  We critically review the QCD predictions for the cross sections sigma_L and
sigma_T for diffractive rho meson electroproduction in longitudinally and
transversely polarised states in the HERA energy region. We show that both
perturbative and non-perturbative approaches which involve convolution with the
rho meson wave function predict values of sigma_T which fall-off too quickly
with increasing Q^2, in comparison with the data. We present a perturbative QCD
model based on the open production of light qqbar pairs and parton-hadron
duality, which describes all features of the data for rho electroproduction at
high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of
sigma_L/sigma_T. We find that precise measurements of the latter can give
valuable information on the Q^2 behaviour of the gluon distribution at small x.
