Sequential Changes in Lipid Fluidity and Phase Properties of Microsomal Membranes from Senescing Rose Petals

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Abstract
Cytoplasmic membrane deterioration during petal senescence of cut rose flowers entails a sequence of interrelated physical changes in the lipid bilayer. Wide angle X-ray diffraction revealed that microsomal membranes from flowers at the tight bud stage (day 1) contained liquid-crystalline lipid and only traces of gel phase lipid. However, with advancing senescence, the proportion of gel phase lipid increased, particularly between days 6 and 9. The lipid phase transition temperature also increased during senescence from 38 °C at day 1 to 48 °C at day 9. An order parameter (S), reflecting molecular order close to the membrane surface and determined by electron spin resonance (ESR) of microsomal membranes labelled with 5-doxyl stearate, increased during the early stages of senescence between days 1 and 3. The rotational correlation time (τc), a motion parameter obtained from microsomal membranes labelled with 16-doxyl stearate, increased throughout senescence, reflecting a rise in lipid microviscosity deep in the hydrophobic core of the membrane bilayer. Activation energies, calculated from continuously linear Arrhenius plots of rc and reflecting the degree of molecular order in the hydrophobic core of the membrane, increased during the later stages of senescence between days 6 and 9. The physiological implications of these physical changes in the lipid bilayer are discussed.