  We study the relations between luminosity and chemical abundance profiles of
spiral galaxies, using detailed models for the chemical and spectro-photometric
evolution of galactic discs. The models are ``calibrated'' on the Milky Way
disc and are successfully extended to other discs with the help of simple
``scaling'' relations, obtained in the framework of semi-analytic models of
galaxy formation. We find that our models exhibit oxygen abundance gradients
that increase in absolute value with decreasing disc luminosity (when expressed
in dex/kpc) and are independent of disc luminosity (when expressed in
dex/scalelength), both in agreement with observations. We notice an important
strong correlation between abundance gradient and disc scalelength. These
results support the idea of ``homologuous evolution'' of galactic discs.
