title: CO2gas exchange and transpiration of Welwitschia mirabilis Hook. fil. in the central Namib desert.
abstract: The diurnal course of CO2gas exchange,14CO2incorporation, 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 CO2fixation pathway is used by this gymnosperm.The CO2gas exchange of Welwitschia is that of a C3plant under arid conditions. Younger leaf parts show a two-peaked pattern of photosynthetic CO2uptake whereas in older parts the morning peak is followed by net CO2release during the rest of the day. The maximum rates of net photosynthesis decrease from 3.4 μmol m-2s-1in 1-year-old parts to 1 μmol m-2s-1in 7-year-old parts. No net CO2uptake was detected during the night. The diurnal CO2balance indicates that the old leaf parts live at the expense of the younger ones. Irrigation of Welwitschia plants resulted in an increased CO2uptake throughout the light period with maximum rate of 4.1 μmol m-2s-1.14CO2was only incorporated during the day.The water loss of Welwitschia by transpiration is considerable, reaching a peak value of 1.9 mmol m-2s-1around noon. Leaf conductance corresponds with the twopeaked pattern of CO2uptake.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-1dry matter) and citrate (up to 250 μmol g-1dry 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 high13C content in a C3plant are discussed.
authors.affiliation: Lehrstuhl Pflanzenökologie der Universität Bayreuth, Postfach 3008, D-8580, Bayreuth, Federal Republic of Germany.
authors.forename: D J
authors.initials: DJ
authors.lastname: von Willert
authors.affiliation: Institute of Plant Biology, University of Zürich, Zollikerstr. 107, CH-8008, Zürich, Switzerland.
authors.forename: B M
authors.initials: BM
authors.lastname: Eller
authors.affiliation: Lehrstuhl Pflanzenökologie der Universität Bayreuth, Postfach 3008, D-8580, Bayreuth, Federal Republic of Germany.
authors.forename: E
authors.initials: E
authors.lastname: Brinckmann
authors.affiliation: Lehrstuhl Pflanzenökologie der Universität Bayreuth, Postfach 3008, D-8580, Bayreuth, Federal Republic of Germany.
authors.forename: R
authors.initials: R
authors.lastname: Baasch