  Let $G$ be a linear algebraic group over an algebraically closed field $k$
and $\mathrm{Aut}_{\mathrm{alg}}(G)$ the group of all algebraic group
automorphisms of $G$. For every $\varphi\in \mathrm{Aut}_{\mathrm{alg}}(G)$ let
$\mathcal{R}(\varphi)$ denote the set of all orbits of the $\varphi$-twisted
conjugacy action of $G$ on itself (given by $(g,x)\mapsto gx\varphi(g^{-1})$,
for all $g,x\in G$). We say that $G$ has the algebraic $R_\infty$-property if
$\mathcal{R}(\varphi)$ is infinite for every $\varphi\in
\mathrm{Aut}_{\mathrm{alg}}(G)$. In \citep{bb} we have shown that this property
is satisfied by every connected non-solvable algebraic group. From a theorem
due to Steinberg it follows that if a connected algebraic group $G$ has the
algebraic $R_\infty$-property, then $G^\varphi$ (the fixed-point subgroup of
$G$ under $\varphi$) is infinite for all $\varphi\in
\mathrm{Aut}_{\mathrm{alg}}(G)$. In this article we show that the condition is
also sufficient. We also show that a Borel subgroup of any semisimple algebraic
group has the algebraic $R_\infty$-property and identify certain classes of
solvable algebraic groups for which the property fails.
