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Kinetic separation of phototropism from blue-light inhibition of stem elongationThese experiments tested the hypothesis that phototropic bending arises when a light gradient across the stem differentially inhibits cell elongation because of direct inhibition of cell elongation by light (the Blaauw hypothesis). Continuous irradiation of dark-grown cucumber seedlings (Cucumis sativus L.) with unilateral blue light inhibited hypocotyl elongation within 30 s, but did not induce phototropic curvature until 4.5 h after the start of irradiation. Marking experiments showed that curvature began simultaneously at the top and bottom of the growing region. In situ measurements of the light gradient across the stem with a glass fiber optic indicated that a 5- to 6-fold difference in fluence rate was established on the two sides of the stem. The light gradient established at the start of irradiation was the same as that after 6 h of irradiation. Changes in gravitropic responsiveness during this period were also ruled out. Calculations show that the light gradient should have caused curvature which would be detectable within 30 to 60 min and which would extrapolate to the start of irradiation--if the Blaauw hypothesis were correct. The long lag for phototropism in this case indicates that rapid inhibition of cell elongation by blue light does not cause the asymmetrical growth of phototropism. Rather, phototropism is superimposed upon this separate light growth response.
Document ID
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Cosgrove, D. J.
(University Park 16802 United States)
Date Acquired
August 21, 2013
Publication Date
January 1, 1985
Publication Information
Publication: Photochemistry and photobiology
Volume: 42
Issue: 6
ISSN: 0031-8655
Subject Category
Life Sciences (General)
Distribution Limits
NASA Discipline Number 40-30
NASA Discipline Plant Biology
Non-NASA Center
NASA Program Space Biology

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