Mass spectrometric determination of O2 gas exchange during a dark-to-light transition in higher-plant cells : Evidence for two individual O2-uptake components.

The exchange of O2 and CO2 by photoautotrophic cells of Euphorbia characias L. was measured using a mass-spectrometry technique. During a dark-tolight transition the O2 uptake rate was little affected whereas CO2 efflux was decreased by 40%. In order to differentiate eventual superimposed O2-uptake processes, the kinetics of O2 exchange resulting from brief illuminations were measured with a highly sensitive device. When the cells were exposed to a saturating light for short periods, the rate of O2 uptake passed through a series of transients: there was first a stimulation occurring 2-3 s after the appearance of O2 from water-splitting, followed 30 s later by an inhibition. These two transients were reduced 80% by 3-(3',4'-dichlorophenyl)1, 1-dimethylurea (DCMU), indicating that they relied on the linear transport of electrons in the chloroplasts. The first transient (stimulation of an O2 uptake) was little affected by mitochondrial inhibitors such as antimycin A and oligomycin or the uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP) but was increased in presence of KCN. When spaced flashes (2 us duration; 100-ms intervals) were used instead of continuous light, this transient was almost suppressed indicating that it was dependent on the saturation of some component of the chloroplastic chain. The second transient (inhibition of O2 uptake) was present when spaced flashes were used instead of continuous light. It was markedly decreased by addition of CCCP and mitochondrial inhibitors (antimycin A, oligomycin, KCN) which strongly indicates that it relied on mitochondrial respiration. It is concluded from these experiments that illumination of the cells resulted in an inhibition of mitochondrial respiration, but the resulting inhibition of O2 uptake was hidden by the appearance of an O2-uptake process of extramitochondrial origin, presumably located in the chloroplast.