EXCITED-STATE BEHAVIOR OF PHYTOCHROME P-R

The primary excited state processes of phytochrome, the plant chromopr otein responsible for most photomorphogenetic responses, are investiga ted by picosecond intensity-dependent transmission measurements. At su fficient high excitation intensity (starting at photon flux densities of about 1 x 10(25) cm(-2) s(-1)) a moderate bleaching of the absorpti on is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation P-r-->lumi-R and the photon transport th rough the sample was utilized. The following was found for the excited state parameters of the phytochrome P-r: (1) an excited state absorpt ion of the same order of magnitude as the ground state absorption (cro ss section: sigma(exc) = 4 x 10(-16)cm(-2) at 650 nm), (2) a fast rela xation of the higher excited singlet state on the femtosecond time sca le.