  The pressure dependence of the magnetic penetration depth in polycrystalline
samples of YBa2Cu3Ox with different oxygen concentrations x = 6.45, 6.6, 6.8,
and 6.98 was studied by muon spin rotation (muSR). The pressure dependence of
the superfluid density (p_s) as a function of the superconducting transition
temperature Tc is found to deviate from the usual Uemura line. The ratio
(dTc/dP)/(dp_s/dP) is factor of 2 smaller than that of the Uemura relation. In
underdoped samples, the zero temperature superconducting gap and the BCS ratio
both increase with increasing external hydrostatic pressure, implying an
increase of the coupling strength with pressure. The relation between the
pressure effect and the oxygen isotope effect on the magnetic penetration depth
is also discussed. In order to analyze reliably the muSR spectra of samples
with strong magnetic moments in a pressure cell, a special model was developed
and applied.
