  We use oxygen and argon abundances for planetary nebulae (PNe) with low
internal extinction (progenitor ages of (>4.5 Gyr) and high extinction
(progenitor ages <2.5 Gyr), as well as those of the H II regions, to constrain
the chemical enrichment and star formation efficiency in the thin and thicker
discs of M31. The argon element is produced in larger fraction by Type Ia
supernovae (SNe) than oxygen. We find that the mean log(O/Ar) values of PNe as
a function of their argon abundances, 12 + log(Ar/H), trace the inter-stellar
matter (ISM) conditions at the time of birth of the M 31 disc PN progenitors.
Thus the chemical enrichment and star formation efficiency information encoded
in the [alpha/Fe] vs. [Fe/H] distribution of stars is also imprinted in the
oxygen-to-argon abundance ratio log(O/Ar) vs. argon abundance for the nebular
emissions of the different stellar evolution phases. We propose to use the
log(O/Ar) vs. (12 + log(Ar/H)) distribution of PNe with different ages to
constrain the star-formation histories of the parent stellar populations in the
thin and thicker M31 discs. For the inner M31 disc (R_{GC} < 14 kpc), the
chemical evolution model that reproduces the mean log(O/Ar) values as function
of argon abundance for the high- and low-extinction PNe requires a second
infall of metal poorer gas during a gas-rich (wet) satellite merger. In M31,
the thin disc is younger and less radially extended, formed stars at a higher
star formation efficiency, and had a faster chemical enrichment timescale than
the more extended, thicker disc. Both the thin and thicker disc in M31 reach
similar high argon abundances ( 12 + log(Ar/H) ) ~ 6.7. The chemical and
structural properties of the thin/thicker discs in M31 are thus remarkably
different from those determined for the Milky Way thin and thick discs.

Astrophysics of Galaxies