  We investigate the spectral flow of the integrable non-hermitian Heisenberg
spin chain under boundary conditions with complex twist angle. It is shown that
the period of the spectral flow is $4\pi$ up to a certain critical imaginary
twist, beyond which the period jumps successively to higher values. We argue
that this phenomenon caused by non-hermitian properties of the system is
closely related to the metal-insulator transition caused by non-hermitian
hoppings for the one-dimensional insulator.
