Introduction - Phenotypic plasticity - the ability of one genotype to produce different phenotypes depending on growth conditions - is a core aspect of the interactions between plants and their environment. For instance, leaf traits define the ability of plants to capture light, as well as their exposition and responses to various signals and stresses. In turn, leaf traits such as dimensions, composition and mass are highly regulated by growth conditions. The explicit description of shoot architecture in functional-structural models (FSPM) open new possibilities to express these feedback loops, which regulate plant fitness and productivity. However, formalizing into models the processes that build the plastic responses of traits to growth conditions is a major bottleneck to date. Most FSPMs that address the coupling between resources availability and growth consider only carbon (C) and drive resource allocation by sink priorities defined from empirical relations. Besides, the determinism of traits such as the areal density, which links mass growth with dimension growth, are poorly understood, so that these traits are frequently approximated as constant, while they have been shown to vary widely with growth conditions. Finally, the lack of process-based formalisms of existing models impairs our ability to simulate morphogenesis under contrasting growth conditions. As a step toward more mechanistic approaches to simulate shoot morphogenesis, we propose a plantscale FSPM of C and nitrogen (N) economy of the growing grass in which the morphogenesis is fully integrated with the plant metabolism. Model description - The model represents the plant as a collection of tillers made of several growing and mature shoot phytomers (identifying lamina, sheath and internode mature tissues and growth zones), a single roots compartment and a shared pool mimicking the phloem. Each compartment has a structural mass and concentrations in mobile and storage metabolites. The plant is seen