Studies oftheMechanism ofEnhancement ofPhytochrome- dependent Lettuce SeedGermination byPrechilling

Temperature andkinetic studies wereperformed toexamine themechanismbywhichprechilling stimulates phytochrome-dependent seedgermination inlettuce, Lactuca sativa, L.cv.GrandRapids. Imbibed seeds weregiven ashort farredirradiation andonedayofdarkincubation at20 C toestablish verylowlevels ofthefarred-absorbing formofphytochrome-(Pfr). Germination wasgreatly stimulated bysubsequent prechilling treatments whenthey werefollowed byasecond short farred irradiation. Prechilling therefore increased germinadon sensitivity tothe low,normally inhibitory Pfrlevels established byfarredirradiatdon. This sensitivity increased withlowered prec l temperature toamaximum near4C.Itwaslinearly dependent uponduration ofprechilling at4Cup toanearmaximal response at10hours, anditdecayed inaconverse mannerwhenseeds werereturned to20Cafter 10hours at4C.Prechilling also increased germination responses tosubsequent periods ofhigh levels ofPfrwhich wereinitiated byredandterminated byfarredirradiations. HighPfrperiods adequate topromote thegermination ofunchilled seeds produced sharp inflections at18C inthedependence ofgermination on prechilling temperature. Rates ofphytochrome potentiation ofgermination werenotaffected byprechilling. Theresponse toprechilling fit amechanisminvolving homeoviscous adaptation ofmembrane lipids totemperature.