Crassulacean acid metabolism (CAM) is a photosynthetic pathway characterized by nocturnal CO₂ uptake.  Some plants in water-limited environments utilize CAM because it increases the amount of carbon assimilated relative to water lost.  Many plant species exhibit facultative CAM, in which they switch between CAM and C3 photosynthesis, depending on drought and other abiotic conditions.  The objective of this study was to observe CAM photosynthesis in a California coastal dune succulent, Dudleya spp. (Crassulaceae).  Due to the significant water costs associated with flowering, we hypothesized that flowering individuals of Dudleya would assimilate relatively more carbon at night than non-flowering individuals.  To test this hypothesis, we measured instantaneous photosynthesis of paired flowering and non-flowering plants in the field during the day and night.  We also measured the carbon isotopic composition (δ¹³C) of plants to assess water-use efficiency and quantified nocturnal acid accumulation via titration.   
Results/Conclusions

We found that flowering and non-flowering plants had different patterns of CO₂ uptake.  While flowering plants took up ~70% (±23%) of their CO₂ at night, non-flowering plants growing under the same soil moisture conditions acquired only ~20% (±7%) of their carbon at night (Paired t-test P < 0.05).  Even during the rainy season, several months after flowering had finished, these relative differences in nocturnal carbon assimilation were maintained (Paired t-test P < 0.01).  Accordingly, δ¹³C values revealed that flowering plants had slightly higher water-use efficiency (-16 ± 0.5 ‰) than non-flowering plants (-17.8 ± 1.6 ‰).  Our results demonstrate that CAM photosynthetic activity is increased in association with flowering in this coastal dune Dudleya. Therefore, we conclude that CAM photosynthesis is a vital source of energy during reproduction in this species.