  Many models of quintessence predict a time variation of the fundamental
constants as well as a composition-dependent gravity like long-range force
mediated by the cosmon. We present bounds for the cosmon coupling to matter and
radiation within a grand unified framework. The unification scale, the unified
gauge coupling and the Fermi scale are allowed to vary independently. We find
that the variation of the weak scale compared to the nucleon mass is severely
restricted. The violation of the equivalence principle turns out to be
substantially larger than in models where only the electromagnetic fine
structure constant varies with time. We also show that in contrast to gravity
the local cosmon field in a condensed object does not decouple from the
cosmological evolution. In consequence, the cosmon interaction constitutes a
possible quantitative link between cosmological observations and several areas
of high precision experiments concerning the local time or space variation of
couplings and tests of the equivalence principle.
