Rottura di supersimmetria e correzioni di soglia in modelli di orientifold

String Theory, as a candidate unified theory of both gravitational and gauge interactions, must be able to reproduce the Standard Model predictions in its low-energy limit. This work has been mainly developped in the context of Type I theory, only consistent in ten space-time dimensions, which contains both oriented and unoriented superstrings, by means of the so-called orientifold projection, and describes a supersymmetric universe: in order to recover the four-dimensional, non supersymmetric observed space-time, different mechanisms concerning compactification of extra dimensions and supersymmetry breaking are investigated. In particular, this thesis presents a four-dimensional T^4/Z_2xT^2 model in whose framework supersymmetry is broken through two different approaches, Scherk-Schwarz mechanism and brane supersymmetry breaking. Moreover, one-loop threshold corrections to the gauge coupling constants of these theories are computed, on the basis of the background field method.