MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) is a SDSS-IV survey that will obtain spatially resolved spectroscopy from 3600 AA to 10300 AA for a representative sample of over 10000 nearby galaxies. In this paper we present the analysis of nebular emission line properties in 14 galaxies obtained with P-MaNGA, a prototype of the MaNGA instrument. Using spatially resolved diagnostic diagrams we find extended star formation in galaxies that are centrally dominated by Seyfert/LINER-like emission, which illustrates that galaxy characterisations based on single fibre spectra are necessarily incomplete. We observe extended LINER-like emission (up to $rm 1 R_{e}$) in three galaxies. We make use of the $rm EW(H alpha)$ to argue that the observed emission is consistent with ionisation from hot evolved stars. We derive stellar population indices and demonstrate a clear correlation between $rm D_n(4000)$ and $rm EW(H delta_A)$ and the position in the ionisation diagnostic diagram: resolved galactic regions which are ionised by a Seyfert/LINER-like radiation field are also devoid of recent star formation and host older and/or more metal rich stellar populations. We also detect extraplanar LINER-like emission in two highly inclined galaxies, and identify it with diffuse ionised gas. We investigate spatially resolved metallicities and find a positive correlation between metallicity and star formation rate (SFR) surface density. We study the relation between N/O vs O/H on resolved scales. We find that, at given N/O, regions within individual galaxies are spread towards lower metallicities, deviating from the sequence defined by galactic central regions as traced by Sloan $3$ fibre spectra. We suggest that the observed dispersion can be a tracer for gas flows in galaxies: infalls of pristine gas and/or the effect of a galactic fountain.
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