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The temperature limits of the order-disorder transition, and the Arrhenius activation energy of succinate oxidase activity for mitochondria of Jerusalem artichoke (Heianthus tuberosus L.) tubers were determined from the initiation to the termination of dormancy. The temperature limits for the transition at the initiation of dormancy were 25 and 3 C. These changed to 9 and -5 C at mid-dormancy and returned to 25 and 2 C at the termination of dormancy. The Arrhenius activation energy measured in the temperature range above the transition was 35 kiiojoules per mole at middormancy and decreased to 17 kilojoules per mole at the termination of dormancy when sprouting was evident. The coincidence of the changes in membrane structure and function with dormancy suggests that artichokes possess a mechanism for regulating membrane lipid structure so that celiular integrity of tuber tissue is maintained even when the tubers are exposed to low temperatures.The tubers of a number of plants undergo dormancy in winter. Low temperatures, such as 2 C experienced during storage of dormant tubers, are a prerequisite for the tubers to sprout in the spring (4). The Jerusalem artichoke (Helianthus tuberosus L.) grows and develops during summer and autumn in temperate climates and dies before the winter. Whereas the vegetative portion ofthe plant is sensitive to low temperatures the tuber can withstand long exposure to these temperatures. The tuber's requirement for low temperature can be replaced by a number of chemical compounds such as gibberellic acid, thiourea, and anesthetics (6) and some of these compounds are known to affect membrane structure and function (9) suggesting that regulation of dormancy may involve structural changes in cell membranes.Both morphology and function of tuber mitochondria change during either the storage of whole tubers (11,21) or the aging of tuber discs (1, 5). The possibility that these changes in tuber mitochondria might reflect alterations in the structure of membranes, prompted an examination of the molecular ordering of lipid components and the function of mitochondrial membranes of artichoke tubers from the initiation to the termination of dormancy.MATERIALS AND METHODS Preparation of Tubers. Artichokes (H. tuberosus L.) were grown in the field from October (spring) to May (autumn) in the 1975 to 1976 and 1976 to 1977 seasons. Harvested tubers were washed, dipped in a suspension ofa fungicide (Benlate, Dupont, Australia), dried and packed into foil-covered trays of dry Vermiculite. The trays were stored at 4 C. Preparation of Mitochondria. Tubers (500 g) were peeled and cooled and then homogenized in a commercialjuice extractor with 500 ml of isolating medium containing 500 mm mannitol, 2 mM HEPES, 10 mm KCI, 1 miM MgCl2, 10 mm KH2PO4, 0.5 mM ethyleneglycol-bis(fi-aminoethyl ether)N,N'-tetraacetic acid, 5.0 mM DTT and 0.5 mg/ml BSA, adjusted to pH 7.2 with KOH. The brei was centrifuged at 2,000g for 5 min and the resulting supernatant centrifuged again at 20,000g for 4 min. The sediment...

Long term feeding of acetate-2-"C, "'CO2, citrate-i , fumarate-2,3-"C, and succinate-2,3-"C to mung bean (Phaseolus aureus L. var. Mungo) leaves in the dark gave labeling predominantly [33] of the results of Barker [1]) and in the tricarboxylic acid cycle in bananas (28).The crossover theorem has been applied in the present study to the tricarboxylic acid cycle intermediates during dark/light/ dark transitions which are likely to result in changes in carbon flux. The main conclusions drawn from the analysis are that the tricarboxylic acid cycle in mung bean leaves undergoes a series of changes in control points at citrate synthase, fumarase, isocitrate and malate dehydrogenases during the transition from dark to light. The changes can be correlated with known lightinduced changes of the adenine nucleotides (34) and of the nicotinamide adenine dinucleotides (13,16,19,29,30). After the major initial changes in the light, the "C-labeled intermediates of the tricarboxylic acid cycle approximate to a new steady state in which it appears probable that the ratio of oxidizedreduced NAD continues to play an important role in the regulation of the cycle. On transition from light to dark the major apparent control point is at isocitrate dehydrogenase. MATERIALS AND METHODSThe growth of mung bean seedlings (Phaseolus aureuis L.var. Mungo) and the experimental and analytical procedures have been described in the previous paper (9). RESULTS

Excised green leaves of mung bean (Phaseolus aureus L. var. Mungo) were used to determine the effect of light on the rate of endogenous respiration via the tricarboxylic acid cycle. ILlumination with white light at an intensity of 0.043 gram calories cm'minn (approximately 8600 lux) of visible radiation (400-700 nm) gave a rate of apparent photosynthesis, measured as net C02 uptake, of 21 mg CO2 dm-2hr-' which was about 11-fold greater than the rate of dark respiration. The feeding of '4CO2 or 14C-labeled acids of the tricarboxylic acid cycle in the dark for 2 hours was established as a suitable method for labeling mitochondrial pools of cycle intermediates.At a concentration of 0.1 mM 3-(3,4-dichlorophenyl)-1,1-dimethylurea, apparent photosynthesis was inhibited 82%, and the refixation of 14CO2 derived internally from endogenous respiration was largely prevented. In the presence of this inhibitor endogenous respiration, measured as "CO2 evolution, continued in the light at a rate comparable to that in the dark. Consequently, under these conditions light-induced nonphotosynthetic processes have no significant effect on endogenous dark respiration. Inhibitors of the tricarboxylic acid cycle, malonate and fluoroacetate, were used to determine the relative rates of carbon flux through the cycle in the dark and in the light by measuring the rate of accumulation of 4(C in either succinate or citrate. Results were interpreted to indicate that the tricarboxylic acid cycle functions in the light at a rate similar to that in the dark except for a brief initial inhibition on transition from dark to light. Evidence was obtained that succinate dehydrogenase as well as aconitase, was inhibited in the presence of fluoroacetate.Controversy has surrounded the question of the effect of light on endogenous aerobic respiration via glycolysis and the tricarboxylic acid cycle in algae and green leaves of higher plants. Reports have included stimulation, inhibition, or no effect of light on respiration. For example, physiological studies, using the mass spectrometer to measure respiratory and photosynthetic gas exchange simultaneously, showed that relatively low light intensities had little or no effect on rate of respiratory oxygen uptake in barley leaves or in suspensions of the alga Chiorella (8). With the green alga Ankistrodesmus braunii (9) or the algal flagellate Ochromonas malhamensis (52) it was found that respiratory CO2 evolution was almost independent of light intensity, and that respiratory oxygen consumption was not affected by low light although it was enhanced at high light intensities. Hoch et al. (24) concluded from their results obtained with the mass spectrometer that low light intensities inhibited endogenous respiration in Anacystis, a blue-green alga, but not in Scenedesmus, a green alga, whereas higher light intensities promoted oxygen uptake in both organisms. The total oxygen consumption or CO2 evolution cannot be separated, however, into dark endogenous respiratory and light-induced photorespirator...