Analysis of chlorophyll a fluorescence in C3 and CAM forms of Mesembryanthemum crystallinum

Comparisons of chlorophyll a fluorescence characteristics of C3 and CAM forms of Mesembryanthemum crystallinum were used to identify features of the photosynthetic mechanism associated with CAM. The reduction status, Q, was lower and predicted PSII activity higher in the C3 form than in the CAM form throughout the photoperiod. These differences were 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 this high proton gradient diminishes PSII activity and serves a protective 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 characteristics between the C3 and CAM forms also indicate modification of the energy transduction mechanisms of the CAM form possibly related to the increased overall demand for ATP in CAM photosynthesis. Total non-photochemical quenching was higher in the CAM form than in the C3 form. An inverse relationship between fast and slowly-relaxing components of non-photochemical quenching can be interpreted in terms of the changing demand for ATP in the different phases of CAM.