C-Glycosylflavones as an accumulation tendency: A critical review

Abstract

C-Glycosylflavones represent a special group of flavonoid glycosides both with respect to their chemical structure and presumed different biogenetic origin. Their accumulation and distribution in various plant groups has frequently been interpreted systematically; however, the application of chemotaxonomic congruence sometimes has led to incoherent results. This indicates the necessity for studying C-glycosylflavones primarily as an accumulation tendency before using them as systematic markers. Taking this into account, the distribution of C-glycosylflavones within the Compositae has been compiled and, on the basis of available data, their integration into the complete flavonoid complement analyzed. These results have been compared to those from other selected angiosperm groups, with special regard to the occurrence of certain combinations of different flavonoid glycosides with C-glycosylflavones. Despite the lack of detailed systematic surveys, the biosynthetic capacity of C-glycosylflavone accumulation in the Compositae was found to be relatively high. Mono-and di-C-glycosylflavones, including some with complex structures, are accumulated in the tribes Anthemideae, Heliantheae, Cynareae and Lactuceae. The genera of the Anthemideae and Heliantheae exhibited a very similar C-glycosylflavone profile, thus suggesting possible affinities. The tribes Eupatorieae, Astereae, Inuleae, Vernonieae and Senecioneae apparently accumulated only a small number of less complex C-glycosylflavones. Strikingly, di-C-glycosylflavones were often combined with a series of flavonoid O-glycosides, whereas predominantly C-glycosylflavone profiles showed a low percentage of co-occurring flavonoid O-glycosides. These combination trends could possibly serve as additional chemical characters. Various combination trends have been found within the angiosperms surveyed here. Mostly C-glycosylflavones co-occurred with flavonol 3-O-glycosides and (or) flavone 7-O-glycosides. Mutual exclusivity appeared to be very rare. A predominantly C-glycosylflavone pattern frequently contained a few flavonoid O-glycosides only. By comparison, the Compositae seemed to accumulate di-C-glycosylflavones as a part of a flavonolor flavone-dominated pattern to a greater extent than the other taxa studied here. Taxa accumulating C-glycosylflavones as a main tendency exhibited different degrees of specialization with respect to the substitution patterns of individual compounds. These patterns are much more conclusive than the mere presence/absence criterion. The available data suggest a reconsideration of the present dogma on flavonoid evolution. There is little evidence that C-glycosylflavones truly represent an intermediate stage in the reduction trend from flavonols to flavones. More knowledge on the biosynthesis and function(s) of C-glycosylflavones would be helpful for their interpretation as systematic features.