Comparative ecophysiology of five species of Sedum (Crassulaceae) under well-watered and drought-stressed conditions

Gas exchange patterns, diurnal malic acid fluctuations, and stable carbon isotope ratios of five species of Sedum were investigated to assess the ecophysiological characteristics of three different photosynthetic pathways under well-watered and drought-stressed conditions. All five species have succulent leaves and stems and were examined under identical environmental conditions. When well-watered, Sedum integrifolium (Raf.) Nels. and S. ternatum Michx. displayed C₃ photosynthesis, S. telephioides Michx. and S. nuttallianum Raf. exhibited CAM-cycling, and S. wrightii A. Gray showed CAM. When grown under a less frequent watering regime, S. integrifolium and S. ternatum exhibited CAM-cycling, whereas S. telephioides and S. nuttallianum displayed CAM-cycling simultaneously with low-level CAM. Sedum wrightii retained its CAM mode of photosynthesis. In general, leaf δ¹³C values reflected these variations in photosynthetic pathways. While all values of water-use efficiency (WUE) were greater than those reported for most C₃ and C₄ species, no correlation of malic acid accumulation in the CAM and CAM-cycling (including low-level CAM) species with increased WUE was found. Sedum wrightii (CAM) had the highest WUE value at night, yet its 24-h WUE was not different from S. ternatum when the latter was in the C₃ mode. Thus, relative water-use efficiencies of these species of Sedum were not predictable based on photosynthetic pathways alone.