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

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.