Gribov's theory of light quark confinement implies the existence of two kinds of scalar bound states. The phase diagram of the three-flavor QCD is mapped out in the (mπ - mK)–plane with help of the SUL(3) × SUR(3) linear sigma model supplemented with the assumption that the masses of the so-called superbound scalars do not change under the variation of the pion and kaon mass. The phase boundary along the mπ = mK line is found in the interval 15 MeV < m_crit < 25 MeV, irrespective which f0 - σ linear combination is identified with the pure superbound state.
