X-ray diffraction evidence for decreased lipid fluidity in senescent membranes from cotyledons

Smooth microsomal membranes were isolated from bean cotyledons at various intervals after planting. Wide-angle x-ray diffraction patterns were recorded at room temperature from oriented specimens of the membranes. Patterns for membranes from 2-day-old cotyledons featured a broad lipid reflection centered about a Bragg spacing of 4.6 Å. This diffraction band derives from the apolar hydrocarbon chains of the membrane phospholipid, and its diffuse nature indicates that these hydrocarbons are in a disordered liquid–crystalline state. By day 4 in the germination sequence a faint sharp band superimposed on the broad lipid reflection was discernable at a Bragg spacing of 4.15 Å. This sharp reflection represents lipid in an ordered, crystalline state and derives from a close hexagonal packing of the hydrocarbon chains. During the later stages of germination the 4.15-Å reflection intensified to the point of becoming very pronounced in patterns for membranes from 9-day-old cotyledons. These patterns also displayed a sharp but weaker reflection at 3.75 Å, which derives from an orthorhombic packing of the hydrocarbon chains in crystalline lipid. By 9 days of age the cotyledons were extensively senescent and beginning to abscise.The data indicate that senescence of membranes involves a phase change whereby the proportion of crystalline to liquid– crystalline lipid progressively increases. This changed physical state of the lipid represents a substantial decrease in membrane fluidity and may well contribute to loss of membrane function in senescing membranes.