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BIOCHEMICAL SYSTEMATICS 



In vascular plants, wherein the production of phenylpropane 

 derivatives in lignin synthesis has been continuously (in evolutionary 

 time) a major metabohc activity, flavonoid compounds are of general 

 occurrence, though it is true that phenolics are far less prominent 

 among vascular cryptogams. In thallophytes, where hgnin does not 

 occur, flavonoid pigments are practically unknown although other 

 phenohcs may be numerous. According to Blank (1947) a report of 

 the occurrence of flavonoids in mosses is probably valid, but no algae 

 or fungi are known to produce flavonoids. The report of flavonoids in 

 Chlamydomonas (Moewus, 1950) was invalid (Kuhn and Low, 1960). 

 Alston (1958) showed that the purple pigment of the green alga, 

 Zygogonium ericetorum, was not an anthocyanin and that early re- 

 ports of the occurrence of anthocyanins in filamentous algae were prob- 

 ably erroneous. It seems therefore that the presence of an enzyme 

 system leading to lignin synthesis has provided an opportunity for 

 the appearance of phenylpropane derivatives to couple with the 

 acetate pathway to form the basic flavonoid nucleus. This step in 

 biochemical evolution may have been acquired quite early in view of 

 the wide distribution of flavonoids among pteridophytes, but it is 

 also possible that it evolved repeatedly. 



The next section will be devoted to biochemical genetical 

 studies of certain classes of complex phenols, the flavonoid pigments. 

 In order to appreciate fully the implications of such studies a brief sur- 

 vey of the chemistry of these compounds might prove helpful. 



Chemical structures of classes 

 of flavonoid compounds 



This group of compounds contains a Cs-Ca-Ce carbon 

 skeleton in which the C3 unit links two aromatic groups. The C3 chain 

 is essentially the key to the different major classes of flavonoids since 

 these classes are recognized on the basis of the oxidation state of the 

 C3 unit in addition to the mode of ring closure to form a heterocyclic 

 middle ring (if ring closure ensues). Flavonoids usually occur as glyco- 

 sides and sometimes also as acylated compounds, the acyl group being 

 in many cases a phenoHc acid. Glycosides are mostly formed as esters 

 at carbons 3, 5, or 7 but some carbon glycosides at position 8 are known 

 (Horhammer and Wagner, 1961). Flavonoid glycosides are usually 

 water soluble and are located in the vacuole of the cell. Classes of 

 flavonoid compounds are discussed below: 



