Membrane fluidity changes during ethanol-induced transition from dormancy to germination in cucumber seeds

Exogenous application of ethanol to secondarily dormant, unimbibed cucumber (Cucumis sativus L.) seeds was found to break their dormancy quite effecti vely. The embryo in cucumber seed is enclosed in a multilayered perisperm-e ndosperm envelope and a testa. The force required to penetrate the perisper m-endosperm envelope was measured using an Instron universal testing machin e. A marked decrease in the penetration force was found to precede the spli tting of the perisperm-endosperm envelope, the first visible sign of germin ation in cucumber. The force could thus serve as a biophysical marker for g ermination before it was physically expressed. The plasma membrane isolated from ethanol-treated, water-imbibed seeds showed a significantly higher fl uidity, as determined by the fluorescence polarization technique, much befo re the weakening of the perisperm-endosperm envelope. In contrast, an incre ase in the fluidity in intracellular membranes in response to ethanol treat ment to seeds was more or less coincident with the splitting of the perispe rm-endosperm envelope. It thus appeared that the increase in plasma membran e fluidity might be highly relevant to the dormancy-breaking process. It is speculated that breaking of seed dormancy in cucumber by ethanol is due ei ther to a greater activity of membrane-bound enzymes or because of a greate r statistical probability of Pfr/Pfr-receptor interaction consequent to a h igher plasma membrane fluidity.