  Spin-changing collisions may lead under proper conditions to the parametric
amplification of matter waves in spinor Bose-Einstein condensates. Magnetic
dipole-dipole interactions, although typically very weak in alkaline atoms, are
shown to play a very relevant role in the amplification process. We show that
these interactions may lead to a strong dependence of the amplification
dynamics on the angle between the trap axis and the magnetic-field orientation.
We analyze as well the important role played by magnetic-field gradients, which
modify also strongly the amplification process. Magnetic-field gradients must
be hence carefully controlled in future experiments, in order to observe
clearly the effects of the dipolar interactions in the amplification dynamics.
