The Physiology of Flowering Plants - KHAM PHA MOI

180 PLANT GROWTH HORMONESFig: 7:1 Development andstructure of a grass seedling. (A, B,C) Stages in germination; stage(C) shows emergence of the firstleaf from the apex of the coleoptile.(D) Section taken at stage A;a ¼ coleoptile; b ¼ first leaf;c ¼ apical meristem; d ¼ node; theregion below the node is referredto as the mesocotyl. (E) Transversesection through the coleoptile;e ¼ coleoptile; f ¼ first leaf; vascularstrands in the coleoptile and firstleaf are shown as filled circles.Redrawn from James (1943).A B C DabcdEefFig: 7:2 The structures of somenatural and synthetic planthormones. Indole-3-acetic-acid(IAA) is the most commonnatural auxin found in plants.The synthetic auxinsnaphthaleneacetic acid (NAA) and2,4-dichlorophenoxyacetic acid(2,4-D) are widely used inagriculture. All of the gibberellinsare based upon the ent-gibberellaneskeleton. The structure of acommon, naturally occurringgibberellic acid, GA 3 , is shown.Ethylene (ethene) has a muchsimpler structure than other planthormones. The natural cytokinin,zeatin, was first identified in maize(Zea mays), although synthetichomologues, such as kinetin, aremore widely used for plant tissueculture. The best knownrepresentative of the abscisins isabscisic acid (ABA).NHIndole-3-acetic acid (IAA)(natural)NCH 2 COOHEnt-gibberellane skeletonCH 3NHCH 2 CH C CH 2 OHNNNZeatinNHCH 2 COOHNaphthaleneacetic acid (NAA)(synthetic)HONH 2 CH 2COOCH 3 COOHGibberellic acid (GA 3 )N ONNHKinetin(synthetic)H 3 CCH 3O CH 3OCH 2 COOHClCl2,4-dichlorophenoxyaceticacid(2,4-D) (synthetic)OHOHCH 2HHCCEthyleneCH 3COOHAbscisic acid (ABA)H H