Membrane turnover in imbibed dormant embryos of the wild oat (Avena fatua L.) : II. Phospholipid turnover and membrane replacement

Long-term storage of dormant Grand Rapids lettuce seeds in the imbibed state: physiological and metabolic changes

Grand Rapids lettuce (Lactuca sativa L.) seeds retained their viability for up to ten months when maintained in the fully imbibed state on moist filter paper at 25°C in darkness. The ability of red light (R) to promote their germination was essentially lost within one week from the start of imbibition; sensitivity to gibberellic acid was retained for two weeks longer. Seeds which did not respond to either treatment had entered the state of secondary dormancy (skotodormancy). This could be relieved at all times by a combination of benzyladenine and R applied to the intact seed, or by isolation of the embryo and incubation on water. Protein synthesis increased initially following imbibition but declined after 72 h to a constant low level. Respiration declined over the first week of storage in the imbibed state to a much-reduced steady level. Cotyledonary lipid declined between four and ten months of storage but the axial lipid remained unchanged. Sucrose in the embryo increased after five months, but no changes in glucose, galactose, fructose or mannose were found. The total N content of the cotyledons declined over the first three months of storage in the imbibed state, with a concomitant rise in axial N; the latter declined slowly thereafter. Basal α-galactosidase (EC 3.2.1.22) activity decreased over seven months, but the phytochrome-induced component could not be raised by a 15-min R treatment even after one month. Germination induced by R and benzyladenine was achieved at later times without a rise in α-galactosidase levels.

Membrene turnover in imbibed and dormant embryos of the wild oat (Avena fatua L.) : I. Protein turnover and membrane replacement

A comparative study of protein synthesis has been carried out with embryos excised from dormant (D) and non-dormant (ND) caryopses of the wild oat. Although D embryos imbibed in water or ND embryos imbibed in abscisic acid do not germinate, they incorporate [(14)C]leucine into TCA-insoluble material for the first 48 h as readily as embryos that do germinate (ND embryos imbibed in water, or D embryos imbibed in gibberellic acid). Pulsechase experiments with [(14)]leucine show that in both D and ND embryos the proteins associated with the membranes undergo turnover. The rates of decay of incorporated radioactivity are similar in both dormant and germinating embryos up to 98 h following embryo excision. Fractionation of the membrane proteins in SDS-polyacrylamide gels indicates that the different polypeptides have different rates of turnover. It is concluded that membrane proteins in imbibed D embryos are in a state of constant turnover, and that this is a part of the replacement processes necessary to maintain the integrity of hydrated cells. The continuation of such synthetic events could account for long term survival of dormant Avena fatua in the imbibed state.