Although the possibility of a low-energy cut-off ( LEC ) of the relativistic electron population ( E _ { min } \sim 0.1 GeV ) in the different components of radio galaxies has been discussed in the literature , from both theoretical and observational viewpoints , its possible role in causing a distinct spectral flattening , and thus reducing the apparent brightness temperature of the radio nucleus , has thus far not been explicitly discussed . Here we point out that such an effect may in fact be quite significant for the parsec-scale , superluminal emission regions associated with the inner radio jets . This is because the observed frequency of the spectral turnover arising from the LEC increases linearly with the bulk Doppler factor ( \delta ) of the jet flow , whereas the frequency of synchrotron self-absorption ( the well-known cause of spectral flattening and turnover at low frequencies ) shows a mild decrease as \delta rises . For an observationally relevant range of source parameters , we investigate here the role of the LEC . We argue that some statistical trends revealed by the recent very long baseline interferometry ( VLBI ) surveys can in fact be understood in terms of effects arising from LEC , including the faster superluminal motion found in the VLBI surveys at higher frequencies , and the apparent inability of most radio cores to even attain the brightness temperatures consistent with the equipartition condition . We also point out some possible implications of the LEC for X-ray observations of kiloparsec scale relativistic jets of quasars .