  It is proved that every series-parallel digraph whose maximum vertex-degree
is $\Delta$ admits an upward planar drawing with at most one bend per edge such
that each edge segment has one of $\Delta$ distinct slopes. This is shown to be
worst-case optimal in terms of the number of slopes. Furthermore, our
construction gives rise to drawings with optimal angular resolution
$\frac{\pi}{\Delta}$. A variant of the proof technique is used to show that
(non-directed) reduced series-parallel graphs and flat series-parallel graphs
have a (non-upward) one-bend planar drawing with $\lceil\frac{\Delta}{2}\rceil$
distinct slopes if biconnected, and with $\lceil\frac{\Delta}{2}\rceil+1$
distinct slopes if connected.
