Photocontrol of Hypocotyl Elongation in Light-Grown Cucumis sativus L. : Responses to Phytochrome Photostationary State and Fluence Rate.

The effects of the calculated photostationary state of phytochrome (phi(c)) and the photon fluence rate on the elongation growth of the hypocotyl of light-grown seedlings of Cucumis sativus L. are examined. Two threshold responses to phi(c) are found at values of 0.06 and 0.43. At phi(c) = 0.06, there is no response at any fluence rate. In the phi(c) range 0.1 to 0.43, elongation growth does not respond to changes in phi(c). Above the second threshold (phi(c) = 0.43), there is a strong response to changes in phi(c). At all values of phi(c) at and above 0.1, there is a response to fluence rate. A linear relationship can be demonstrated between a factor comprised of the logarithm of phytochrome cycling rate (a fluence-rate-dependent process) and phi(c), and the growth response.