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 this study, individuals of the C3-CAM intermediate plant Clusia minor were subjected to drought. The characteristics of chlorophyll fluorescence, gas exchange and organic acid content were investigated in individuals performing CAM under controlled laboratory conditions. The organic acid content increased 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 suppressed. The highest levels of non-photochemical quenching (measured as qN and NPQ were observed on day 16. However, increased capacity to dissipate the energy in excess to drive photosynthesis was not enough to maintain a low reduction state of the primary electron acceptor of photosystem II (measured as 1-qp) at late afternoon under drought stress. Sustained decreases in predawn Fv/FM ratio were observed even though organic acid accumulation increased after 16 d without irrigation. Despite non-photochemical quenching remaining high after rewatering, the decline in FvFM ratio was relatively rapidly reversible. Considering the partitioning of qN into its fast (qF) and slow (qs) components, it was observed that the proportion of the two components was dependent on both the number of days without watering and the different CAM phases.