Flowers plant hormones, auxin

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

Ethylene is a hydrocarbon plant hormone that comes from the methionyl moiety of S-adenosylmethionine. Ethylene stimulates fruit n 2 ripening and the aging of flowers, and it inhibits seedling growth. It also redirects auxin transport (another plant hormone) to promote transverse, rather than longitudinal, growth of plants. 

O. have been shown to inhibit flowering, promote vegetative growth and control organ development in plant tissue culture. Effective concentrations of O. are 100-1000-fold lower than those of other plant hormones, such as auxins, cytokinins and gibberellins. 

On the one hand this has to be attributed to the simple structure of the plant hormone and on the other hand to its functional poly-valency, viz. the fact that apart from promoting growth (cell elongation) this auxin acts in differentiation (initiation of root formation, control of flower initiation) too. 

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