  We consider $2$-colourings $f : E(G) \rightarrow \{ -1 ,1 \}$ of the edges of
a graph $G$ with colours $-1$ and $1$ in $\mathbb{Z}$. A subgraph $H$ of $G$ is
said to be a zero-sum subgraph of $G$ under $f$ if $f(H) := \sum_{e\in E(H)}
f(e) =0$. We study the following type of questions, in several cases obtaining
best possible results: Under which conditions on $|f(G)|$ can we guarantee the
existence of a zero-sum spanning tree of $G$? The types of $G$ we consider are
complete graphs, $K_3$-free graphs, $d$-trees, and maximal planar graphs. We
also answer the question of when any such colouring contains a zero-sum
spanning path or a zero-sum spanning tree of diameter at most $3$, showing in
passing that the diameter-$3$ condition is best possible. Finally, we give, for
$G = K_n$, a sharp bound on $|f(K_n)|$ by which an interesting zero-sum
connectivity property is forced, namely that any two vertices are joined by a
zero-sum path of length at most $4$.
  One feature of this paper is the proof of an Interpolation Lemma leading to a
Master Theorem from which many of the above results follow and which can be of
independent interest.
