Seasonal and diurnal changes in cell sap titratable acidity, the content of organic acids, and phosphoenolpyruvate (PEP) carboxylase activity in leaves of orpine (purple stonecrop) Hylotelephium triphyllum (Haw.) Holub were examined. During reproductive plant development, the cell sap acidity increased in morning hours in parallel with an increase in malate content and the activity of PEP-carboxylase increased in evenings on the background of the decreased malate concentration. These changes provide evidence for the transition from C3 photosynthesis to the crassulacean acid metabolism (CAM photosynthesis). The β-carboxylation of PEP was most pronounced at the phase of abundant flowering when the monthly average air temperature rose along with the decrease in water availability. The contribution of CAM photosynthesis to carbon balance and plant productivity was insignificant, as evidenced from discrimination of 13C isotope in the organic matter of leaves (δ13C of approximately –27‰) and other organs. These δ13C values are typical for the isotopic signature of C3 species. The functioning of CAM photosynthesis during flowering and fruiting periods protects the photosynthetic apparatus and promotes the reproductive potential of H. triphyllum plants inhabiting sandy loamy soils that easily lose moisture in the humid climate of the taiga zone of northeastern Europe.