The variation in peak intensity of parametrically excited second-harmonic emissions, as a function of laser intensity, was measured from plasmas produced from planar, slab targets of carbon, aluminium and copper, using a 20 J-5 ns, p-polarized Nd:glass laser. At and beyond a laser intensity of 1014 W cm-2, the second-harmonic emission showed saturation behaviour in the cases of all three materials. The observed behaviour is attributed to the saturation of the convective parametric decay instability due to ion-nonlinear Landau damping of plasmons. The consequences of plasmon convection have also been considered. It is inferred that the harmonic generation occurs within the parametric decay instability region.