It is demonstrated that phytochrome-mediated anthocyanin synthesis in the epidermal cells of mustard seedling cotyledons takes place only 27 h after sowing onwards (at 25 ~ C). This "starting point" cannot be shifted by light treatments or by nutrients.
The late appearance of competence for efr (er and Pfr, red-and far-red absorbing forms of phytochrome, respectively) with regard to anthocyanin synthesis is not related to the phytochrome system per se (Pr~-Pfr) as this is fully functional immediately after sowing of the seed; nor is it related to the primary reaction of phytochrome: Pfr q-~---~ PfrX'--+Pfr X' (X, X', two forms of a receptor for Pf~) or to the initial action of Pf~X': PfrX'+K'~ Y' (K', coupling element, leading to the product Y', which is no longer photoreversible). Rather, the "starting point" is determined by internal factors only and is thus not accessible to any specific control by external factors. On the other hand, however, the beginning of the initial action of Pf,.X' (" coupling point") can be shifted by light via phytochrome under high irradiance conditions. Moreover, it is shown that there is no phytochrome-independent effect of blue light on photomorphogenesis in the young mustard seedling and that there is no "rapid dark reversion of Pfr" which can be detected by physiological means, at least during the first 27 h after sowing.