Motivated by several recent data, we test the QCD spectral sum rules (QSSR) predictions based on different proposals (qq, qqqq, and gluonium) for the nature of scalar mesons. In the I = 1 and 1/2 channels, the unusual wrong splitting between the a0(980) and κ(900) and the a0(980) width can be understood from QSSR within aqq assignement. However, none of theqq andqqqq results can explain the large κ width, which may suggest that it can result from a strong interference with non-resonant backgrounds. In the I = 0 channel, QSSR and some low-energy theorems (LET) require the existence of a low mass gluonium σB(1 GeV) coupled strongly to Goldstone boson pairs which plays in the U (1)V channel, a similar role than the η ′ for the value of the U (1)A topological charge. The observed σ(600) and f0(980) mesons result from a maximal mixing between the gluonium σB andqq(1 GeV) mesons, a mixing scheme which passes several experimental tests. OZI violating J/ψ → φπ + π − , Ds → 3π decays and J/ψ → γS glueball filter processes may indicate that the f0(1500), f0(1710) and f0(1790) have significant gluonium component in their wave functions, while the f0(1370) is mostlyqq. Tests of these results can be provided by the measurements of the pure gluonium η ′ η and 4π specific U (1)A decay channels.