  Three pomerons are known to couple via the gluon loop and interactions of
this kind which are responsible for high-mass diffraction are described in
terms of the gluonic structure function of the QCD pomeron. We show that the
triple-pomeron coupling via the light quark loop associated with the
``valence'' in the pomeron is of the same strength as the purely gluonic
coupling. The large $Q^2$ behavior of this new contribution is described by the
DLLA evolution from the non-perturbative $f\bar f$ valence state of the
pomeron. Numerical estimates of the high-mass diffraction structure functions
based on the consistent account of both couplings are in good agreement with
experimental data.
