부산대학교 도서관

The diurnal course of CO₂ gas exchange, 14 CO 2 incorporation, malate and citrate content, and transpiration 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 throught the light period with maximum rate of 4.1 μmol m -2 s -1 . ${}^{14}CO{}_{2}$ 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 two-peaked 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 δ 13 C values are in the range of -17.77 to -19.64‰. Possible reasons for such a high 13 C content in a C₃ plant are discussed.