EXCITED STATE BEHAVIOR OF PHYTOCHROME Pr

The primary excited state processes of phytochrome, the plant chromoprotein responsible for most photomorphogenetic responses, are investigated by picosecond intensity-dependent transmission measurements. At sufficient high excitation intensity (starting at photon flux densities of about 1′1025 cm−2 s−l) a moderate bleaching of the absorption is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation Pr lumi-R and the photon transport through the sample was utilized. The following was found for the excited state parameters of the phytochrome Pr: (1) an excited state absorption of the same order of magnitude as the ground state absorption (cross section: Sexc= 4′10−16 cm−2 at 650 nm), (2) a fast relaxation of the higher excited singlet state on the femtosecond time scale.