The Physiology of Flowering Plants - KHAM PHA MOI

PLANT GROWTH HORMONES 181system provided a quantitative and sensitive bioassay for auxin whichwas used for many years although it has now been superseded bymodern analytical techniques. If agar blocks are placed asymmetricallyon decapitated oat (Avena sativa) coleoptiles, the degree ofcurvature under standardized conditions is related to the auxin concentration.Similarly if coleoptile or hypocotyl segments are floatedon different concentrations of auxin solution, the growth rate islogarithmically related to the auxin concentration. Such a systemillustrates well the essential features of a plant growth hormone –auxin is produced in the tip of the coleoptile and transported downthe coleoptile stimulating a response, cell elongation. The growthresponse which is observed depends upon both the concentration ofthe hormone and the target tissue. Figure 7.3 illustrates the growthresponses of roots and shoots to different concentrations of auxin.Note that the concentration ranges which stimulate growth are quitedifferent, and in both roots and shoots either promotion or inhibitionof growth can result from exposure to auxin depending upon theconcentration.The structure of IAA is relatively simple and it is the most commonlyfound natural auxin although many other related compoundswith auxin-like actions can be found within plants. A general featureof these compounds is that they have a carboxylic acid group linkedto an aromatic ring. Many synthetic auxins have been produced suchas naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid(2,4-D) (Fig. 7.2), and these are widely used in agriculture. As well asthese simple, free auxins plants also contain large amounts of conjugatedauxins in which the auxin is covalently bound to anothermolecule (ranging from simple sugars to proteins). These conjugatedauxins are inactive and are transported via the phloem to other partsof the plant where they may be deconjugated, and hence activated.However, the growth responses of coleoptiles described above resultfrom the basipetal (basewards) transport of free auxin from cell tocell (discussed in more detail in Chapter 12).As well as stimulating growth, auxins have many other effectson plant development. For example, auxin plays a key role in apicaldominance. As seen in later chapters, auxin is also importantin regulating the formation of lateral roots (Section 9.6.3), the developmentof the vascular system (Section 9.5.3), parthenocarpy(Section 11.11.3) and in senescence. This diversity of function ledK. V. Thimann to state ‘The trouble with auxin is that its actions areso numerous and apparently unrelated’ (in Palme & Galweiler 1999).Although its actions are certainly numerous, it is clear that its functionscan be understood only when interactions with other plantgrowth hormones are considered. This interaction between plantgrowth hormones is a theme which will continually arise, and it isfundamental to the way in which plants respond to interacting, andat times conflicting, environmental signals.Inhibition PromotionRootsStems10 –11 10 –9 10 –7 10 –5 10 –3 10 –1Molar concentration of IAAFig: 7:3 Growth responses ofroots and shoots to differentconcentrations of auxin. Indole-3-acetic acid (IAA) can promote orinhibit the linear growth of rootsand shoots depending uponconcentration. Note that the auxinconcentration is shown on alogarithmic axis. After Thimann(1937).