CO gas exchange and transpiration of Welwitschia mirabilis Hook. fil. in the central Namib desert.

The diurnal course of CO gas exchange, CO incorporation, malate and citrate content, and traspiration of Welwitschia mirabilis were measured in one of its natural habitats, the Welwitschia-Vlakte in the central Namib desert (Namibia), in order to decide which CO fixation pathway is used by this gymnosperm.The CO gas exchange of Welwitschia is that of a C plant under arid conditions. Younger leaf parts show a two-peaked pattern of photosynthetic CO uptake whereas in older parts the morning peak is followed by net CO release during the rest of the day. The maximum rates of net photosynthesis decrease from 3.4 μmol m s in 1-year-old parts to 1 μmol m s in 7-year-old parts. No net CO uptake was detected during the night. The diurnal CO balance indicates that the old leaf parts live at the expense of the younger ones. Irrigation of Welwitschia plants resulted in an increased CO uptake throughout the light period with maximum rate of 4.1 μmol m s. CO was only incorporated during the day.The water loss of Welwitschia by transpiration is considerable, reaching a peak value of 1.9 mmol m s around noon. Leaf conductance corresponds with the twopeaked pattern of CO uptake.Although there is no sign of a crassulacean acid metabolism in Welwitschia the leaf contains rather high amounts of malate (up to 200 μmol g dry matter) and citrate (up to 250 μmol g dry matter), which depend on leaf age but do not show any significant day-night oscillation.In spite of all this the δC values are in the range of-17.77 to-19.64‰. Possible reasons for such a high C content in a C plant are discussed.