Date of Award

Spring 2006

Document Type

Restricted Thesis

Terms of Use

© 2006 Adam B. Roddy. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts


Biology Department

First Advisor

Rachel Merz

Second Advisor

Nicholas J. Kaplinsky


The success of a plant depends in part on its ability to locate water while securely anchoring in the soil. Cellular growth in roots is driven by water uptake and resulting increases in turgor pressure. Low water potentials slow the rate of primary root growth and induce a suite of changes in the growth zone. To better understand the effects of low water potentials on the growth and structure of primary roots, Zea mays seedlings were grown in different concentrations of polyethylene glycol solution. The effects of low water potentials on the water content, the spatial distribution of growth, and the mechanical properties of the root throughout the growth zone were quantified. Low water potentials decreased the total amount of water in roots and the percent water composition of roots. Low water potentials also decreased relative elemental growth rate and shifted the region of maximal growth rate towards the root tip. Because of water stress induced root thinning and cell wall stress relaxation at the cellular level, low water potentials were expected to alter the mechanical properties of roots. Engineering beam theory was adapted to test for water stress induced changes in the spatial distributions of flexural stiffness (EI), the product of the material term E (Young's modulus of elasticity) and the geometrical term I (second moment of area). Low water potentials decreased EI of roots, due mostly to root thinning (and thus reductions of I). Increasing water stress had no consistent effects on E, suggesting the existence of compensatory mechanisms to maintain root material properties despite cellular changes in wall properties due to water stress. Root thinning in low water potentials results from the promotion of longitudinal over lateral growth to facilitate finding deeper water sources. Though water stress weakens roots by reducing EI, soil impedance has been shown previously to cause root thickening. Soil impedance could thus offset root thinning by low water potentials and increase EI of roots by increasing I. Further studies should test the hypothesis that water stress and impedance have counterbalancing effects on root mechanical properties, thus promoting deep penetration of soils for water while maintaining the mechanical properties necessary for penetrating hard soils.