The pivotal role played by radial electric fields in the development of turbulence associated with anomalous transport is examined by means of global gyrokinetic simulations. It is shown that the stabilizing effect of E x B flows on ion temperature gradient (ITG) modes is quadratic in the shearing rate amplitude. For a given shearing rate it leads to an increase in the critical gradient. The electric fields (zonal flows) self-generated by ITG modes interact in a nonlinear way and it is shown that a saturated level of both the zonal flow and ITG turbulence is reached in the absence of any collisional mechanism being included in the model. The quality of the global nonlinear simulations is verified by the energy conservation which is allowed by the inclusion of nonlinear parallel dynamics. This demonstrates the absence of spurious damping of numerical origin and thus confirms the nonlinear character of zonal flow saturation mechanism.