Background and Aims :The dynamical system of plant growth GreenLab was originally developed for individual plants, without explicitly taking into account inter-plant competition for light. Inspired by the competition models developed in the context of forest science, we propose to adapt the method of crown projection onto the x-y plane to GreenLab, in order to study the effects of density on resource acquisition and on architectural development. Materials and methods : The empirical production equation of GreenLab is extrapolated to the stand level by computing the exposed photosynthetic foliage area of each plant. The computation is based on the combination of Poisson models of leaf distribution for all the neighbour plants, in the overlapping crown projection surfaces. To study the effects of density on architectural development, we link the proposed competition model to the model of interaction between functional growth and structural development introduced by Mathieu (2006). Key Results and Conclusions : The model is applied to determine the production of field crops according to density and its validity is discussed by comparing it to the classical equation of field crop production given by Howell and Musick (1985). The application of the model to trees leads us to derive a new general equation of resource acquisition. It is used to illustrate the expression of plant plasticity in response to competition for light. Density strongly impacts tree architectural development through interactions with the source-sink balances during the growth. Well-known properties of forest stands are reproduced concerning the effects of density on tree height and radial growth.