Concerns about the medium-term evolution of phosphorus (P) resources and the impact of excessive P fertilization on the environment lead to the search for genotypes having a good P use efficiency. We evaluated the effect of P availability on the aerial growth and root architectural traits of 14 genotypes of the genus Lactuca, including wild species and ancient and modern cultivars. We identified substantial genotypic variation in most of the investigated traits. A limitation in P tended to increase the dominance of the taproot over the lateral roots. Despite a limited genotype x P interaction, we were able to observe compromises in the adaptations to low P of the different traits according to the genotypes. P use efficiency was mainly associated with traits related to the taproot. This pre-eminence of the taproot in the adaptation of Lactuca genotypes to low P contrasts with other species which rely more on lateral roots to adapt to P stress. Finally, by comparing total masses of observed and simulated root systems, we investigated the ability of the Archisimple root architecture model to represent the effects of genetic variability and P availability on lettuce root growth. A discussion is proposed regarding the experimental conditions required to compare the efficiency of highly contrasting genotypes, and about the root traits to be selected for a greater P use efficiency in modern varieties of lettuce.