Green roofs mitigate many negative environmental effects of urbanization. The most common plants found on green roofs are species of Sedum (Crassulaceae), which are popular primarily because they are able to grow rapidly in favorable conditions and survive extended water deficit. It has previously been hypothesized that this is due to its photosynthetic plasticity. Several Sedum species have been shown to switch between C3 and CAM photosynthesis in response to changes in water availability. In this experiment, we examined the following research questions: Is there variation in photosynthetic pathway within the genus Sedum? Do any of the Sedum species exhibit photosynthetic plasticity in response to short term changes in water availability? Do predominantly C3 species of Sedum use more water than predominantly CAM congeners? To explore these questions, we measured photosynthesis and water use of eight species of Sedum under wet and dry conditions in a greenhouse. After two weeks of the watering treatments, we measured photosynthesis for three consecutive days. We measured night-time and daytime whole-plant carbon dioxide (CO₂) exchange as well as dawn and dusk leaf acidity. We measured whole-plant water use by repeating weighing of pots before and after watering.     

Results/Conclusions

There was variation in photosynthetic pathway among the eight species of Sedum tested. Although four species showed nocturnal acid accumulation, consistent with CAM, none of the species tested exhibited nocturnal uptake of CO₂. S. album and S. rupestre exhibited CAM idling, recycling respiratory CO₂ behind closed stomata. S. spectabile and S. spurium exhibited CAM cycling, taking up CO₂ during the day (typical of C3) but also accumulating malic acid at night (typical of CAM). S. cauticolum and S. ternatum exhibited C3 photosynthesis with daytime CO₂ uptake and minimal diel acid fluctuation. There was no evidence for a rapid shift in photosynthetic pathway in response to short-term water availability for any of the species tested. For most individuals, gas exchange and acidity was consistent across the three days, despite substantial changes in soil moisture and air temperature. Surprisingly, photosynthetic pathway had no effect on whole-plant water use. Furthermore, water loss by each species was similar to that of evaporation from a pot with soil but no plant. These results increase our understanding of plant performance on green roofs as well as our understanding of the role of plants on stormwater retention of green roofs.