Changes in Membrane Fluidity and Protein Composition during Release of Cucumber Seeds from Dormancy by a Higher Temperature Shift

Dormancy of seeds of cucumber (Cucumis sativus L.) was inducedby imbibing in -1.8 MPa polyethylene glycol 6000 (PEG) solutionand pulsing with far red light for 15 min prior to washing anddrying. When re-imbibed with water at 20 °C, dormancy wasbroken by raising the temperature to 30 °C for 6 h. Thistreatment was also effective when -0.9 MPa PEG was present duringre-imbibition and high temperature. Seeds with broken dormancywere found to germinate in water over a smaller temperaturerange than seeds in which dormancy had not been induced. Whenthe duration of the temperature shift to 30 °C was varied,germination percentage increased from 7 to 60% after 6 h, butlonger exposures up to 12 h had no further promoting effect.The time course of germination after transfer to water following6 h at 30 °C in PEG showed piercing of the perisperm-endospermenvelope after 9–12 h and radicle protusion after 12–15h. If PEG was retained after high temperature treatment no visiblegermination was observed. Thus, to study membrane fluidity andthe protein content associated with germination, seeds weresampled 9 h after high temperature treatment. To study the germinablebut not germinating state, seed held in PEG for 9 h rather thanin water was used. Dormant seed was sampled before the hightemperature treatment. Membrane fluidity was assessed usingfluorescence polarization of membrane fractions treated withDPH (1,6-diphenyl-1,3,5-hexatriene) or its derivatives. Membraneproteins were compared using one-dimensional SDS-PAGE electrophoresis.Intracellular membrane fluidity was not increased in the transitionfrom the dormant to germinable state, but did increase in thetransition to germination. There were no detected changes inintracellular membrane proteins during either transition. Inplasma membrane fractions, fluidity increased during both transitions,while a marked increase in 21, 18 and 17 kD proteins was observedin the transition from germinable to germinating state. Thusmodification of plasma membrane fluidity rather than changesin protein profile is associated with the high temperature releaseof cucumber seeds from dormancy. Copyright 2000 Annals of BotanyCompany     

Effect of Acetone on the Induction and Breakage of Secondary Dormancy in Seeds of Cucumber

The dark-germinating seeds of cucumber (Cucumis sativus L. cv.Saharanpur Long green) developed secondary dormancy when giventwo brief far-red light (FR) irradiations during a 4 d incubationin osmoticum at 20°C. Acetone treatment of seeds was foundnot only to break FR-induced dormancy but also to prevent itsdevelopment. The progressive effect of increasing the durationof treatment or the concentrations of acetone, as well as anappreciable activity of the latter, irrespective of its applicationto dry or hydrated seeds, is consistent with the Taylorson hypothesisof anaesthetic action at the membrane level. Contrary to thegeneral consensus that it is only the cells of the embryo whichrespond to an active dormancy-breaking factor, termination ofdormancy as well as prevention of its development by acetonein cucumber seeds was accompanied by a striking change in thepermeability of the cell membranes of the perisperm-endospermenvelope enclosing the embryo. This change in the permeabilityof the cell membranes brought about by acetone appeared to bepermanent in nature as it was not affected by hydration or dehydrationof the treated seeds. Key words: Acetone, anaesthetic substances, secondary seed dormancy, triphenyl tetrazolium chloride, benzyladenine, Evan's blue