Oxygen abundance gradient as a function of the stellar mass and the D(4000) index for abundances derived either from the R calibration (top panels) or using the HCM method (bottom panels). Left panels: O/H gradient as a function of the stellar mass of a galaxy with colour-coded D(4000) for individual galaxies; crosses are median values in each mass bin. Right panels: median O/H gradient with 2σ CI in each mass bin for three sub-samples of galaxies with different ranges of the D(4000) index: 1.0–1.4 (black), 1.4–1.6 (blue), and 1.6–2.2 (red).