Date of Award

Spring 8-1-2009

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

William J. Brett

Second Advisor

James Hughes

Third Advisor

Timothy Mulkey

Abstract

The mechanism by which plants sense gravity is not fully understood. The statolith theory states that plants sense gravity based on the sedimentation of starch granules called statoliths. However, not all plants contain statoliths. The hydrostatic model was proposed to explain gravitropic sensing in systems which lack statoliths. This model states that the gravity sensor in a plant cell is the entire protoplast; gravitropic sensing depends on the buoyancy of the protoplast relative to the density of its external environment. The ability of maize roots to sense gravity when grown in media of differing densities is examined in these studies. Primary roots of maize with a length of about 1 em were used. The roots were placed in environments of various densities using air, oxygenated water, sucrose/polyethylene glycol (PEG), and Ficoll PM 400 (polysucrose). The rates of growth and curvature were monitored using digital recordings. Roots grown in air show growth rates comparable to roots grown in oxygenated water, but roots grown in air show greater curvatures compared to roots grown in water medium. Sucrose solutions proved unsuitable; they produced erratic gravitropic responses in roots related to detrimental effects of high osmolarity. Roots show no significant curvature in a 1 0% sucrose solution with 5% polyethylene glycol (PEG). Elongation of the roots is inhibited for approximately 2 hrs, but the elongation rate recovers to within 1 0% of the control rate. Substituting Ficoll for sucrose eliminates some of the interfering effects of high osmolarity on elongation and decreases gravitropic curvature. Altering the environmental density does not alter sedimentation of statoliths within the root tip. In conclusion, the hydrostatic model is supported with these data. When the density of the external media was changed, the gravitropic response of roots was changed.

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