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A previous method for handling bound states in QCD is briefly revisited.Taking advantage of the Feynman-Schwinger representation for the iterated quarkpropagator in an external field, it is possible to give closed representationsfor certain appropriate (second order) two point and four point Greenfunctions, H^(2)(x-y) and H^(4)(x_1,x_2,y_1,y_2), as path integrals on quarkworld lines. Then, starting from reasonable assumptions on the Wilson linecorrelators, a Bethe-Salpeter equation for H^(4) and a Dyson-Schwinger equationfor H^(2) can be obtained, which are consistent with the Goldstone theorem inthe chiral limit. Such equations are too complicate to be solved directly.However, a reduced Salpeter equation can be derived which is tractable and hasbeen applied to a calculation of the meson spectrum. The results are in generalgood agreement with the data, but with the important exceptions of the lightpseudo scalars (that are related to the breaking of the chiral symmetry). Inthis scenario two important improvements can be introduced: a) the fixedcoupling constant can be replaced by a running coupling constant\alpha_{s}(Q^2) appropriately modified in the infrared region; b) the fixedmass in the reduced equation can be replaced for light quarks by an effectivemass depending on the momentum of the particle, as suggested by the form of theDS equation. Then even the light pseudo scalar mesons can be made to agree withto their experimental value.Comment: 20 pages, 3 figures. This is an expanded version of the proceeding of  the talk given at the "International Conference on Color Confinement and  Hadrons in Quantum Chromodynamics (Confinement 2003)", Riken,Tokyo, July  21-24, 200

LIGHT MESONS AND INFRARED BEHAVIOR OF THE RUNNING COUPLING CONSTANT IN QCD