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PLANT BIOCHEMISTRY 



"florigens" which induce flower formation. Akhough all attempts to 

 isolate these florigens have been unsuccessful, many workers feel that 

 the flowering hormones are produced in the leaves and are trans- 

 located to the growing points. Moreover, it is generally agreed that 

 these hormones are not species specific but rather that the same sub- 

 stances are active in long-day and short-day plants. 



The fungus Gibberella fujikiiroi produces the substances gibberellic 

 acid, gibberellin Aj, and gibberelin A2, which are active in promoting 

 shoot growth by cell elongation and, in addition, stimulate flower 

 formation. Although the chemical formulas for gibberellins Aj and Ao 

 have not been conclusively demonstrated, gibberellic acid appears to 

 be a tetracyclic dihydroxylactonic acid with the structural formula 



gibberellic acid 



Root-forming Hormones 



The portion of a plant that is severed from the parent plant for the 

 purpose of propagation is called a cutting. Thus the formation of 

 roots on such cuttings is an important method of plant propagation. 



Shortly after 3-indoleacetic acid was recognized as a plant-growth 

 regulator, it was discovered that this same substance promoted root 

 formation when applied to stems and cuttings of some plants. Today 

 such closely related compounds as 3-indolebutyric acid (IBA) and 

 1-naphthaIeneacetic acid (NAA) are commonly used in horticultural 

 practice. The amide derivative of naphthaleneacetic acid, naphthalene 

 acetamide (NAd), is also in common use. The potassium salts of 

 IBA and NAA, as well as the methyl esters of these compounds, are 

 the most effective forms, since they have the advantage of sufficient 

 solubility in water. 



2,4-Dichlorophenoxyacetic acid (2,4-D) promotes root growth at low 

 concentrations but at the same time inhibits bud growth. Trichloro- 

 phenoxyacetic acid (TCP) does not inhibit bud growth and therefore 

 is generally more suitable for rooting practices. 2,4-Dichlorophen- 

 oxypropionic acid and 2,4-dichlorophenoxybutyric acid are also quite 

 effective substances in promoting root formation and growth. Owing 

 to the relatively low cost of production, the last four compounds are 

 used in large-scale plant-propagation operations. 



There are a large number of ways to introduce the root-growth 



