  Aims. In this paper we study the influence of the ionizing cluster mass on
the emission line spectrum of Hii regions in order to determine the influence
of low mass clusters on the integrated emission line spectra of galaxies.
Methods. For this purpose, we present a grid of photoionization models that
covers metallicities from Z = 0.001 to Z = 0.040, ages from 0.1 to 10 Ma (with
a time step of 0.1 Ma), and cluster initial masses from 1 to 107 Mo. The
stellar masses follow a Salpeter initial mass function (IMF) in an
instantaneous burst mode of star formation. We obtain power-law scale-relations
between emission-line luminosities and ionizing cluster masses from the grids
and we evaluate the dependences on the ionizing cluster mass for some line
luminosities, equivalent widths and line ratios. Results. Power-law
scale-relations are shown to be useful tools to obtain robust diagnostics, as
examples: (a) H?/H? ratio varies from the usually assumed value of 2.86, these
variations imply the existence of a lower limit to the attainable precision in
extinction estimations of ?E(B - V) ~ 0.1.; (b) EW(H?) is a good age indicator
with a small dependence on cluster mass, while EW([O iii] 5007) shows a
noteworthy mass dependence; (c) abundance estimations from R23 are practically
unaffected by variations of the cluster mass; (d) estimations from S 23 and ?'
would improve if the cluster mass dependences were considered and (e) [Oii]
3727/H? is a good star formation rate indicator for ages older than -4.5 Ma. We
also show that the ionizing cluster mass dependence explains why empirical
calibrations produce more reliable diagnostics of some emission lines than
photoionization models grids. Finally, we show preliminary results about the
contribution of low mass clusters (M < 104 Mo) to the integrated emission line
spectra of galaxies, which can be as high as 80% for some relevant lines.
