  Ordering and quantum fluctuations of orbital degrees of freedom are studied
theoretically for LaVO$_3$ in spin-C-type antiferromagnetic state. The
effective Hamiltonian for the orbital pseudospin shows strong one-dimensional
anisotropy due to the negative interference among various exchange processes.
This significantly enhances the instability toward lattice distortions for the
realistic estimate of the Jahn-Teller coupling by first-principle LDA+$U$
calculations, instead of favoring the orbital singlet formation. This explains
well the experimental results on the anisotropic optical spectra as well as the
proximity of the two transition temperatures for spin and orbital orderings.
