  Turaev defined a function on the first homology of a rational homology
3-sphere $Y$ as the minimal rational Seifert genus of all knots in this
homology class. Ni and the first author discovered a lower bound of this
function using the Heegaard Floer $d$-invariant and showed that Floer simple
knots are rational Seifert genus minimizers. In this paper, we give a simple
reproof of the above results. We then define a version of rational slice genus
for knots in the product 4-manifold $Y\times I$ and investigate the analogous
minimal genus problem. We prove the same lower bound in terms of the
$d$-invariant formula and the same genus minimizers given by Floer simple
knots.
