gt;2&lt;/inf&gt;, glycine accumulation was maximal at about 475 μl CO&lt;inf&gt;2&lt;/inf&gt;1−1 and was reduced at higher or lower CO&lt;inf&gt;2&lt;/inf&gt; concentrations, being almost abolished at 3000μ1 CO&lt;inf&gt;2&lt;/inf&gt;1−1. These observations are discussed in the context of a model of photorespiration based on the properties of ribulose 1, 5-bisphosphate carboxylase&amp;sol;oxygenase, and in relation to the results of previous attempts to measure photorespiration. Preliminary evidence from 14CO&lt;inf&gt;2&lt;/inf&gt;-labelling experiments which suggests a non-photorespiratory pathway of serine synthesis is also presented.