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Gravity Effects on Growth Form of Brassica rapa L. and Arabidopsis thaliana (L.) Heyn.    J. Allen¹, A. Kuang², M. Regan³, and M.E. Musgrave¹.  ¹Department of Plant Science, University of Connecticut, Storrs, CT, ²Biology Department, University of Texas Pan American, Edinburg, TX, ³Bionetics Corp., Kennedy Space Center, FL

   To investigate effects of hypergravity on plant growth and development, we conducted a series of experiments on large-diameter centrifuges at NASA Ames Research Center during summer and fall 2006 using experiment conditions from STS-87 (B-STIC) and STS-68 (CHROMEX-5).  Brassica rapa L. cv. Astroplants and Arabidopsis thaliana (L.) Heyhn. var. Columbia were grown for 16 or 11 days, respectively, inside plant growth chambers mounted in powered swinging buckets or in stationary units.  Responses of the plants grown in the hypergravity treatments (2-g and 4-g) were compared with the 1-g controls.  For purposes of growth form analysis, Brassica plants were photographed daily, during short-duration rotor stops, while Arabidopsis plants were photographed only at the beginning and the end of the 11-d continuous runs. Stem angles were measured using Image-J software.  At both 2-g and 4-g, Brassica plants assumed a more horizontal growth form as their exposure to hypergravity progressed, with a ~ 45° plant angle after 16 days at 4-g.  A significant but less dramatic effect of g-force on growth form was seen in Arabidopsis (~25° from vertical at 4-g).  Growth form changed less at 2-g than at 4-g.  In Brassica, stem diameter increased in both hypergravity treatments.    Also in Brassica, stem lengths/heights were not significantly different in hypergravity. The results will be discussed in the context of gravitropic set point angle and how g-force, across its continuum, affects plant growth form.  (Supported by NASA grant NAG10-329.)