Accumulation of Sugars and Plasmalemma Alterations: Factors Related to the Lack of Cold Acclimation in Young Roots1

The possibility that low cellular concentration of sucrose was limiting the expression of hardiness in young roots of Pyracantha coccinea Roem. ‘Lalandii’ Dipp. was investigated. While the sucrose content of young roots increased four-fold following exposure to 4°C, the highest concentration was not higher than that found in non-acclimated mature roots. Attempts to increase hardiness by incubating young roots on sucrose solutions were unsuccessful. However, intracellular sucrose concentrations were not significantly increased by this treatment. Cytochrome oxidase incorporated into a membrane fraction containing plasmalemma vesicles isolated from Pyracantha young roots or from tissue capable of acclimation (Hedera helix L. ‘Thorndale’ callus cultures) was used as a probe for architectural alterations of this membrane following exposure to 4° and 5°. The apparent first order rate constant of the cytochrome oxidase reaction was used to indicate membrane fluidity. Above the Arrhenius discontinuity, membrane fluidity in both species was greatest when plants were grown at 4° or 5°. However, below the Arrhenius discontinuity fluidity remained greater in ivy callus grown at 5°, but not in Pyracantha young roots exposed to 4°. Altered properties of the membrane surface, inferred from the second order rate constant, were observed only in plasmalemma of young roots. Several possibilities to account for the lack of young root hardiness are presented.