Auxin - Chapter 5 Plant Growth Regulators I Introduction Auxins their Analogues and Inhibitors 1 HORMONES GROWTH SUBSTANCES AND GROWTH REGULATORS

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Chapter 5 Plant Growth Regulators I : Introduction ; Auxins, their Analogues and Inhibitors 1. HORMONES, GROWTH SUBSTANCES AND GROWTH REGULATORS Some chemicals occurring naturally within plant tissues ( i.e. endogenously), have a regulatory, rather than a nutritional role in growth and development. These compounds, which are generally active at very low concentrations, are known as plant hormones (or plant growth substances ). Synthetic chemicals with similar physiological activities to plant growth substances, or compounds having an ability to modify plant growth by some other means, for example polyamines, are usually termed plant growth regulators . Some of the natural growth substances are prepared synthetically or through fermentation processes and can be purchased from chemical suppliers. When these chemicals have been added to plant tissue culture media, they are termed plant growth regulators in this book, to indicate the fact that they have been applied from outside the tissues ( i.e. exogenously). There are several recognised classes of plant growth substance. Until relatively recently only five groups were recognised namely: auxins cytokinins gibberellins ethylene abscisic acid Auxins and cytokinins are by far the most important for regulating growth and morphogenesis in plant tissue and organ cultures; in these classes, synthetic regulators have been discovered with a biological activity, which equals or exceeds that of the equivalent natural growth substances. No chemical alternatives to the natural gibberellins or abscisic acid are available, but some natural gibberellins are extracted from cultured fungi and are available for use as exogenous regulants. However, several classes of chemicals, which are highly effective in blocking the synthesis of gibberellins within the plant, are very effective growth regulators. They are usually termed anti- gibberellins (or growth retardants). These however, can also affect the synthesis of other classes of hormone or growth regulator such as abscisic acid, sterols or brassinosteroids. Exogenous ethylene can be used as a growth regulant, but being a gas, it is difficult to administer and to control the available concentration, except in tightly sealed vessels, this is also true of other alkynes and alkenes which mimic ethylene action such as acetylene and propylene. However, some chemicals have been invented which are capable of releasing ethylene; effective compounds are taken up into plants as intact molecules, but then break down to release ethylene within the tissue of a plant. One of these ethylene-releasing chemicals – ‘ethephon’ (2-chlorethanephosphonic acid) is used as a growth regulator for tissue cultures. There are now also some very specific inhibitors both of ethylene biosynthesis and of its action.
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This note was uploaded on 07/23/2011 for the course HOS 6737c taught by Professor Moore during the Spring '09 term at University of Florida.

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Auxin - Chapter 5 Plant Growth Regulators I Introduction Auxins their Analogues and Inhibitors 1 HORMONES GROWTH SUBSTANCES AND GROWTH REGULATORS

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