Five drops of the cell suspensions treated as described above were pipetted on pieces of filter paper and incubated in darkness for 20 min. A minimum of 7 parallel measurements was performed. During this incubation the cells were kept moist in the sample holders. PS II activity was expressed as Fv/Fm (variable over maximum fluorescence as an introduced parameter) and the shape of the fluorescence induction curves was used to demonstrate the differences in fast energy conversion after the treatments in comparison to the controls, as has been described in detail by