... We applied microscopy-pulse amplitude modulation chlorophyll fluorometer (Microscopy PAM, Walz, Effeltrich, Germany) combined with an Axiovert 25 CFL inverted microscope (Carl Zeiss, Germany) to assess photosynthesis at the subcellular level by chlorophyll fluorescence using the saturation pulse method. ... When the weak measuring light is switched on, the minimal fluorescence yield, Fo, is assessed which is characteristic for the dark-adapted state with all PSII reaction centers (RC) being open. ...