An analysis of phytochrome action in the 'high-irradiance response'

Using a model based on spectrophotometric measurements of the phytochrome of Sinapis alba L. seedlings we have used the published rate constants for the component reactions of this system to calculate the behaviour of the far-red absorbing form of phytochrome (Pfr), and other parameters, over a period of continuous irradiation. We have then analysed two of the most intensively studied photomorphogenetic responses, hypocotyl extension and anthocyanin synthesis, of these seedlings in terms of various parameters of the model. The results show that many of the main features of the high-irradiance response are correlated with Pfr level. However, a more detailed analysis leads to the conclusion that Pfr alone cannot account for some critical aspects of the response. In particular, it is necessary to hypothesize an additional 'active' component which is a function of the rate of photoconversion.