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CHARACTERIZATION OF NEGATIVE PHOTOTROPISM IN PRIMARY ROOTS OF MAIZE.  M.L. Evans¹, C. Wolverton¹, J.L. Mullen², H. Ishikawa¹, R. Hangarter^{2   1}Dept of Plant Biology, The Ohio State University, Columbus and ²Department of Biology, Indiana University, Bloomington.

     Using customized video analysis software, we measured the kinetics and spatial distribution of the negative phototropic response of primary roots of maize (Zea mays, L. cv Merit).  We also scanned the root for zones of phototropic sensing using fiber optics for localized application of light.  Application of broad beam unilateral white light to the root resulted in negative phototropism following a latent period of 39 ± 8 min.  The zone of differential growth driving curvature was located approximately 4-6 mm from the tip, within the central elongation zone (CEZ).  Application of continuous unilateral blue (450 nm) light also caused a negative phototropic curvature in the CEZ.  The threshold for blue-light-induced negative phototropism was approximately 0.01 µmol m^-2 s^-1, with the response saturating near 10 µmol m^-2 s^-1.  Roots from which the cap had been surgically removed continued to grow vigorously but failed to show a phototropic response.  Localized unilateral illumination of the cap of intact roots induced negative phototropism while localized illumination of either the distal elongation zone (2 mm behind the root tip) or the CEZ (4 mm behind the root tip) did not induce curvature.  Our findings indicate that the root cap is the site of photoperception for negative root phototropism and that the responding cells are located remote from this site, within the central elongation zone.

     (Supported by NASA grants NAG5‑6385 and NAG2-1411)