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We study the acceleration of heavy nuclei at SNR shocks when the process ofionization is taken into account. Heavy atoms ($Z_N >$ few) in the interstellarmedium which start the diffusive shock acceleration (DSA) are never fullyionized at the moment of injection. The ionization occurs during theacceleration process, when atoms already move relativistically. For typicalenvironment around SNRs the photo-ionization due to the background galacticradiation dominates over Coulomb collisions. The main consequence of ionizationis the reduction of the maximum energy which ions can achieve with respect tothe standard result of the DSA. In fact the photo-ionization has a timescalecomparable to the beginning of the Sedov-Taylor phase, hence the maximum energyis no more proportional to the nuclear charge, as predicted by standard DSA,but rather to the effective ions' charge during the acceleration process, whichis smaller than the total nuclear charge $Z_N$. This result can have a directconsequence in the prediction of the knee structure of the cosmic ray spectrum.Moreover the acceleration of ultra-heavy elements beyond the Iron's maximumenergy is very hard to achieve making unlikely their possible contribution tothe cosmic ray spectrum in the transition region between Galactic andextragalactic component.

Ionization and maximum energy of nuclei in shock acceleration theory