  We considered two sequences of spiral galaxies with different shapes of the
radial gas-phase oxygen abundance distributions from the galaxies in the MaNGA
survey: (1) Galaxies in which the gradient is well approximated by a single
linear relation across the whole disc, that is, galaxies with an S (slope)
gradients, (2) galaxies in which the metallicity in the inner region of the
disc is at a nearly constant level and the gradient is negative at larger
radii, that is, galaxies with level-slope (LS) gradients. We also selected
galaxies with a nearly uniform oxygen abundance across the whole galaxy, that
is, galaxies with level (L) gradients that can be the final evolutionary stage
of the two galaxy sequences described above. The radial nitrogen abundance
distributions in galaxies with LS oxygen abundance distributions also show
breaks at radii smaller than the O/H distribution breaks. The observed
behaviour of the oxygen and nitrogen abundances with radius in these galaxies
can be explained by the time delay between the nitrogen and oxygen enrichment
together with the variation in the star formation history along the radius.
These galaxies clearly show the effect of the inside-out disc evolution model.
We find that the shape of the radial abundance distribution in a galaxy is not
related to its macroscopic characteristics (rotation velocity, stellar mass,
isophotal radius, and star formation rate). The correlations between the
gradient slopes and macroscopic characteristics of galaxies are weak in the
sense that the scatter of the points in each diagram is large. We also examined
the properties of the Milky Way in the context of the considered galaxy
samples.
