In Vivo Measurements of the Phytochrome Photostationary State in Far Red Light

The in vivo photostationary state, ϕfr= ([Pfr]fr/[P]), of phytochrome in far red light has been determined in mustard seedling cotyledons by three different methods. The ϕfr is a function of the length of time of etiolation (t= 36 hr, ϕfr= 0·14; t= 72 – 120 hr, ϕfr= 0·075). The calculated ϕr= 0·8. The amount of Ptot is strongly dependent on the time of onset of far red light. These data imply that it would be almost impossible to maintain a constant level of Pfr in mustard cotyledons over a considerable period of time.