Abstract. The incorporation of some genetic and physiological processes in a developmental model of Hordeum vulgare L. (barley) is presented. The model exhibits different hierarchical scales and has been conceived as a Relational Growth Grammar (RGG). RGG is a new formalism that has been developed as an extension of L-Systems and implemented using a new modelling language, eXtended Lsystems (XL). Models written in XL can be executed using the interactive, Java-based modelling platform GroIMP, which has been developed for this purpose.The barley model proper is a set of morphogenetic rules. These are combined with a set of rules representing a metabolic network simulating some key steps of the biosynthesis of gibberellic acid (GA 1 ). The transport of GA 1 and GA 19 (a metabolic precursor of GA 1 ) along the developing simulated structure has also been foreseen. In the model, the local concentrations of GA 1 in a module induce the elongation of internodes. In an extension of this base model, called "BarleyBreeder", individual virtual specimens are chosen interactively from a population, followed by the simulation of reproduction. Both genotype and phenotype of the population are visualized. So far, the model is restricted to a few Mendelian genes, one of which (the dwarfing gene Zeocrithon) is directly interacting with the biosynthesis of GA 1 .