Convective line asymmetries in the optical spectrum of two metal-poor stars, Gmb 1830 and HD 140283, are compared to those observed for solar metallicity stars. The line bisectors of the most metal-poor star, the subgiant HD 140283, show a significantly larger velocity span that the expectations for a solar-metallicity star of the same spectral type and luminosity class. The enhanced line asymmetries are interpreted as the signature of the lower metal content, and therefore opacity, in the convective photospheric patterns. These findings point out the importance of the three-dimensional convective velocity fields in the interpretation of the observed line asymmetries in metal-poor stars and, in particular, urge caution when deriving isotopic ratios from observed line shapes and shifts using one-dimensional model atmospheres. The mean line bisector of the photospheric atomic lines is compared with those measured for the strong Mg I b1 and b2 features. The upper parts of the bisectors are similar, and-assuming they overlap-the bottom ends of the stronger lines, which are formed higher in the atmosphere, go much farther to the red. This is in agreement with the expected decreasing of the convective blueshifts in upper atmospheric layers, and is compatible with the high-velocity redshifts observed in the chromosphere, transition region, and corona in late-type stars.