Time threshold for second positive phototropism is decreased by a preirradiation with red light.
A second positive phototropic response is exhibited by a plant after the time of irradiation has exceeded a time threshold.  The time threshold of dark-grown seedlings is about 15 minutes for Arabidopsis thaliana.  This threshold is decreased to about 4 minutes by a 669-nanometer preirradiation.  Tobacco (Nicotiana tabacum) seedlings show a similar response.  The time threshold of dark-grown seedlings is about 60 minutes for tobacco, and is decreased to about 15 minutes after a preirradiation with either 450- or 669- nanometer light.  The existence of a time threshold for second positive phototropism and the dependence of this threshold on the irradiation history of the seedling contribute to the complexity of the fluence response relationship for phototropism.
Janoudi A-K
Michigan State University-Department of Energy Plant Research Laboratory, East Lansing 48824.
Konjevic
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D017360:Arabidopsis / Q000254:growth & development / Q000502:physiology* / Q000528:radiation effects; D003624:Darkness; D018546:Hypocotyl / Q000254:growth & development / Q000502:physiology / Q000528:radiation effects; D008027:Light; D017440:Photoperiod; D018524:Phototropism / Q000502:physiology* / Q000528:radiation effects; D010947:Plants, Toxic; D013997:Time Factors; D014026:Nicotiana / Q000254:growth & development / Q000502:physiology* / Q000528:radiation effects