Monoterpenes

A large number of monoterpenes have been reported to be phytotoxic, and they have been proposed as potential starting structures for herbicides.44 Nevertheless, of the monoterpenes, we only know anything of significance about the mode of action of the cineoles. Early work showed that relataively high concentrations of 1,8-cineole (Fig. 10.1) inhibit mitochondrial respiration of isolated organelles. The high concentration used makes this mechanism of action suspect as a primary site of action. Both 1,8-cineole and 1,4-cineole (Fig. 10.1) are strong growth inhibitors, but only 1,8-cineole strongly inhibits all stages of mitosis (Fig. 10.2).39 Swollen root tips have also been reported in 1,8-cineole-treated onion root tips.27 Also, 1,4-cineole causes growth abnormalities in shoots, such as helical growth, that 1,8-cineole does not cause. Thus, despite the similar structures, the two compounds apparently have different modes of action. Camphor (Fig. 10.1) has effects on mitosis and respiration that are similar to those of 1,8-cineole.27, 28

cinmethylin

1,4-cineole 1,8-cineole artemisinin OH

camphor cinmethylin

1,4-cineole 1,8-cineole

artemisinin OH

dehydrozaluzanin C

dehydrozaluzanin C

podolactone B OH

HH glaucarubolone

podolactone B OH

holacanthone betulin

Figure 10.1

fVOH

holacanthone betulin

Figure 10.1

Structures of some of the compounds mentioned in the text.

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