Fig. S6.

Citrus flower buds appear to accumulate xanthine, 1-methylxanthine, theophylline, and caffeine as well as minor amounts of 3-methylxanthine. Mass spectrometry scans show parent ion–fragment ion pairs unique for each xanthine alkaloid product detected in Citrus flower buds. The presence of these metabolites is consistent with the enzyme assays (Fig. 2D and Fig. S5) and suggests that the primary pathways by which caffeine is produced in Citrus flowers are those shown in Fig. 2D. Table S2 shows parent ion–fragment ion masses for xanthine alkaloid standards.