Natural variation of water use and water productivity in Arabidopsis thaliana

Plant performance under reduced water availability has traditionally been assessed as drought resistance and more recently as water use efficiency (WUE). An extensive body of work has been established over the past 15 years where the natural variation of water use efficiency has been studied in the model species Arabidopsis thaliana (Arabidopsis). At the same time, a substantial degree of criticism has arisen with respect to the use of drought resistance and WUE as measures of plant performance, due to the lack of relatedness of these parameters to reproductive performance, i.e. yield. The work in this thesis is centered on understanding the physiological and genetic basis of water use and water productivity as alternative measures of plant performance under the context of reduced water availability. The first part of this study describes an extensive assessment of the natural variation of water use and water productivity in Arabidopsis in relation to numerous key physiological, phenological, and developmental parameters. Furthermore, this work concisely relates plasticity of key traits to historical climatic variation. A fundamental aspect of this work was the clarification that it is possible to estimate long term water use to a high degree of accuracy based on short term water use, i.e. soil drying rate, and flowering time. Flowering time was demonstrated to be the predominant driver of vegetative performance and water use, however it appeared to be genetically uncoupled from reproductive performance. This is in contrast to previous work that suggests WUE, measured as the ratio of C12 to C13 isotopes (δ13C), is positively associated with flowering time. Additionally, it was demonstrated that multiple commonly employed proxies of reproductive performance including total biomass, WUE, and flowering time, were not sufficient at predicting seed yield in Arabidopsis across multiple environments. The second part of this study involved the genetic dissection of water use and productivity related traits in Arabidopsis through a quantitative trait loci (QTL) mapping study and a genome wide association study (GWAS). QTL mapping using a recombinant inbred line (RIL) population developed from the ecotypes Col-0 and C24 revealed two key flowering time genes, FLOWERING LOCUS C (FLC) and FRIGIDA (FRI), as key regulators of water use. It was demonstrated that a combination of non-functional alleles of both FLC and FRI reduced long term water use via a shorted life cycle, which is again in contrast to previous work relating to the genetic dissection of WUE in Arabidopsis. Crucially, it was observed that reduced water use mediated in this fashion did not detrimentally impact upon reproductive performance. GWAS was employed subsequent to the QTL mapping in order to identify candidate genes underlying the variation for productivity as a unique trait and also as a factor of water use, i.e. water productivity. GWAS identified multiple promising candidate genes that potentially underlie the heritable genetic variation for flowering time, water use, and water productivity.