  Spinor ultracold gases in one dimension represent an interesting example of
strongly correlated quantum fluids. They have a rich phase diagram and exhibit
a variety of quantum phase transitions. We consider a one-dimensional spinor
gas of bosons with a large spin $S$. A particular example is the gas of
chromium atoms (S=3), where the dipolar collisions efficiently change the
magnetization and make the system sensitive to the linear Zeeman effect. We
argue that in one dimension the most interesting effects come from the pairing
interaction. If this interaction is negative, it gives rise to a
(quasi)condensate of singlet bosonic pairs with an algebraic order at zero
temperature, and for $(2S+1)\gg 1$ the saddle point approximation leads to
physically transparent results. Since in one dimension one needs a finite
energy to destroy a pair, the spectrum of spin excitations has a gap. Hence, in
the absence of magnetic field there is only one gapless mode corresponding to
phase fluctuations of the pair quasicondensate. Once the magnetic field exceeds
the gap another condensate emerges, namely the quasicondensate of unpaired
bosons with spins aligned along the magnetic field. The spectrum then contains
two gapless modes corresponding to the singlet-paired and spin-aligned unpaired
bose-condensed particles, respectively. At T=0 the corresponding phase
transition is of the commensurate-incommensurate type.
