  We investigate the contributions from the $3\sigma_{g}$ and $1\pi_{u}$ and
molecular orbitals in high-order harmonic generation in $N_{2}$, with
particular emphasis on quantum-interference effects. We consider both the
physical processes in which the electron is freed and returns to the same
orbital, and those in which it is ionized off one orbital and recombines with
the other. We show that the quantum-interference patterns observed in the
high-order harmonic spectra are predominantly determined by the $3\sigma_{g}$
orbital. This holds both for the situation in which only the $1\pi_{ux}$
orbital is considered, and the dynamics of the electron is restricted to the
plane $p_{x}p_{z},$ or in the full three-dimensional case, if the azimuthal
angle is integrated over and the degeneracy of $1\pi_{u}$ is taken into
account.
