Changes in membrane fluidity and protein composition during release of cucumber seeds from dormancy by a higher temperature shift

Abstract

Dormancy of seeds of cucumber (Cucumis sativus L.) was induced by imbibing in - 1·8 MPa polyethylene glycol 6000 (PEG) solution and pulsing with far red light for 15 min prior to washing and drying. When re-imbibed with water at 20 °C, dormancy was broken by raising the temperature to 30 °C for 6 h. This treatment was also effective when -0·9 MPa PEG was present during re-imbibition and high temperature. Seeds with broken dormancy were found to germinate in water over a smaller temperature range than seeds in which dormancy had not been induced. When the duration of the temperature shift to 30 °C was varied, germination percentage increased from 7 to 60 % after 6 h, but longer exposures up to 12 h had no further promoting effect. The time course of germination after transfer to water following 6 h at 30 °C in PEG showed piercing of the perisperm-endosperm envelope after 9-12 h and radicle protusion after 12-15 h. If PEG was retained after high temperature treatment no visible germination was observed. Thus, to study membrane fluidity and the protein content associated with germination, seeds were sampled 9 h after high temperature treatment. To study the germinable but not germinating state, seed held in PEG for 9 h rather than in water was used. Dormant seed was sampled before the high temperature treatment. Membrane fluidity was assessed using fluorescence polarization of membrane fractions treated with DPH (1,6-diphenyl-1,3,5-hexatriene) or its derivatives. Membrane proteins were compared using one-dimensional SDS-PAGE electrophoresis. Intracellular membrane fluidity was not increased in the transition from the dormant to germinable state, but did increase in the transition to germination. There were no detected changes in intracellular membrane proteins during either transition. In plasma membrane fractions, fluidity increased during both transitions, while a marked increase in 21, 18 and 17 kD proteins was observed in the transition from germinable to germinating state. Thus modification of plasma membrane fluidity rather than changes in protein profile is associated with the high temperature release of cucumber seeds from dormancy.