  We present the Fundamental Plane (FP) of early-type galaxies in the clusters
of galaxies RXJ1415.1+3612 at z=1.013. This is the first detailed FP
investigation of cluster early-type galaxies at redshift z=1. The distant
cluster galaxies follow a steeper FP relation compared to the local FP. The
change in the slope of the FP can be interpreted as a mass-dependent evolution.
To analyse in more detail the galaxy population in high redshift galaxy
clusters at 0.8<z<1, we combine our sample with a previous detailed
spectroscopic study of 38 early-type galaxies in two distant galaxy clusters,
RXJ0152.7-1357 at z=0.83 and RXJ1226.9+3332 at z=0.89. For all clusters
Gemini/GMOS spectroscopy with high signal-to-noise and intermediate-resolution
has been acquired to measure the internal kinematics and stellar populations of
the galaxies. From HST/ACS imaging, surface brightness profiles, morphologies
and structural parameters were derived for the galaxy sample. The least massive
galaxies (M=2x10^{10}M_{\sun}) in our sample have experienced their most recent
major star formation burst at z_{form}~1.1. For massive galaxies
(M>2x10^{11}M_{\sun}) the bulk of their stellar populations have been formed
earlier z_{form}>~1.6. Our results confirm previous findings by Jorgensen et
al. This suggests that the less massive galaxies in the distant clusters have
much younger stellar populations than their more massive counterparts. One
explanation is that low-mass cluster galaxies have experienced more extended
star formation histories with more frequent bursts of star formation with
shorter duration compared to the formation history of high-mass cluster
galaxies.
