In-vitro binding of phytochrome to a particulate fraction: A function of light dose and steady-state P fr level.

The binding of phytochrome to a particulate fraction in extracts from hypocotyl hooks of etiolated Cucurbita pepo L. seedlings has been examined as a function of the light dose and P fr level established in vitro. As the steady-state level of P fr transiently established in the 500×g supernatant is increased, so the level of P r subsequently pelletable at 20 000×g increases up to a saturation level. Increasing both the time and irradiance parameters of the light dose while holding the steady-state P fr level constant, results similarly in increasing P r pelletability. This agrees with results obtained previously with in-vivo irradiations of maize coleoptiles. Thus, like the in-vivo response, phytochrome binding in vitro appears to be a function of the total number of molecules converted to the P fr form during the irradiation period.