with regard to the sequence: — cinnamic acid ^ cinnamyl alcohol ^ allyl derivative. The favoured pathway for eugenol biosynthesis is accepted to be: L-phenylalanine ^ cinnamic acid ^ P-coumaric acid ^ caffeic acid ^ ferulic acid ^ eugenol.
Manitto et al. also found that L-tyrosine was not incorporated into eugenol. Thus it is in agreement with the lack of L-tyrosine ammonia-lyase in dicolyledonous plants (Young et al., 1966).
In cinnamon, cinnamic aldehyde was formed from phenylalanine without randomisation of the 14C label. The same degree of labelling was observed when PA1, PA2 and PA3 were separately used as a precursor (Senanayake et al., 1977; Angmor et al., 1979). Such a uniform labelling would not be observed if randomisation of the label occurred. The experimental evidence suggests that the C6-C3 skeleton of PA was retained as one unit in the biosynthesis of cinnamic aldehyde from PA. This is in agreement with the suggestion of Birch (1963) that cinnamic aldehyde can be formed from PA via cinnamic acid and cinnamic aldehyde, which can be further reduced to cinnamyl alcohol that then goes for lignin biosynthesis (Neish, 1968). The retention of the C6-C3 skeleton in the biosynthesis of propenylphenols, such as anethole (P-methoxy propenyl benzene), from PA has been observed by other workers. Kaneko (1960, 1961) and Manitto et al. (1974) observed that anethole was formed in Foeniculum vulgare and Pimpinella anisum (Umbelliferae) from PA without label randomisation, suggesting that the C6-C3 skeleton was retained as one unit. Thus the experimental evidence indicates that in the cinnamon plant cinnamic aldehyde is formed from pheylalanine via the shikimic acid pathway (Fig. 3. 4).
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