ANALYSIS OF CHLOROPHYLL-A FLUORESCENCE IN C-3 AND CAM FORMS OF MESEMBRYANTHEMUM-CRYSTALLINUM

Comparisons of chlorophyll a fluorescence characteristics of C-3 and C AM forms of Mesembryanthemum crystallinum were used to identify featur es of the photosynthetic mechanism associated with CAM. The reduction status, Q, was lower and predicted PSII activity higher in the C-3 for m than in the CAM form throughout the photoperiod. These differences w ere particularly pronounced during the first hour of illumination when non-photochemical quenching attributable to the intrathylakoid proton gradient was also at its highest in the CAM form. It is argued that t his high proton gradient diminishes PSII activity and serves a protect ive role against photoinhibition at a time in the CAM cycle when both CO2 concentration within the leaf, and carbon cycle enzyme activation levels are likely to be low. Differences in fluorescence characteristi cs between the C-3 and CAM forms also indicate modification of the ene rgy transduction mechanisms of the CAM form possibly related td the in creased overall demand for ATP in CAM photosynthesis. Total non-photoc hemical quenching was higher in the CAM form than in the C-3 form. An inverse relationship between fast and slowly-relaxing components of no n-photochemical quenching can be interpreted in terms of the changing demand for ATP in the different phases of CAM.