Polycrystalline aggregates lacking four independent systems for the glide of dislocations can deform in a purely viscoplastic regime only if additional deformation mechanisms are activated. Since this is the case of most minerals of the upper mantle (olivine, pyroxene, …), we anticipate that, besides dislocation creep that is responsible for the development of strongly pronounced lattice preferred orientations, other deformation mechanisms such as diffusion, grain boundary sliding, disclinations, … must be active in-situ. It has been established that such an accommodation mechanism completely controls the effective flow stress of the mantle rock. In this presentation, using a scale transition model that allows relating polycrystal and grain behaviors, we will show that they also have a huge influence of the effective stress sensitivity (stress exponent). For example if one consider that n=3.5 as for the slip of dislocation in individual grains, one cannot reach the commonly adopted value n=3.5 at the polycrystal scale, if the accommodation mechanism is linear (n=1) as for e.g. diffusion creep. This raises the question about possible deformation mechanisms active in-situ.