  We study the one-dimensional spin-1/2 model with nearest and
next-to-nearest-neighbor couplings exposed to a homogeneous magnetic field
$h_{3}$ and a dimer field with period $q$ and strength $\delta$. The latter
generates a magnetization plateau at $M=(1-q/\pi)/2$, which evolves with
strength $\delta$ of the perturbation as $\delta^{\epsilon}$, where
$\epsilon=\epsilon(h_{3},\alpha)$ is related to the $\eta$-exponent which
describes the critical behavior of the dimer structure factor, if the
perturbation is switched of ($\delta=0$). We also discuss the appearance of
magnetization plateaus in ladder systems with $l$ legs.
