  We derive a compact expression for the Borel sum of a QCD amplitude in terms
of the inverse Mellin transform of the corresponding Borel function. The result
allows us to investigate the momentum-plane analyticity properties of the
Borel-summed Green functions in perturbative QCD. An interesting connection
between the asymptotic behaviour of the Borel transform and the Landau
singularities in the momentum plane is established. We consider for
illustration the polarization function of massless quarks and the resummation
of one-loop renormalon chains in the large-$\beta_0$ limit, but our conclusions
have a more general validity.
