Flavonoid evolution in land plants is discussed from an enzymic point of view, based on the present day distribution of the major subgroups of flavonoids in bryophytes, lower and higher vascular plants. The importance of varied functions in the origin of pathways with a series of sequential steps leading to end-products is considered; it is argued that the initial function is that of an internal regulatory agent, rather than as a filter against ultraviolet irradiation. The basic syntheses, hydroxylases, and reductases of flavonoid pathways are presumed to have evolved from enzymes of primary metabolism. A speculative scheme is presented of flavonoid evolution within a primitive group of algae derived from a Charophycean rather than a Chlorophycean line, as a land environment was invaded. Flavonoid evolution was preceded by that of the phenylpropanoid and malonyl-coenzyme A pathways, but evolved prior to the lignin pathway.Flavonoid evolution in the past has been considered mainly from a chemotaxonomic or phylogenetic point of view, based on end products accumulated in various plant groups, especially within angiosperms (1 1). Recently, similar chemotaxonomic studies of the distribution of flavonoids within bryophytes and lower, non-seed-bearing vascular plants have been summarized (18,19). The enzymology of the major steps leading to flavonoid subgroups has either been demonstrated in cell-free systems or has at least been characterized as to potential mechanisms. There is now a need to consider flavonoid evolution from an enzymic point of view and to speculate how such pathways leading to varied end-products may have arisen during evolution. An earlier attempt at this was made by Swain (26) Monohydroxy B-ring flavonoids were implicated as cofactors of peroxidase functioning as an IAA oxidase that destroys the hormone, whereas dihydroxy B-ring forms inhibited the IAA degrading activity (7). More recent work has implicated both mono-and dihydroxy forms as inhibitors of IAA transport across the plasma membrane by binding to a plasma membrane protein (15). In addition, the growth inhibition in the Hepaticae (a bryophyte) by lunularic acid, a possible early stilbene or C6-C3 derivative, has been postulated to be comparable to that of ABA in vascular plants (8). Gottlieb (9) has also argued the primacy ofinternal rather than environmental factors in phytochemical evolution, but his driving force is based on a metabolic function in which less degradable secondary metabolites ultimately replenish primary metabolites; I find this a less plausible internal function.Although

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