One difficulty when analyzing the determinants at the origin of plant phenotypic differences is that measured plant traits are frequently integrative: they result from the integration of a large number of physiological processes under the control of genetic and environmental factors. In a previous report, we demonstrated that dissecting integrative traits into simpler components using a simple crop physiology model was a valuable method for detecting quantitative trait loci (QTL) related to the nitrogen nutrition for a recombinant inbred lines population of Medicago truncatula. Here, using the same data set, we demonstrate the relevance of decomposing integrative traits for understanding biological differences among phenotypes, independently of QTL detection. Two examples are given to demonstrate that the dissection of integrative traits (i.e., plant leaf area and nitrogen nutrition index) into variables representing the efficiency of the plant to extract and valorize (carbon and nitrogen) resources is an effective method to determine the stream of physiological events that leads to the final phenotype.