  Most Galactic Globular Clusters (GCs) harbour multiple populations of stars
(MPs), composed of at least two generations: the first characterized by a
"standard" $\alpha$-enhanced metal mixture, as observed in field halo stars of
the Milky Way, and the second displaying anti-correlated CN--ONa chemical
abundance pattern in combination with an enhanced helium fraction. Adequate
collections of stellar spectra are needed to characterize the effect of such
stellar abundance changes on the integrated light of GCs. We present a grid of
synthetic stellar spectra covering the atmospheric parameters relevant to old
stellar populations at four subsolar metallicities and two abundance patterns,
representative of first- and second-generations of stars in GCs. Integrated
spectra of populations were computed using our stellar grid and empirical
stellar populations, namely, colour-magnitude diagrams from literature for
Galactic GCs. The spectra range from 290 to 1000nm, where we measured the
effect on several spectrophotometric indices due to the surface abundance
variations attributed to MPs. We find non-negligible effects of the MPs on
spectroscopic indices sensitive to C, N, Ca, or Na, and on Balmer indices; we
also describe how MPs modify specific regions in the near-UV and near-IR that
can be measured with narrow or medium photometric passbands. The effects vary
with metallicity. A number of these changes remain detectable even when
accounting for the stochastic fluctuations due to the finite nature of the
stellar population cluster.
