Understanding the genetic basis for phenotypic variation is a central question in modern biology with wide-ranging application from medical research (e.g. why some people get cancer) to plant breeding (e.g. why are some crop species more resistant to pathogen infection). Plants stay in one place and unlike animals can not move away from unfavourable growth conditions. Consequently plants have developed sophisticated adaptation and defence mechanisms to cope with external conditions such as extreme temperatures or lack of water. Conventional methods for finding and characterizing genes involved in defence responses often rely on creating mutant versions of genes that are non-functional. Although this has been an extremely successful strategy in the past, most mutant screens are at this stage saturated and new procedures will be required to further understand plant responses to the external environment. A complementary approach is to use natural variation between individuals of the same species to find genes important for adaptation and survival. Plant science as such is currently in a revolutionary phase, mostly because of the adaptation of new technological developments in molecular biology including microarray analysis and next generation sequencing technology. The objective of this application is a three fold approach based on quantitative trait loci mapping to better understand plant adaptation to environmental stress conditions. The natural genetic diversity in Arabidopsis thaliana ecotypes will be used to map, identify and characterize genes and gene networks that function in determining ozone responses, important antioxidants and adaptation to extreme climate conditions. Due to consumer resistance to genetically modified organisms this research will open the possibility to breed for improved agronomical traits.