  Bar-induced gas inflows towards the galaxy centres are recognized as a key
agent for the secular evolution of galaxies. One immediate consequence is the
accumulation of gas in the centre of galaxies where it can form stars and alter
the chemical and physical properties. We use a sample of nearby face--on disc
galaxies with available SDSS spectra to study whether the properties of the
ionised gas in the central parts (radii <~0.6-2.1 kpc) of barred galaxies are
altered by the presence of a bar, and whether the bar effect is related to bar
and/or parent galaxy properties. The distributions of all parameters analysed
are different for barred and unbarred galaxies, except for the R23 metallicity
tracer and the oxygen abundance (from photoionisation models). The median
values point towards (marginally) larger dust content, star formation rate per
unit area, electron density and ionisation parameter in the centres of barred
galaxies than in the unbarred counterpart. The most remarkable barred/unbarred
difference appears in the [NII]6583/Ha line ratio, which is on average ~25%
larger in barred galaxies, due to a larger N/O in the centres of these
galaxies. We observe an enhancement of the central gas differences in
later-type galaxies or galaxies with less massive bulges. However the bar seems
to have a lower impact on the central gas properties for galaxies with more
massive bulges (M_bulge > 10^10 M_sun) or galaxies with total stellar mass
above ~ 10^10.8 M_sun. In conclusion, we find observational evidence that the
presence of a galactic bar affects the central ionised gas properties of disc
galaxies, where the most striking effect is an enhancement in the N/O abundance
ratio, which can be qualitatively interpreted as due to a different origin or
evolutionary processes for less and more massive bulges, with the gaseous phase
of the former having currently a closer relation with bars.
