The time course of appearance of competence towards phytochrome (Pfr) was studied in cotyledons of mustard (Sinapis alba L.) with regard to the light-mediated formation of anthocyanin (aglycone cyanidin) and NADP-dependent plastidal glyceraldehyde-3-phosphate dehydrogenase (GPD, EC 1.2.1.13). The experiments were performed to answer the following question: Does phytochrome act to turn responses on (induction), or - as an alternative - does phytochrome cause an amplification of processes already occurring in absolute darkness albeit at low rates once competence is reached (modulation)? The data show that in the case of GPD, phytochrome causes an amplification of the rate of synthesis once the competence point is reached at approximately 36 h after sowing at 25° C. In the case of anthocyanin, it was found that two distinct points of competence exist (26 h and 39 h after sowing, 25° C). In the case of 'early anthocyanin' (competence point at 26 h), synthesis does not occur in darkness without Pfr, while in the case of 'late anthocyanin' (competence point at 39 h), phytochrome causes an amplification of a process occurring in complete darkness albeit at a very low rate. It is concluded that in phytochrome-mediated photomorphogenesis, modulation as well as induction of biosynthetic processes plays a role.