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Aims. In this paper we study the influence of the ionizing cluster mass onthe emission line spectrum of Hii regions in order to determine the influenceof low mass clusters on the integrated emission line spectra of galaxies.Methods. For this purpose, we present a grid of photoionization models thatcovers metallicities from Z = 0.001 to Z = 0.040, ages from 0.1 to 10 Ma (witha time step of 0.1 Ma), and cluster initial masses from 1 to 107 Mo. Thestellar masses follow a Salpeter initial mass function (IMF) in aninstantaneous burst mode of star formation. We obtain power-law scale-relationsbetween emission-line luminosities and ionizing cluster masses from the gridsand we evaluate the dependences on the ionizing cluster mass for some lineluminosities, equivalent widths and line ratios. Results. Power-lawscale-relations are shown to be useful tools to obtain robust diagnostics, asexamples: (a) H?/H? ratio varies from the usually assumed value of 2.86, thesevariations imply the existence of a lower limit to the attainable precision inextinction estimations of ?E(B - V) ~ 0.1.; (b) EW(H?) is a good age indicatorwith a small dependence on cluster mass, while EW([O iii] 5007) shows anoteworthy mass dependence; (c) abundance estimations from R23 are practicallyunaffected 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. Wealso show that the ionizing cluster mass dependence explains why empiricalcalibrations produce more reliable diagnostics of some emission lines thanphotoionization models grids. Finally, we show preliminary results about thecontribution of low mass clusters (M < 104 Mo) to the integrated emission linespectra of galaxies, which can be as high as 80% for some relevant lines.

On emission-line spectra obtained from evolutionary synthesis models II. Scale-relations and the estimation of mass dependences