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In sweetclover (Melilotus aIba Desr.) the cis and trans isomers of o-hydroxycinnamic acid occur primarily as the respective β-D-glucosides (Rudorf and Schwarze 1958; Kosuge 1961; Haskins and Gorz 1961a; Stoker and Bellis 1962). The review of Brown (1963) summarizes evidence that these glucosides are formed via the following pathway: phenylalanine (formed from shikimic acid) → trans-cinnamic acid → trans-o-hydroxycinnamic acid (o-coumaric acid) trans-β-D-glucosyl-o-hydroxycinnamic acid (o-coumaric acid glucoside) → cis-β-D-glucosyl-o-hydroxycinnamic acid (coumarinic acid glucoside). Sweetclover plants of the Cu Cu genotype contain substantial amounts of both o-coumaric acid glucoside and coumarinic acid glucoside, and cu cu plants are low in content of both glucosides (Haskins and Gorz 1961a,b). The suggestion has been made that the cu gene influences content of the glucosides by controlling the o-hydroxylation of cinnamic acid (Brown 1963). Another possibility is that the cu allele somehow effects the destruction of o-coumaric acid as rapidly as this compound is formed, thus preventing the synthesis of o-coumaric acid glucoside. Still other possibilities could be suggested. In the present study, the role of the Cu/cu gene pair in the formation of o-coumaric acid glucoside and coumarinic acid glucoside was investigated by comparing plants of the cu cu and Cu Cu genotypes with respect to the ability to incorporate 14C from 14C-labeled phenylalanine, trans-cinnamic acid, and o-coumaric acid into the two glucosides.