Seasonal Changes in the Structure and Function of Mitochondrial Membranes of Artichoke Tubers

Abstract

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...

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