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PLANT PHYSIOLOGY , Vol 102, Issue 3 947-955, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Mitochondrial Contribution to Photosynthetic Metabolism (A Study with Barley (Hordeum vulgare L.) Leaf Protoplasts at Different Light Intensities and CO2 Concentrations)
S. Kromer, G. Malmberg and P. Gardestrom
Department of Plant Physiology, University of Umea, S-901 87 Umea, Sweden
An oligomycin concentration that specifically inhibits oxidative
phosphorylation was added to isolated barley (Hordeum vulgare L.) leaf
protoplasts at various irradiances and carbon dioxide concentrations. At
saturating as well as low light intensities, photosynthetic oxygen
evolution was decreased as a result of the oligomycin treatment, whereas no
effect was observed at intermediate light intensities. This was the same
for photorespiratory and nonphotorespiratory conditions. These results were
confirmed by measurements of fluorescence quenching under the same
conditions. Metabolite analysis in the presence of oligomycin revealed a
drastic decrease in the mitochondrial and cytosolic ATP/ADP ratios, whereas
there was little or no effect on the chloroplastic ratio. Concomitantly,
sucrose phosphate synthase activity was reduced. Under high irradiances,
this inhibition of sucrose synthesis by oligomycin apparently caused a
feedback inhibition on the Calvin cycle and the photosynthetic activity.
Under low irradiances, a feedback regulation compensated, indicating that
light was more limiting than the activity of regulative enzymes. Thus, the
importance of mitochondrial respiratory activity might be different in
different metabolic situations. At saturating light, the oxidation of
excess photosynthetic redox equivalents is required to sustain a high rate
of photosynthesis. At low light, the supply of ATP to the cytosol might be
required to support biosynthetic reactions.
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