Ecophysiological studies on Welwitschia mirabilis in the Namib desert

To determine which CO2 fixation pathway is used by the gymnosperm Welwitschia mirabilis Hoof. f. an extensive field study on diurnal CO2 gas exchange, 14CO2 incorporation, chemical composition, distribution pattern and variation of organic acids, and transpiration was made in the Welwitschia Flats of the central Namib desert.The CO2 gas exchange of W. mirabilis is that of a C3 plant under arid conditions. No CO2 uptake during the night was detected and the organic acids showed no day-night fluctuations that significantly exceeded the differences observed between the values obtained in a grid sampling over a whole leaf.The daily water loss of W. mirabilis by transpiration is considerable, reaching a peak value of 1,9mmol m−2 s−1 around noon. Problems with respect to water supply to maintain such transpiration rates are discussed. In spite of the evidence for a C3-pathway the δ13C values are in the range of CAM plants. Reasons for such values in W. mirabilis are discussed