CO2gas exchange and transpiration ofWelwitschia mirabilisHook. fil. in the central Namib desert

✍ Scribed by D. J. Willert; B. M. Eller; E. Brinckmann; R. Baasch

• Book ID: 104750259
• Publisher: Springer-Verlag
• Year: 1982
• Tongue: English
• Weight: 878 KB
• Volume: 55
• Category: Article
• ISSN: 0029-8549
• DOI: 10.1007/bf00386713

✦ Synopsis

The diurnal course of CO 2 gas exchange, 14CO2 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 I~mol m -2 s -1 in 1-year-old parts to 1 gmol m 2 s-~ 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 gmol 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-Zs 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 lamol g-~ dry matter) and citrate (up to 250 gmol g-a dry matter), which depend on leaf age but do not show any significant day-night oscillation.

In spite of all this the fi13C values are in the range of -17.77 to -19.64 %o. Possible reasons for such a high 13C content in a C 3 plant are discussed.