[2′,3′,5′,6′- 2 H 4 ]-2-Hydroxynaringenin is synthesised and incubated with commercially available UDP-glucose and the crude protein extract from Desmoduim uncinatum leaves. The organic extract produces isotopically labelled [2′,3′,5′,6′- 2 H 4 ]-vitexin and [2′,3′,5′,6′- 2 H 4 ]-isovitexin. Repeating the experiment with denatured protein or replacing the 2-hydroxynaringenin with [2′,3′,5′,6′- 2 H 4 ]-apigenin or [2′,3′,5′,6′- 2 H 4 ]-naringenin results in no observable incorporation. 2-Hydroxynaringenin is therefore the substrate for C-glucosylflavonoid biosynthesis in D. uncinatum.