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Effect Of Solution Density On Growth And Gravitropic Response Of Maize Roots.     Timothy J. Mulkey.  Life Science Dept., Indiana State University, Terre Haute, IN  47809.

   The ability of roots to perceive and respond to gravity has been investigated for many years.  The sensory mechanism for the detection of gravity in roots has been suggested as the sedimentation of amyloplasts in the root cap.  An alternative model suggests that plants sense the gravitational vector via a pressure differential exerted on the cell membrane.  Thus the entire cell, instead of specific organelles, functions as statoliths.  Elongation and gravitropic response by intact primary roots of Zea mays L. is examined under various solution densities.  Ten percent sucrose solution with 5% polyethylene glycol (PEG) inhibits elongation of roots by approximately 80% for 2 hrs after the root is submerged in the solution.  After 2 hr the elongation rate recovers to within 10% of the control rate.  Comparisons of roots in humid air (RH >98%), distilled water and sucrose/PEG solutions (up to 10%; after recovery) are made to determine the effect of solution density/protoplast buoyancy on gravicurvature.  Horizontally oriented roots in humid air exhibit 80±5° positive curvature within 90 min.  Horizontally-oriented roots in distilled water exhibit weak positive gravicurvature (40±15°) after 3 hr.  Horizontally oriented roots in 10% sucrose/PEG solutions exhibit no significant curvature after 8 hrs.  Microscopic examination of the root cap indicate that in all treatments the amyloplast sediment normally.  These results support the hypothesis that roots respond to gravity by sensing gravitational pressures exerted by the protoplast.