Breeding Lettuce for Improved Water and Nitrogen Use Efficiency

Lettuce crops require high quality water and relatively high nitrogen fertilizer inputs.  Competing demands for a diminishing water supply and environmental impacts of nitrogen fertilizers have increased the pressure to use less of these resources.  To map quantitative trait loci (QTL) associated with water use efficiency (WUE) and nitrogen use efficiency (NUE), a lettuce recombinant inbred line (RIL) population was grown in fields under a control or deficit water/nitrogen regime.  Control plots received irrigation to replace 130% of reference crop evapotranspiration (ETo) and 100% nitrogen (168 kg/ha) while stress plots received water to replace 75% of ETo and nitrogen at 50% of control levels.  Carbon and nitrogen isotope discrimination were used as proxies for WUE and nitrogen metabolism, respectively.  At market maturity, fresh weight, leaf carbon and nitrogen content were obtained.  Three major QTL associated with 13C isotope discrimination  were mapped, with the largest effect accounting for 24% of the observed variation.  A single QTL was associated with nitrogen content and another QTL associated with 14N isotope discrimination (nitrogen metabolism).  Each of these QTL accounted for 17% of the variation observed.  Environmental influence and treatment effects on fresh weight were highly variable across production years.  Environmental variation for WUE and NUE was significant, especially NUE.  Controlled experiments of N uptake using gel plates were useful in eliminating environmental effects, clarified N metabolism, and led to identification of QTL associated with N metabolism, root growth and WUE that differed from that observed in the field.