Physiological and anatomical investigations of CAM-cycling in Lewisia cotyledon (Portulacaceae)

We investigated leaf morphology, CO ₂ uptake, and the diurnal acid fluctuation of the montane species, Lewisia cotyledon Robinson. This species belongs to the Portulacaceae, a small family with ~ 425 species, which exhibit photosynthetic flexibility. Lewisia cotyledon was found in the Siskiyou Mts. in Josephine County, OR along with other montane species that utilize the CAM pathway. Plants were collected and returned to the greenhouse to study the photosynthetic metabolism of this species under both well-watered and drought conditions. Lewisia cotyledon showed primarily daytime CO₂ gas exchange. Well-watered plants exhibited a significant diurnal acid fluctuation during the day. Phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) activity was low in both control and water-stressed plants. Leaf anatomy showed a CAM-like spongy mesophyll tissue with palisade mesophyll tissue atypical of CO₃plants. The percentage of leaf air space was 16.8%, which is characteristic of CAM and CAM-cycling plants. Water stress did not induce CAM metabolism. Our results suggest that: 1) L. cotyledon exhibited the CAM cycling pathway that would better enable the plant to withstand water stress and high light environments; 2) CAM is more widespread in the Portulacaceae than was previously thought; and 3) the evolution of the CAM pathway in the Portulacaceae appears to require the acquisition of both physiological and anatomical modifications