The international consensus on Medicago truncatula as a model system has lead to the development of powerful approaches for dissecting the genetic and molecular bases of legume nitrogen nutrition. However, such approaches now come up against a poor knowledge of the phenotypic traits that should be used for the large-scale screening of the genotypic variability associated with nitrogen nutrition. This issue was unravelled in a previous report, in which an ecophysiological approach allowed a better understanding of the relationships between plant nitrogen nutrition and plant growth traits, for the model symbiotic association between M. truncatula cv. Jemalong and Rhizobium meliloti strain 2011. From this analysis, phenotypic traits were identified as potentially relevant for the large-scale screening of the genotypic variability. Here, by the phenotyping of a recombinant inbred lines population, we show that the proposed methodology provides a valuable support for assisting the detection of genetic variants affected for nitrogen uptake. Especially, the relative expansion rate of plant leaf area is identified as a good proxy for ranking genotypes according to their ability to uptake nitrogen in given environmental conditions. As leaf area can be measured non-destructively, such finding should pave the way for a more efficient evaluation of the genotypic variability.