Control by light of hypocotyl growth in de-etiolated mustard seedlings : II. Sensitivity for newly-formed phytochrome after a light to dark transtition.

After sowing, mustard (Sinapis alba L.) seedlings were grown for 48 h in white light (25°C). These fully de-etiolated, green seedlings were used as experimental material between 48 and 72 h after sowing to study the control by phytochrome (Pfr) of hypocotyl elongation after a white light→dark transition. It was found that the transfer to darkness leads to a complete loss of sensitivity towards newlyformed Pfr for a period of 12 h while the effectiveness of the Pfr, which was established at the end of the 48 h white light pretreatment was maintained. It is concluded that two kinds of Pfr must be kept apart: "old" Pfr which operates during the first 12 h after the light→dark transition and "new" Pfr the appearance of which is totally ignored by the plant. Between 12 and 24 h after the white light→dark transition the sensitivity of the seedling towards "new" Pfr is restored while an effect of the "old" Pfr is no longer detectable. The restoration process is accelerated by light which operates through phytochrome ("positive feedback"). The implications of these findings for plant growth under natural light/dark regimes are discussed.