Exploitation of Natural Variation in Nitrogen Uptake and Metabolism to Develop Lettuce Cultivars with Improved Nitrogen Use Efficiency

Lettuce is an intensively managed crop requiring high quality water and nitrogen (N) to ensure the size and quality required by the market. Excess N applied through fertilizers can lead to leaching of nitrates into groundwater and the release of N₂O into the atmosphere and contributes significantly to greenhouse gases that are driving climate change. Improving nitrogen use efficiency (NUE) in lettuce will reduce the environmental impacts of lettuce production and ensure the economic sustainability of the lettuce industry in California. To identify the genetic and physiological basis of NUE in lettuce, we evaluated N uptake and N metabolism using N isotope discrimination of lettuce grown in the field under non-limiting and limited N. A recombinant inbred line (RIL) population from a cross of two domesticated lettuce types and a diversity panel of Lactuca species was used to identify QTL associated with N uptake, N metabolism and other measures of NUE. Numerous high-effect QTL were detected that were specifically associated with field N levels. NUE was evaluated over several years and the environmental variation was surprisingly high. The wide range in N metabolism within the RIL and among the different Lactuca species suggests different metabolic pathways or genes were utilized depending on N availability and genetic composition. These data are guiding the development of germplasm with improved NUE and to pinpoint the genes underlying the major effect QTL.