Studies on Phytochrome Photoconversions in Vitro With Laser‐induced Optoacoustic Spectroscopy*

After excitation at room temperature with a 15 ns dye laser pulse, phytochrome (60 kDalton) from etiolated oat seedlings was studied by optoacoustic spectroscopy, which records the heat emission caused by non‐radiative deactivation processes of the photoexcited molecule. The action spectrum for the heat emitted by Pr deviates from the absorption spectrum around 610 and 695 nm, indicating the build‐up of photoproduct(s) within 15 ns after excitation. It is proposed that the 695 nm product is identical with I700 and lumi‐R known to be the first intermediate on the Pr× Pfr pathway, and that the photochemical back reaction of I700 to Pr occurs on the same time scale. About 90% of the absorbed light energy was lost by radiationless deactivation. Values for the rate constants of excited state deactivation of Pr and for the internal energy content difference of the primary photoproducts are calculated.