Fig. 2a and b. Top : The slope of the relation in dependence on the slope, , of the IMF in the range . Bottom : The element ratio at as a function of the upper stellar mass contributing to the ISM enrichment via SN II for . The solid curves are for the SN II yields from Tsujimoto et al. (1995 b), the dashed curves for the yields from Woosley & Weaver (1995).