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The Role of Phytochrome in an Interaction with Ethylene and Carbon Dioxide in Overcoming Lettuce Seed Thermodormancy ¹

^a Department of Plant Sciences, University of California, Riverside, California 92502

Ethylene and CO₂ were used to control induction of germination in thermodormant lettuce seed (Lactuca sativa L.). These experiments ultimately showed that germination depends on the presence of an active form of the phytochrome. The phytochrome system is functional and stable at 35 C, a temperature which completely inhibits germination. Phytochrome responses to red or far red light and darkness showed that this inhibition of germination under light must be due to some other block(s) rather than to a direct inactivation of the phytochrome system itself. A postred radiation increase in lettuce seed germination that is not reversed by far red light was observed. The CO₂ requirement for C₂H₄ action is not due to a change in the medium's pH; addition of C₂H₄ plus CO₂ at the start of imbibition did not result in as much germination as when they were added several hours after imbibition. This reduction in germination, when the gases are added at the start of imbibiton, is due to CO₂.