[Further Investigations on the phytochrome-mediated accumulation of ascorbic acid in the mustard seedling (Sinapis alba L.)]

The accumulation of ascorbic acid (AS) in the young mustard seedling is greatly increased by the action of the active form of phytochrome (P730) (see SCHOPFER, 1966). There is no photosynthesis in continuous far-red light, which was used throughout the experiments. The phytochrome-mediated increase in AS approximately parallels the synthesis of anthocyanin in the seedling, although the onset of AS-accumulation precedes the anthocyanin synthesis by 2-3 hours (Fig. 4 and 5).-The action of P730 increases the amount of AS in every part of the seedling (cotyledons, hypocotyl, radicula) (Fig. 1-3). This increase in AS parallels the formation of P730 rather than the different growth responses of these organs (enlargement of the cotyledons, inhibition of hypocotyl and radicula lengthening). The lag in AS-accumulation after onset of far-red irradiation is the same in all 3 parts of the seedling (about 1 hour under the experimental conditions; Fig. 4).-Actinomycin D (10 μg/ml), which strongly inhibits anthocyanin synthesis (Fig. 9 and 10), has no corresponding effect on P730-dependent increase in AS-accumulation (Fig. 7 and 8). This result support the hypothesis that already active genes are only slightly influenced by actinomycin D (see LANGE and MOHR, 1965). It also shows that, in contrast to its role in anthocyanin synthesis, P730 probably does not act by initiating "potentially active" genes in the case of AS-accumulation. - A dark synthesis of anthocyanin in the cotyledons can be obtained by application of AS to the seedlings (Fig. 11). Glucose and sucrose are ineffective in this respect (Table 2). The effect of AS-feeding on anthocyanin synthesis can be inhibited by actinomycin D in very much the same way as light induced anthocyanin synthesis is inhibited (Table 3). - Also the RNA content of the cotyledons is increased by feeding AS in the dark (Table 4).These results are in line with the earlier suggested hypothesis (see SCHOPFER, 1966) that increase in AS-accumulation is a very early event in the mediation of some "positive" photoresponses, e.g. anthocyanin synthesis. According to the hypothesis of MOHR (1966 a, b) it has been concluded that AS functions as a part of the "signal chain" of phytochrome-mediated photomorphogenesis. This "signal chain" is thought to start differential gene activation to bring about "positive" photoresponses.