We provide new data on the mechanism of Noccaea caerulescens acclimation to Cd exposure by elucidating the process of photosystem II (PSII) acclimation by chlorophyll fluorescence imaging analysis. Seeds from the metallophyte N. caerulescens were grown in hydroponic culture for 12 weeks before exposure to 40 and 120 mu M Cd for 3 and 4 days. At the beginning of exposure to 40 mu M Cd, we observed a spatial leaf heterogeneity of decreased PSII photochemistry, that later recovered completely. This acclimation was achieved possibly through the reduced plastoquinone (PQ) pool signaling. Exposure to 120 mu M Cd under the growth light did not affect PSII photochemistry, while under high light due to a photoprotective mechanism (regulated heat dissipation for protection) that down-regulated PSII quantum yield, the quantum yield of non-regulated energy loss in PSII (Phi(NO)) decreased even more than control values. Thus, N. caerulescens plants exposed to 120 mu M Cd for 4 days exhibited lower reactive oxygen species (ROS) production as singlet oxygen (O-1(2)). The response of N. caerulescens to Cd exposure fits the 'Threshold for Tolerance Model', with a lag time of 4 d and a threshold concentration of 40 mu M Cd required for the induction of the acclimation mechanism.