Chlorophyll fluorescence and organic acid oscillations during transition from CAM to C3-photosynthesis in Clusia minor L. (Clusiaceae)

In species of Clusia, switching from C3-photosynthesis (C3-PS) to crassulacean acid metabolism (CAM) may be a means of optimizing water use, plant carbon balance and photon utilization during periods of stress. We ask whether, in perennial species of Clusia, the switch from CAM back to C3-PS is also of ecophysiological significance. Our objective was to investigate the performance of C. minor L. during a short-term shift from CAM to C3-PS. During the transition from CAM to C3-PS, nocturnal malate and citrate accumulation decreased whereas CO2 uptake increased during the daytime. However, after 7 d, marked nocturnal accumulation of citrate and 24 h CO2 uptake occurred. In contrast to C3-like photosynthesis, a pronounced reduction in the effective quantum yield of photosystem II, deltaF/F'(m), together with a sharp increase in non-photochemical quenching were observed during CAM at the beginning and end of the day. After 7 d, integrated CO2 uptake over 24 h approximately doubled; however, water use efficiency was reduced three-fold due to increased rates of daytime transpiration. Despite very similar maximum deltaF/F'(m) between CAM and C3-PS, the results suggest that daily photon utilization increases when there is unrestricted CO2 uptake directly from the atmosphere. Thus, under well-watered conditions and with increased rates of transpiration, C. minor performing C3-like photosynthesis may overcome the limitations of the storage capacity of the vacuole for overnight organic acid accumulation, improving its daily carbon balance.