  The physics of nuclear reactions in stellar plasma is reviewed with special
emphasis on the importance of the velocity distribution of ions. Then the
properties (density and temperature) of the weak-coupled solar plasma are
analysed, showing that the ion velocities should deviate from the Maxwellian
distribution and could be better described by a weakly-nonexstensive
(|q-1|<0.02) Tsallis' distribution. We discuss concrete physical frameworks for
calculating this deviation: the introduction of higher-order corrections to the
diffusion and friction coefficients in the Fokker-Plank equation, the influence
of the electric-microfield stochastic distribution on the particle dynamics, a
velocity correlation function with long-time memory arising from the coupling
of the collective and individual degrees of freedom. Finally, we study the
effects of such deviations on stellar nuclear rates, on the solar neutrino
fluxes, and on the pp neutrino energy spectrum, and analyse the consequences
for the solar neutrino problem.
