Chlorophyll fluorescence during CAM-phases in Clusia minor L. under drought stress

Some species of Clusia show a high flexibility in regulating carbon uptake during the day-night cycles in response to environmental conditions. In thi s study, individuals of the C-3-CAM intermediate plant Clusia minor were su bjected to drought. The characteristics of chlorophyll fluorescence, gas ex change and organic acid content were investigated in individuals performing CAM under controlled laboratory conditions. The organic acid content incre ased after 16 d of drought, however, the malate/citrate ratio showed a 2.6- fold decrease. After 13 d of drought, phase IV of CAM was completely suppre ssed. The highest levels of non-photochemical quenching (measured as q(N) a nd NPQ) were observed on day 16. However, increased capacity to dissipate t he energy in excess to drive photosynthesis was not enough to maintain a lo w reduction state of the primary electron acceptor of photosystem II (measu red as 1-q(p)) at late afternoon under drought stress. Sustained decreases in predawn F-V/F-M ratio were observed even though organic acid accumulatio n increased after led without irrigation. Despite non-photochemical quenchi ng remaining high after rewatering, the decline in F-V/F-M ratio was relati vely rapidly reversible. Considering the partitioning of q(N) into its fast (q(F)) and slow (q(S)) components, it was observed that the proportion of the two components was dependent on both the number of days without waterin g and the different CAM phases.