Date of Award

Spring 1-1-2012

Document Type


Degree Name

Master of Science (MS)

First Advisor

Franck Vernerey

Second Advisor

Ronald Y. S. Pak

Third Advisor

Mark Stoykovich


The utilization of the surrounding environment is a distinct feature that plants harness to sustain equilibrium and continual maintenance. Plants undergo rapid changes and various stresses that add to an already complex system consisting of elaborate functions. Phototropism involves the activation of photoreceptive pigments, cell proteins and plant hormones through light reception which brings about curvature. Gravitropism is induced through the sedimentation of starch sacs that activate cells and allows for curvature response to gravity. Both processes are vital to the plant and can result in various structural responses. Recent research points to a specific plant hormone specified as auxin as the catalyst for differential growth and curvature.

An extensive analysis reveals the exact mechanisms behind phototropism and gravitropism from the molecular level to the global level. A plant experiencing a phototropic response utilizes specific pigments that differentiate the spectra of light. These protein based pigments localize to the plasma membrane of the plant cells and induces a state of phosphorylation. More proteins are activated and begin to interact with the naturally occurring auxin. Auxin then moves laterally across the different cell layers of the plant and produces curvature. Gravitropism follows a similar pattern but it is the endodermis of the plant that recognizes gravity. There are distinct organelles within the vacuoles of the cell that sediment in response to the vector of gravity. Upon sedimentation, an interaction with the cell's cytoskeleton takes place and proteins are again activated. Interaction with auxin occurs resulting in lateral movement that initiates curvature.

Cell expansion is the result of the interaction between auxin and the cell. Internal turgor pressure and cell wall compliance are the two main factors involved in cell expansion. A model is created with the relationship of auxin and cell expansion in terms of the different tropisms. In the model, the stem of a plant is considered and assumptions of curvature are made. Experimental data is gathered and comparisons with the model are drawn. The model is a substantial starting point but has room for improvement. The proper curvature response is simulated and potential applications are explored.