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Crassulacean acid metabolism (CAM) and succulence were compared in two species of Peperomia (Piperaceae) and six species of Cissus and Cyphostenz ma (Vitaceae). The species include thin-leaved plants, leaf succulents, and stem succulents. Succulence at the organ or the cellular level mesophylll succulence [Sm]) does not appear to be causally related to CAM. Peperomia, with high Sm, shows low CAM activity, while Cypizostenima, with relatively low Sm, exhibits very high CAM activity. Overnight accumulation of malic acid as high as 332 µeq/g fresh weight was measured in the nonsucculent, thin leaves of Cissus sp., while only 50 µeq of malic acid per gram fresh weight accumulated in the succulent, photosynthesizing stem of the same plant. In the thin-leaved species of Cissus, CAM does not appear to be related to drought adaptation. Succulence in the CAM species Cissus may be a new acquisition that has allowed the genus to spread from the wet tropics to arid environments. We suggest that CAM and succulence are often associated in drought-adapted plants because both characters have been selected for in arid environments.

Pattern and magnitude of stem gas exchange were studied under controlled conditions on two CAM species of the genus Cissus differing in morphological features. In the cactus-like liana Cissus quadrangularis, at water vapour deficit of the air (VPD) lower than 400 mPa Pa-1 during daytime, under 24/16°C and 27/16°C temperature regime, CO2 uptake occurred during daytime and nighttime but night fixation was responsible for 74 and 77% respectively of the CO2 fixed during the whole diurnal cycle; the contribution of night fixation increased up to 84% at VPD of 590 mPa Pa-1 under 27/15°C. In Cissus sp., a slightly succulent xerophytic liana with mesophytic deciduous leaves, at 27°C day temperature and VPD of 520-540 mPa Pa-1, under both 12 and 16°C night temperature, CO2 uptake occurred exclusively during the night; however at lower day temperatures (18-20°C) and lower VPD (169-269 mPa Pa-1) substantial CO2 uptake was observed in the light. Transpiration was higher in Cissus sp. than in Cissus quadrangularis under all of the conditions applied; moreover night transpiration in Cissus sp. contributed more to total day transpiration as compared with Cissus quadrangularis. The results support the opinion that the nocturnal gas exchange has a preeminent role in Cissus sp. as compared with Cissus quadrangularis. Indeed under conditions which enhance evaporative demand even well-watered plants of Cissus sp. resort totally to night fixation. In contrast Cissus quadrangularis which shows a larger resistance to water loss and possesses an abundant water storing parenchyma as compared with Cissus sp. resorts totally to night CO2 fixation only under drought stress. © 1987 Martinus Nijhoff Publishers.