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Rapid Initiation of Thymidine Incorporation into Deoxyribonucleic Acid in Vegetative Tobacco Stem Segments Treated with Indole-3-acetic Acid ¹

^a Department of Biological Sciences, University of Maryland Baltimore County, Catonsville, Maryland 21228

The short term effect of 11.4 µM indoleacetic acid on the incorporation of (methyl-³H)thymidine into DNA in vegetative tobacco (Nicotiana tabacum cv. Wis. 38) stem segments has been investigated. In segments that are defoliated, inverted, and kept in the dark for 7 hours, indoleacetic acid very rapidly (about 60 minutes) and strikingly initiates thymidine incorporation into DNA. The time required before enough indoleacetic acid (2.8 µM) to enhance thymidine incorporation moves into a segment has been found to be about 35 minutes. The initiation response time for segment tissue that already contains 2.8 µM indoleacetic acid should be no more than about 25 minutes. The rate of labeled thymidine incorporation into DNA is affected by physiological treatments of segments. Moving segments from the light into the dark or defoliating segments or inverting defoliated segments decreases the rate of thymidine incorporation. For segments given all three treatments, indoleacetic acid restores the rate of thymidine incorporation as compared to controls. Darkness, or defoliation or inversion of segments, therefore, may decrease thymidine incorporation into DNA by effecting reduced auxin levels in stem segments.