The role of mitochondrial electron transport during photosynthetic induction. A study with barley (Hordeum vulgare) protoplasts incubated with rotenone and oligomycin

Mitochondrial contribution to photosynthetic metabolism during photosynthetic induction was investigated in protoplasts from barley leaves (Hordeum vulgare L. cv. Gunilla, Svalof) by using an inhibitor of mitochondrial Complex I (rotenone) and an inhibitor of the mitochondrial ATPase (oligomycin). Both inhibitors increased the lag phase of photosynthetic induction after the transition of protoplasts from darkness to light. This effect was not observed with broken protoplasts or isolated chloroplasts. Using the method of rapid fractionation of protoplasts it was shown that the delay in photosynthetic induction was accompanied by a decrease in ATP/ADP ratios of the cytosol and mitochondria, whereas the ratio in chloroplasts was not affected. A delay in activation of chloroplastidic NADP-dependent malate dehydrogenase (EC 1.1.1.82) was observed in the presence of either inhibitor. A delay was also observed in the rise of photochemical quenching of chlorophyll fluorescence in the presence of rotenone or oligomycin during photosynthetic induction. The results indicate that during the transition from dark to light the mitochondrial electron transport chain and its Complex I participate in the reoxidation of excessive redox equivalents from photosynthetic electron transport.