This PhD thesis focusses on the complex traits associated with root systems that have been neglected by breeders. However, breeders are becoming aware that what is happening below-ground is still a black box but could be of interest for developing resource efficient and resilient crops. This is also true for a crop like lettuce which could serve as a model of a crop which strongly depends on a continuous and abundant supply of water and nutrients.
To develop tools for breeders that are more practical than digging up roots, this research included masses of root and soil samples to be able to assess whether there is a good correlation between the genetic variation in root mass per soil layer and the amount of nitrogen and water left in these soil layers. This is a great achievement in science as these soil parameters are much easier and cheaper to measure in breeding programmes. This research also showed that the resource use efficiency is not a matter of having more roots per se, but that it is important to select for cultivars that are flexible in root formation at the right time and speed and in the right layers, when resource availability shows spatial and temporal variation. But you also proved that selection for below ground traits is more complex than we expected at the start of the project. Cultivars have different mechanisms to cope with resource shortage, and respond differently to different types of abiotic stress, especially when they are present in specific soil horizons. In case of nitrogen shortage, root formation will be triggered in the layers where more nitrogen is available. In case of drought stress, the lettuce will make more roots in the layer where water is in short supply. This thesis provided the conceptual base for an ecophysiological model as tool for breeders but more research needs to be done to realize a functional model that can be used for analysis of the genotype-by-environment interaction regarding below-ground resource capture. Pauline Kerbiriou has produced a very rich data set that can be further explored and mined (for example, matching above ground nitrogen accumulation with below-ground efficiency of capturing the nitrogen) but also provided clear suggestions for additional experiments to improve our insight, experiments that we plan to conduct in the near future.