Assessing Genetic Variation in Lettuce for Traits Related to Nitrogen-Use-Efficiency Using Susceptibility and Relative Efficiency Indices.

Lettuce has a market value of ~2.4 billion dollars and ranks as one of the top three vegetable commodities in the USA. California is the largest producer of lettuce and currently faces problems of nitrate leaching into ground water from agricultural land. We conducted field experiments to assess the genetic variation of lettuce in traits related to nitrogen-use-efficiency (NUE). We utilized indices to measure the relative efficiency of biomass production and the susceptibility to stress due to low nitrogen conditions. These indices were associated with the amount of biomass production per nitrogen content. The results from a field experiment in 2011 suggested that the relative efficiency index of a plant could explain 92% of the variation of the biomass a lettuce variety produced given the same nitrogen content in the plant (intercept). The relative efficiency index explained less of the variation in biomass per nitrogen content (slope) in the plant (R² = 0.64). The susceptibility index explained less of variation for the intercept (R² = 0.10) compared to the slope (R² = 0.32). In 2012, some of these results were reproducible using a subset of the lettuce varieties analyzed in 2011. The regressions of the relative efficiency indices were significant for the intercept but not for the slope. The regressions of the susceptibility indices were not significant for the intercept or slope. An ideal lettuce variety would have a big value for the intercept yet a small value for the slope. The lettuce variety Costa Rica No. 4 demonstrated the most favorable combination of traits. These data suggest that there is useful genetic variation in traits that affect NUE in lettuce. The indices can be used as a proxy for the efficiency of a line to produce biomass and its susceptibility to be stressed due to low nitrogen availability.