Effects of root architecture, plasticity, and tradeoffs on water and phosphorus acquisition in heterogeneous environments

by Ho, Melissa Deanne.

The primary objective of my dissertation research was to investigate the relationship between root architecture form and function for resource acquisition under suboptimal environmental conditions, particularly looking at the interaction of phosphorus deficiency and water stress. My specific goals were: 1) to examine the importance of root architectural plasticity for adaptation to low phosphorus environments, and 2) to examine the tradeoffs limiting root architectural plasticity to phosphorus availability, specifically drought and competition. My research consisted of evaluating parent genotypes and recombinant inbred lines of common bean (Phaseolus vulgaris L.) having contrasting root architecture traits in three unique environments: 1) in a stratified water and phosphorus system in the greenhouse, 2) in the field at Zamorano, Honduras, and 3) in a theoretical economic modeling environment. Genetic variation for root architectural plasticity was shown to exist in common bean. Results from the greenhouse and field experiments showed that root architectural tradeoffs do exist for water and phosphorus acquisition in heterogeneous environments, where shallow rooted genotypes were better adapted to low phosphorus environments and deep rooted genotypes were better adapted to terminal drought environments. Optimization modeling of an individual plant supported the experimental data. Theoretical modeling of a population of plastic and nonplastic plants showed that both drought and competition can limit the adaptive advantage of plasticity. More experimental work needs to be conducted examining the adaptive importance of root architectural plasticity, as well as the effect of root plasticity on interplant competition. The overall outcome of this work will contribute to a greater understanding of the fundamental physiological mechanisms of environmental stress tolerance by crop plants. Understanding the role of root architecture and its functional significance for environmental adaptation will aid in the development of bean genotypes that have greater productivity in low-input subsistence agricultural systems. It will also result in the development of genotypes that have a greater resource acquisition efficiency, which will ultimately reduce the need for excessive, high-input agricultural practices, a leading cause of extensive soil and water quality degradation worldwide. iii