MITOCHONDRIAL CONTRIBUTION TO PHOTOSYNTHETIC METABOLISM - A STUDY WITH BARLEY (HORDEUM-VULGARE L) LEAF PROTOPLASTS AT DIFFERENT LIGHT INTENSITIES AND CO2 CONCENTRATIONS

An oligomycin concentration that specifically inhibits oxidative Phosp horylation was added to isolated barley (Hordeum vulgare L.) leaf prot oplasts at various irradiances and carbon dioxide concentrations. At s aturating as well as low light intensities, photosynthetic oxygen evol ution was decreased as a result of the oligomycin treatment, whereas n o effect was observed at intermediate light intensities. This was the same for photorespiratory and nonphotorespiratory conditions. These re sults were confirmed by measurements of fluorescence quenching under t he 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 ra tio. Concomitantly, sucrose phosphate synthase activity was reduced. U nder high irradiances, this inhibition of sucrose synthesis by oligomy cin apparently caused a feedback inhibition on the Calvin cycle and th e photosynthetic activity. Under low irradiances, a feedback regulatio n compensated, indicating that light was more limiting than the activi ty of regulative enzymes. Thus, the importance of mitochondrial respir atory activity might be different in different metabolic situations. A t saturating light, the oxidation of excess photosynthetic redox equiv alents is required to sustain a high rate of photosynthesis. At low li ght, the supply of ATP to the cytosol might be required to support bio synthetic reactions.