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

Changes in the temperature response, fluidity, function and the acyl fatty acid composition, were determined for a mitochondria-rich membrane fraction from Jerusalem artichoke (Helianthus tuberosus L.) tubers during dormancy for a crop which matured in midsummer. The temperature of both the upper and lower limits of the membrane lipid transition decreased during dormancy from 26 C and 1 C to 4 C and -5 C, respectively. This was similar to the changes observed with crops maturing in late autumn. The order parameter of a spin label intercalated into the membrane lipids decreased from about 0.6 to 0.5 during dormancy and returned to the original value before sprouting, showing that membrane fluidity increased during dormancy. The activation energy of succinate oxidase of tuber mitochondria was generally high at middormancy when membrane lipids were more fluid and decreased as the membranes became more rigid at the end of dormancy. The fatty acid composition of the membrane lipids did not alter significantly during dormancy. The results indicate that neither decreasing day length nor low soil temperature during tuber maturation is essential for the initiation of the membrane changes necessary for tubers to avoid low temperature injury during dormancy. The increase in membrane fluidity during dormancy could not be accounted for by an increase in the proportion of unsaturated fatty acids in the membrane lipids.