CO2 gas exchange and transpiration of Welwitschia mirabilis Hook. fil. in the central Namib desert

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Abstract
The diurnal course of CO2 gas exchange, 14CO2 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 CO2 fixation pathway is used by this gymnosperm. The CO2 gas exchange of Welwitschia is that of a C3 plant under arid conditions. Younger leaf parts show a two-peaked pattern of photosynthetic CO2 uptake whereas in older parts the morning peak is followed by net CO2 release during the rest of the day. The maximum rates of net photosynthesis decrease from 3.4 μmol m-2 s-1 in 1-year-old parts to 1 μmol m-2 s-1 in 7-year-old parts. No net CO2 uptake was detected during the night. The diurnal CO2 balance indicates that the old leaf parts live at the expense of the younger ones. Irrigation of Welwitschia plants resulted in an increased CO2 uptake throughout the light period with maximum rate of 4.1 μmol m-2 s-1. 14CO2 was only incorporated during the day. The water loss of Welwitschia by transpiration is considerable, reaching a peak value of 1.9 mmol m-2 s-1 around noon. Leaf conductance corresponds with the twopeaked pattern of CO2 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-1 dry matter) and citrate (up to 250 μmol g-1 dry matter), which depend on leaf age but do not show any significant day-night oscillation. In spite of all this the δ13C values are in the range of-17.77 to-19.64‰. Possible reasons for such a high 13C content in a C3 plant are discussed.