The Physiology of Flowering Plants - KHAM PHA MOI

SEED DORMANCY 313ABALeaching ofABA or otherinhibitorsaleuronehyaline layertestalayerFig: 11:23 Breaking dormancy.Environmental factors and plantgrowth regulators interact tomaintain dormancy or triggergermination. Redrawn fromDebeaujon & Koornneef (2000).GROWTHPOTENTIALOthertargetsCHILLINGAFTER-RIPENINGGAPhytochromeLIGHTEMBRYOPhysicalrestraintgermination is stimulated. This result demonstrates that it is therelative concentrations of ABA and GA which are important in maintaining,or breaking, dormancy. If a seed is treated with inhibitors ofGA synthesis as it imbibes, germination is prevented, demonstratingthat de novo GA synthesis is required, rather than the utilization ofpre-formed GA. GA is an important signal which stimulates both celldivision and the mobilization of seed reserves.Figure 11.23 shows how environmental and endogenous signalsmay interact to regulate seed dormancy. Not all of these will operatein one species and they may have different relative importancesdepending upon how long the seed has been dormant. The growthpotential of the embryo is stimulated by GA, and inhibited by ABA(and potentially other substances), with the seed coat acting as aphysical restraint to germination as well as preventing the loss ofinhibitory substances. Removal, or damage to, the seed coat (a processcalled scarification) will stimulate germination in manyspecies, as will a period of after-ripening or cold treatment (stratification).These requirements allow seeds to germinate at an appropriatetime. At temperate latitudes the cold requirement fordormancy breaking is fulfilled by overwintering; hence seeds willgerminate in the spring rather than the autumn. In hot arid regionssubject to periodic fires, seeds may need to be cracked open byfire (e.g. in the sumach, Rhus ovata) ensuring that seed germinationoccurs at an optimal time. Smoke alone has been identified as animportant factor which triggers germination in many species foundBox 11.7The terms stratification and vernalizationare often misused.Stratification refers to the coldtreatment of seeds stimulatinggermination whilst vernalizationrefers to the cold treatment ofplants stimulating flowering. Theterm stratification comes from thecommercial practice of spreadingseeds between layers (strata) ofmoist substrate and exposure tolow temperatures.