[The inhibition of phytochrome-mediated photomorphogenesis ("positive" photoresponses) by actinomycin D and puromycin in the mustard seedling (Sinapis alba L.)].

The many photochrome-mediated photoresponses of a seedling (Sinapis alba L., white seeded mustard) can be divided into 3 categories: positive, negative, and complex photoresponses. "Positive" photoresponses are those which are characterized by an initiation or a promotion of biosynthetic or growth processes (MOHR, 1966b). Phytochrome-mediated anthocyanin synthesis is the prototype of a "positive" photoresponse. It has been shown in previous papers (e.g. LANGE and MOHR, 1965; MOHR et al., 1965) that "positive" photoresponses can be specifically inhibited by actinomycin D and puromycin. It has been concluded that in the case of "positive" photoresponses P730 (the active phytochrome) exerts its function through differential gene activation.-In the present paper it has been demonstrated that phytochrome-mediated "positive" photoresponses of the mustard seedling like "opening of the hypocotylar hook" and "unfolding of the cotyledons" can be inhibited by relatively low doses of actinomycin D and puromycin in very much the same way as anthocyanin synthesis or cotyledon enlargement is inhibited. It has been concluded that in these cases too the action of P730 must be attributed to an activation of "potentially active genes" in the manner postulated on the basis of the data on anthocyanin synthesis.