QUINONES 225 



found in chloroplasts having an absorption maximum at 254 m/x has 

 been called "plastoquinone" (Crane, 1959). 



The presence of a quinone coenzyme involved in an important 

 electron transport system in plants suggests that the ability to syn- 

 thesize the basic naphthoquinone nucleus is not limited but is charac- 

 teristic of plants in general. Therefore, those groups of plants which 

 accumulate naphthoquinones otherwise substituted than in the 

 vitamin K pattern may not possess a uniquely new enzyme system 

 for the formation of the naphthoquinone ring structure, but rather 

 may possess a metabolic system which permits the accumulation of 

 naphthoquinones, which, when coupled with appropriate enzymes, 

 provide for secondary structural modifications. It is well established 

 that some quinones are fungicidal. If then, there is some positive 

 selective value correlated with quinone accumulation, and the basic 

 quinone pathway pre-exists (even though production is hmited) among 

 green plants in general, it is not surprising to find distantly related 

 plants producing the same compound. Considered in this light there 

 is no reason to suspect cryptic phylogenetic association between taxa 

 possessing such compounds. For example, the quinones lawsone, and 

 its methyl ether (the latter is fungicidal) are found in Lawsonia alba 

 (Lythraceae) and in Impatiens balsamina (Balsaminaceae) respec- 

 tively. Thomson says that "it is noteworthy that such closely related 

 quinones occur in distantly related plant families." Actually, other, 

 even more complex quinones occur in equally distantly related families, 

 for example, lapachol (Bignoniaceae, Verbenaceae, Sapotaceae). 



O 



"oh ^ X /OCH3 ^^^A.OH 



CH3 

 CH2CH=CCH3 







lawsone lawsone methyl ether lapachol 



Some quinones are physiologically active (as purgatives), and 

 others are valued as dyes. Despite their economic significance, how- 

 ever, relatively little is known of quinone biosynthesis, and practically 

 no genetic studies on quinones have been reported. The favored 

 hypothesis to account for the important anthraquinone group involves 

 the same mechanism as that producing the A ring of flavonoid com- 

 pounds, namely, the condensation of acetate units. Acetate-2-Ci4 has 

 been used to investigate the biosynthesis of emodin by Penicillium 

 islandicum, and the results suggest that head to tail condensation of 

 eight acetate groups was involved (Friedrich, 1959). Hegnauer (1959), 

 in contrast, emphasized the fact that compounds such as xanthones, 



