BY

ii The root system is important for plant adaptability to changing environments. Although the importance of the root system is well reported in the research literature there is still a considerable lack of knowledge with regard to the development of root systems per se, and how root system complexity contributes to plant performance under abiotic and biotic stresses. In order to close this knowledge gap, we first investigated the genetic basis of embryonic and post-embryonic maize root systems complexity. We define root complexity as the number of root branching points per soil volume. In subsequent studies, we evaluated the change of maize roots under low and adequate nitrogen levels and determined relationships between root characteristics and agronomic traits. The use of segregating populations (Chapter 1) and a North Carolina III crossing design (Chapter 2) allowed us to map regions in the maize genome involved in the inheritance of maize root complexity. In addition, we were able to estimate for the first time quantitative genetic parameters that characterize maize root complexity. This study was only possible because we developed an image based high-throughput root evaluation system. The application of this evaluation system allowed us to determine root complexity measures with yet unparalleled accuracy. We determined the complexity of a root system by calculating its fractal dimension (FD). We assumed that roots with a higher FD value explore a larger portion of the soil profile than roots with a smaller FD value. In addition, we accessed for the embryonic roo