Panel a: central oxygen abundance as a function of rotation velocity for our sample of MaNGA galaxies. Depending on their angular size, the number of fibres covering the galaxies varied during the observations with the integral field units employed by MaNGA. The differing numbers of fibres are indicated by different symbols. Panel b: same as panel a but for the oxygen abundances at the isophotal radius. Panel c: central oxygen abundance as a function of stellar mass M_sp. Again the different numbers of fibres are indicated by different symbols. Panel d: same as panel c but for the oxygen abundances at the isophotal radius. The value of the M_sp for the galaxy measured with 37 fibres is not available.