Plant growth retardants action

The Action of Plant Growth Retardants at the Biochemical Level... 

J.W. Dicks, Modes of Action of Growth Retardants , British Plant Growth Regulator Group , Monograph no. 4, 1979, pp. 1-14. 

In order to understand the growth retardation mechanism of S-uniconazole, the shoots of Pisum sativum L. treated with S- and R-uniconazoles were analyzed in terms of the levels of the endogenous GAs, BRs, and phytosterols. Only referring to BRs, it is of interest to examine whether uniconazoles modify the biosynthesis of BRs. BRs contained in the shoots of P. sativum L. were extracted, purified, and analyzed by the GC/MS. GC/MS analysis of the active fraction led to the identification of CS m/z (rel. int.) 512 (M+, 54%), 155 (100%). GC/SIM quantitation using an internal standard (d6-CS) revealed that the content of CS in the control plants was 0.9 ng/g fir. wt. and, after treatment with S- and / -uniconazoles, reduced to 54% and 34% of the controls, respectively. The result suggests that the altered metabolism of BRs is likely to be involved in the action mechanism of S-uniconazole. 

Xenobiotics which reduce plant growth without affecting the pattern of development have been available for almost 40 years. A group of nicotinium growth retardants was described by Mitchell et al. in 1949 [36], followed shortly by the quaternary ammonium carbamates, including AMO-1618 [47]. Partial reversal of the dwarfing effect of these compounds by GA3 [32] and the demonstration that the retardants reduced GA production by the fungus Fusarium moniliforme [30] implicated GA biosynthesis as a site of action. This was confirmed by Dennis et al. 

The triazoles and related growth retardants are not general inhibitors of plant cytochrome P-450 monooxygenases [39]. Their action appears to be confined to methyl hydroxylases, with which they interact to differing extents depending on structure. Apart from e f-kaurene oxidase, there are several other such enzymes that are involved in important metabolic pathways (Fig. 5), the products of which may influence growth. These are potential sites of action and will be considered below. 

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