A common fluence threshold for first positive and second positive phototropism in Arabidopsis thaliana.
The relationship between the amount of light and the amount of response for any photobiological process  can be based on the number of incident quanta per unit time (fluence rate-response) or on the number of incident  quanta during a given period of irradiation (fluence-response).  Fluence-response and fluence rate-response relationships have been measured for second positive phototropism by seedlings of Arabidopsis thaliana.  The fluence-response relationships exhibit a single limiting threshold at about 0.01 micromole per square meter when measured at fluence rates from 2.4 x 10(-5)  to 6.5 x 10(-3) micromoles per square meter per second.  The threshold values in the  fluence rate-response curves decrease with increasing time of irradiation, but show a common  fluence threshold at about  0.01 micromole per square meter.  These thresholds  are the same as the threshold of about 0.01 micromole per square meter measured for first positive phototropism.  Based on these data, it is suggested that second positive curvature has a threshold in time of about 10 minutes.  Moreover, if the times of irradiation exceed the time threshold, there is a single limiting fluence threshold at about 0.01 micromole per square meter.  Thus, the limiting fluence  threshold for second positive phototropism is the same as the fluence threshold for first positive phototropism.  Based on these data, we suggest that this common fluence threshold for first positive and second positive phototropism is set by a single photoreceptor pigment system.
Janoudi
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Michigan State University, Department of Energy Plant Research Laboratory, East Lansing 48824.
Poff
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KL
D017360:Arabidopsis / Q000254:growth & development / Q000502:physiology* / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D008027:Light; D017785:Photons; D018524:Phototropism / Q000502:physiology* / Q000528:radiation effects; D010834:Phytochrome; D013997:Time Factors
D010834:Phytochrome