Dependence of phytochrome dark reactions on the initial photostationary state

Under conditions of continuous irradiation, the P jr destruction rate constants (k d ) of phytochrome in hooks and cotyledons of squash (Cucurbita pepo L.) seedlings do not depend on the photostationary state ϕλ and are the same in both organs. On the other hand, the rate constants of the dark reversion and the first destruction step, plotted as a function of ϕ λ (0) , show optimum curves with maxima between ϕ λ (0) and 0.5. Similar results were obtained for dark reactions of mustard (Sinapis alba L.)-hook phytochrome in vivo. This indicates a cooperative behaviour of these phytochrome dark reactions.