  This paper investigates the peculiar behaviour of the light even
(alpha-elements) and odd atomic number elements in red giants in the galactic
bulge, both in terms of the chemical evolution of the bulge, and in terms of
possible deep-mixing mechanisms in these evolved stars. Using UVES on VLT, we
measure the abundances of the four light elements O, Na, Mg and Al in 13 red
clump and 40 red giant branch stars in four fields spanning the bulge from b=-3
to -12\deg. We show that the resulting abundance patterns point towards a
chemical enrichment dominated by massive stars at all metallicities. O, Mg and
Al ratios with respect to iron are overabundant with respect to both galactic
disks (thin and thick) for [Fe/H]$>-0.5$. A formation timescale for the
galactic bulge shorter than for both the thin and thick disks is therefore
inferred. Using Mg as a proxy for metallicity (instead of Fe) we further show
that: (i) the bulge stars [O/Mg] ratio follows and extend to higher
metallicities the decreasing trend of [O/Mg] found in the galactic disks. (ii)
the [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three
distinct sequences in the thin disk, the thick disk and the bulge. The bulge
trend is well represented by the predicted metallicity-dependent yields of
massive stars, whereas the galactic disks have too high Na/Mg ratios at low
metallicities, pointing to an additional source of Na from AGB stars. (iii)
Contrary to Na, there appears to be no systematic difference in the [Al/Mg]
ratio between bulge and disk stars, and the theoretical yields by massive stars
agree with the observed ratios, leaving no space for AGB contribution to Al.
