Upper panel: adopted N yield as a function of the progenitor mass. For reference, new nucleosynthesis results are attached (circles: Ekström et al. 2008; triangles: Hirschi 2007). Middle panel: N/O evolution predicted by the models with a Salpeter IMF (x =  −1.35; blue curve) and a flatter one (x =  −1.1; red curve). The symbols of the observed data are the same as in Fig. 1. Lower panel: N/O evolution calculated with the models in which the upper mass of the IMF ends at 25 M_⊙ (red curve) and 20 M_⊙ (blue curve).