  Although the possibility of a low energy cut-off (LEC) of the relativistic
electron population (E_{min} ~ 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 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 the kiloparsec scale relativistic jets of
quasars.
