Diurnal Pattern of Transpiration,Water Uptake and Water Budget of Succulents with Different CO2 Fixation Pathways

The water fluxes and the CO₂ exchange of three leaf succulents, Othonna opima, Cotyledon orbiculata and Senecio medley-woodii, with different leaf anatomy, growth form and CO₂ fixation pathways (C₃, CAM) were monitored with a gas exchange cuvette which was combined with a potometric system to quantify water uptake. Measurements, which are primarily valid for plants with a sufficient water supply, were made during 6 to 10 consecutive days under constant experimental conditions. Water uptake for 24 h exceeded water loss by transpiration only for a S, medley-woodii plant with 10 expanding but only 7 mature leaves. In this case the gained water evidently is put into leaf expansion. All other plants showed balanced transpiration and water uptake rates. O. opima and C. orbiculata have a similar life form, similar water storage volumes and the same natural habitat but their diurnal water uptake patterns differ significantly. In the C₃ plant O. opima water uptake increased when the transpiration increased or transpiration rates were higher than uptake rates and vice versa. On the contrary the CAM plant C. orbiculata transpired during the dark period at constant or decreasing rates but showed steadily increasing uptake rates. Senecio medley-woodii- and C. orbiculata are CAM plants with similar diurnal water uptake patterns with its maximum in uptake during or towards the end of the CO₂ dark fixation period. Water uptake of C. orbiculata was at its minimum at the end of the light period despite transpiration being maximal. The results were discussed considering the different CO₂ fixation pathways. In the investigated CAM succulents, C. orbiculata and S. medley-woodii, the CAM influenced water uptake throughout the whole day and not only during the CO₂ dark fixation period.