Light responses, growth factors and phytochrome transformations of Cucumis seedling tissues

The elongation of hypocotyl segments cut from etiolated Cucumis sativus seedlings is not affected by a single red light exposure at the start of the 20-hour growth period, but is inhibited by brief exposures repeated each hour or two even though these give no greater total energy. The inhibition is annulled if each red exposure is followed by far-red. The time course of phytochrome transformations in this tissue after a single red light exposure, followed spectrophotometrically, shows no anomalous characteristics that might correlate with the unusual pattern of responsiveness to red light. In intact seedlings, hypocotyl elongation responds similarly, but the opening of the hypocotyl hook is saturated by a single initial red light treatment. Excised hypocotyl segments on water alone appear insensitive to repeated red light treatment, but the growth increments caused by the addition of potassium ion, 2-propanol or cobaltous ion, or by leaving the cotyledons attached, are all inhibited roughly 40%. However, continuous white light inhibits the entire growth increment, reducing elongation to that of the water controls. Some implications of these results for current hypotheses and future investigations on the mechanisms of growth regulation by light are discussed.