In situ studies on crassulacean acid metabolism in Sedum acre L. and Sedum mite Gil

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Abstract
CO2 exchange, the diurnal variations in the levels of malic, citric and isocitric acid, and the labelling pattern after 14CO2 fixation were measured in Sedum acre and Sedum mite growing in situ. As predicted from laboratory experiments, drought changed the gas exchange pattern from a C3 type to a crassulacean acid metabolism (CAM) type. This shift correlated with the development of a diurnal rhythm in the malic acid content. The results of 14CO2 pulse-chase experiments suggest that in well-watered plants a CAM pattern of carbon flow already exists; hence water stress might enhance latent CAM rather than induce it. The in situ CAM performance by the Sedum species appeared to be highly susceptible to modulation by season and external factors, particularly light and temperature. CAM did not substantially contribute to total carbon gain in S. acre and S. mite. During most of their lifecycles the plants grow under conditions that favour CO2 uptake by the C3 pathway rather than by CAM. Hence, despite a capability to feature CAM, the δ13C values found in S. acre and S. mite are those of C3 plants.