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Metabolism and Enzymology

Effects of Glucose Feeding on Respiration and Photosynthesis in Photoautotrophic Dianthus caryophyllus Cells

Mass Spectrometric Determination of Gas Exchange

Département de Biologie, Service de Radioagronomie, C.E.N. Cadarache, 13108 St. Paul Lez Durance, Cedex, France

When glucose (20 millimolar) was added to photoautotrophic cell suspension cultures of Dianthus caryophyllus, there was during the first 10 hours an accumulation of carbohydrates and phosphorylated compounds. These biochemical changes were accompanied by a progressive decrease of net photosynthesis and a twofold increase of the dark respiratory rate. The rise of respiration was associated with a rise of fumarase and cytochrome c oxidase activities, two mitochondrial markers. Gas exchange of illuminated cells were performed with a mass spectrometry technique and clearly established that during the first hours of glucose feeding, the decrease of net photosynthesis was essentially due to an increase of respiration in light, whereas the photosynthetic processes (gross O₂ evolution and gross CO₂ fixation) were almost not affected. However, after 24 hours of experiment, O₂ evolution and CO₂ fixation started to decline in turn. While ribulose-1,5-bisphosphate carboxylase activity was little affected during the first 48 hours of the experiment, the maximal light-induced phosphoribulokinase activity dramatically decreased with time and represented after 48 hours only 30% of its initial activity. It is postulated that the decrease in phosphoribulokinase activity was at least partially responsible for the decrease of CO₂ fixation and the metabolic events involved in this regulation are discussed.