Abstract
To determine which CO 2 fixation pathway is used by the gymnosperm Welwitschia mirabilis Hoof. f. an extensive field study on diurnal CO 2 gas exchange, 14 CO 2 incorporation, chemical composition, distribution pattern and variation of organic acids, and transpiration was made in the Welwitschia Flats of the central Namib desert. The CO 2 gas exchange of W. mirabilis is that of a C 3 plant under arid conditions. No CO 2 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 1,9 mmol 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 C 3 -pathway the δ 13 C values are in the range of CAM plants. Reasons for such values in W. mirabilis are discussed.
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