  The influence of hydrostatic pressure up to P=1.05 GPa on resistivity R,
excess conductivity ${\sigma}'(T)$ and pseudogap ${\Delta}*(T)$ is studied in
slightly doped single crystals of YBa2Cu3O7-${\delta} (Tc=49.2 K, {\delta}~
0.5)$. The critical temperature Tc is found to increase with increasing
pressure at a rate dTc/dP = 5.1 K/GPa, while R(300)K decreases at a rate d(ln
R)/dP = 0.19% 1/GPa. Near Tc, independently on pressure, the ${\sigma}'(T)$ is
well described by the Aslamasov-Larkin and Hikami-Larkin fluctuation theories,
demonstrating a 3D-2D crossover with increase of temperature. The crossover
temperature To determines the coherence length along the c-axis ${\xi}c(0) \sim
3.43 A$ at P=0, which is found to decrease with increasing P. At the same time,
both, ${\Delta}*$ and the BCS ratio $2{\Delta}*/kTc$ increase with increasing
hydrostatic pressure at a rate $d(ln {\Delta}*)/dP= 0.36 1/GPa$, implying an
increase of the coupling strength with increasing P. At low temperatures below
Tp, the shape of the ${\Delta}*(T)$ curve is found to be almost independent on
pressure. At high temperatures, the shape of the ${\Delta}*(T)$ curve changes
noticeably with increasing P, suggesting a strong influence of pressure on the
lattice dynamics. This unusual behavior is observed for the first time.
