Actton Spectra of Phytochrome in Vitro*

Summary First‐order rate constants, K λ , for the photochemical conversions of both forms of phytochrome, P R and P FR , were measured at various wavelengths between 300 and 800 nm. The product of the extinction coefficient, ε, and the quantum yield, ø, at any wave‐length, λ, could be determined from the value of K λ and the mole fractions of P R and P FR present at the photostationary state set by λ. The determination of the action (εø) spectra required that the photostationary states in red and far‐red light be known. These were determined from the absorption spectra and kinetic data.