  We present ALMA observations of two dense gas tracers, HCN(1-0) and
HCO$^{+}$(1-0), for three galaxies in the green valley and two galaxies on the
star-forming main sequence with comparable molecular gas fractions as traced by
the CO(1-0) emissions, selected from the ALMaQUEST survey. We investigate
whether the deficit of molecular gas star formation efficiency (SFE$_{\rm
mol}$) that leads to the low specific star formation rate in these green valley
galaxies is due to a lack of dense gas (characterized by the dense gas fraction
$f_{\rm dense}$) or the low star formation efficiency of dense gas (SFE$_{\rm
dense}$). We find that SFE$_{\rm mol}$ as traced by the CO emissions, when
considering both star-forming and retired spaxels together, is tightly
correlated with SFE$_{\rm dense}$ and depends only weakly on $f_{\rm dense}$.
The specific star formation rate (sSFR) on kpc scales is primarily driven by
SFE$_{\rm mol}$ and SFE$_{\rm dense}$, followed by the dependence on $f_{\rm
mol}$, and is least correlated with $f_{\rm dense}$ or the dense-to-stellar
mass ratio ($R_{\rm dense}$). When compared with other works in the literature,
we find that our green valley sample shows lower global SFE$_{\rm mol}$ as well
as lower SFE$_{\rm dense}$ while exhibiting similar dense gas fractions when
compared to star-forming and starburst galaxies. We conclude that the star
formation of the 3 green valley galaxies with a normal abundance of molecular
gas is suppressed mainly due to the reduced SFE$_{\rm dense}$ rather than the
lack of dense gas.
