Abstract. Tightly coupled mitochondria have been prepared from a variety of plant sources: white potato (Solanum tuberosum), Jerusalem artichoke (Heliantus tuberosus), cauliflower buds (Brassica oleracea), and mung bean hypocotyls (Phaseolus aureus). Mitochondria with no appreciable coupling were also prepared from skunk cabbage spadices (Symplocarpus foetidus).Room temperature difference spectra show that these mitochondria are very similar in the qualitative and quantitative composition of their electron carriers. The For all kinds of mitochondria, the rates of oxidation of succinate are similar as well as the turnover of cytochrome oxidase sec'1), regardless of the metabolic activities of the tissues. The number of mitochondria per cell appears to be the controlling factor of the intensity of tissue respiration.Higher plant tilssues are characterized by a grea'ter number of components in the mitochondrial respiratory chain than i,s the case wi'th animal tissues (6,7,37,44 (20). However, necessary adjustments had to be made to take into account the fact that the absorption peaks of plant electron carriers are not always the same as those of simitlar carriers of animail mi;tochondria.The identifi'cation of the various cytochromes present in plant mitochondria was best performed at liquid nitrogen temperature (77°K). The oxidized and reduced samples were placed in a 3 mm light path cuvette and cooled in a Dewar flask filled with liquid nitrogen during the scanning of the spectrum (6, 21). Low temperature brings a higher resolution as wdlHl as an enhancement of the absorption peak. By scanning the same sample, first at room temperature and then at low temperature, it is possible to measure the magnitude of the low temperature en'hancement and, by using the same extinction coefficients as before (20) Figure 1 shows a series of room tempera,ture difference spectra obtained with Jerusa,lem artichoke mitoohondria. The tipper spectrum shows a clear identification of the cytochromes after reduotion by dithionite. The a!bsorption peak of cytochrome oxidase (cytochrome a + cytoohrome a3) is located at 602 mju, a few mjm closer to the blue region of the spectrum than cytoc-hrome oxidase from animal mitochondria whose a-band is generally found at 605 mu (52). T.he a-band
Bean plants (Phaseolus vulgaris) were grown for 16–20 days with or without phosphate in Knop nutrient medium. It was found in previous experiments that for roots grown on a Pi‐deficient medium respiration is mainly carried out by the cyanide‐insensitive pathway. Mitochondria isolated from—Pi, roots had poor respiratory control and their respiration exhibited 62% inhibition by cyanide and was inhibited (30%) by salicylhydroxamic acid (SHAM). In contrast, mitochondria obtained with control (+Pi) roots had respiratory control and ADP/O ratios typical for succinate as the substrate; their respiration was inhibited to 95% by cyanide and insensitive to SHAM. The integrity of mitochondrial membranes was similar in both types of mitochondria. Cytochrome oxidase activity, however, was about 20% lower in ‐Pi mitochondria, but the cytochrome composition was the same in both types of mitochondria. The cytochrorae pathway was not operating at full capacity in mitochondria isolated from—Pi roots but the alternative oxidation pathway participated in a great part in mitochondrial respiration, similar to in vivo whole roots. The participation of the non‐phosphorylating., alternative pathway decreased the respiratory control ratio in mitochondria and had an effect on the total adenine nucleotide pool and energy charge values which were lower (16 and 13% respectively) in ‐Pi roots. About 50% lower ADP and 20% lower ATP levels were observed whereas AMP levels were several times higher.
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