[The DNA contents of cotyledons and hypocotyl of the mustard seedling (Sinapis alba L.) during phytochrome-mediated photomorphogenesis].

DNA contents and accordingly cell numbers of cotyledons and hypocotyl of the mustard seedling were virtually constant during the experimental period (between 36-60 hours after sowing) in the dark as well as under the influence of P730, the active phytochrome (Table).-Therefore the phytochromemediated increase of the RNA and protein contents (WEIDNER et al., 1965) must be understood as an increase of RNA and protein per cell. This fact is in agreement with the conception of differential gene activation mediated by P730 (MOHR, 1966). The virtually constant DNA contents during the period of time which is regularly used for experimentation on photomorphogenesis in our laboratory (36-60 hours after sowing; MOHR, 1966) is an important prerequisite for comparing biochemical data under the point of view of differential gene activation, e.g. in kinetical studies in the dark and under continuous far-red light.