Lettuce Seed Germination: Evidence for a Reversible Light-Induced Increase in Growth Potential and for Phytochrome Mediation of the Low Temperature Effect

Abstract

Photoblastic germiinlationi in lettuce has been very extensively sttudiedl (7, 17). The process is phyto-chromiie-miiedliated, that is, brief irradiances with red lighlt (RZ) promote subsequent germiniation, and the red effect is repeatedly reversible by immlliediate reirradliation witlh far-redl light (F). Some lots of freslylv harvested seed of the variety Grand(Rapids germinate little or Inot at all wihen imlHbibed in the (lark at 260. Dark germiniatioln is someiehat higher at lowver temperatures, alnd inlcreases slowlxv with time (luirinig storage of the dry see(ls (6. 19. 21). P'romiiotioni by low temperature is thoughlt to be ini-deplendent of l)hytochrome (13). 'l'he iiatture letttuce achenie conlsists of ani embryo closelv invested r'ith a 2-cell-layer endospermnl which is in tuirn sturrounll(led 1y the frtuit coat (2). Under ordinarv coln(litionis (lark dormanlcv (lepends onl the strtucttural integrity of the endosperm alone. If this layer is removed, the radicle anid, subsequently. the hvpocotyl will elonigate in colntiniuous (larkuiess. after RZ, anid after F, alnd the gro-vth rate is the samiie for all 3 cases (12). It has been proposed (12) that the promiiotive effect of R is dtle to a weakening of the mlechaniical resistanice of the end6spermn to embryo exl)ansion, l)resuimal)ly throtughi secretion of cytolytic enlzyrmes by the emnbryo. I)ark germiinationi is promoted by gibberellic aci(d (10, 14) alnd kinetini (e.g.. 11. 18) at relatively low. anid by thiourea (20) at relatively high concenitra-tion