  We measure the evolution of the rest-frame K-band Fundamental Plane from z=1
to the present by using IRAC imaging of a sample of early-type galaxies in the
Chandra Deep Field-South at z~1 with accurately measured dynamical masses. We
find that $M/L_K$ evolves as $\Delta\ln{(M/L_K)}=(-1.18\pm0.10)z$, which is
slower than in the B-band ($\Delta\ln{(M/L_B)}=(-1.46\pm0.09)z$). In the B-band
the evolution has been demonstrated to be strongly mass dependent. In the
K-band we find a weaker trend: galaxies more massive than
$M=2\times10^{11}M_{\odot}$ evolve as $\Delta\ln{(M/L_K)}=(-1.01\pm0.16)z$;
less massive galaxies evolve as $\Delta\ln{(M/L_K)}=(-1.27\pm0.11)z$. As
expected from stellar population models the evolution in $M/L_K$ is slower than
the evolution in $M/L_B$. However, when we make a quantitative comparison, we
find that the single burst Bruzual-Charlot models do not fit the results well,
unless large dust opacities are allowed at z=1. Models with a flat IMF fit
better, Maraston models with a different treatment of AGB stars fit best. These
results show that the interpretation of rest-frame near-IR photometry is
severely hampered by model uncertainties and therefore that the determination
of galaxy masses from rest-frame near-IR photometry may be harder than was
thought before.
