Welwitschia mirabilis: CAM or not CAM — what is the answer?
Dieter J. von Willert A B , Nicole Armbrüster A , Tobias Drees A and Maik Zaborowski AA Institute for Plant Ecology, Westfälische Wilhelms-University, Hindenburgplatz 55, D-48143 Münster, Germany.
B Corresponding author. Email: Willert@uni-muenster.de
C This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001
Functional Plant Biology 32(5) 389-395 https://doi.org/10.1071/FP01241
Submitted: 19 November 2001 Accepted: 8 December 2004 Published: 27 May 2005
Abstract
After more than 20 years of extensive study we found clear evidence that Welwitschia mirabilis Hook.f. is able to take up CO2 at night in both of its natural ecosystems, the Namib desert and the Mopane savannah, and hence should be classified a crassulacean acid metabolism (CAM) plant. At six different sites, 85 W. mirabilis plants were marked and the growth rate of their leaves and leaf ribbons were measured over a period of 2.5 years. The slowest and the fastest growing plant of these 85 plants were from the Mopane savannah and from the north-west of the Brandberg massif, respectively. These were selected for the gas-exchange measurements of this study. Within the course of a year nocturnal CO2 uptake was found only in December and January when the nights were shortest and plants were flowering. CO2 uptake during the night was not pronounced and never accounted for more than 4% of the total CO2 uptake over 24 h. Maximum rates of nocturnal CO2 uptake never exceeded 0.1 µmol m–2 s–1 for the slowest and 0.2 µmol m–2 s–1 for the fastest growing plant. Neither water availability in the soil nor night temperature was found to determine nocturnal CO2 uptake in terms known for CAM plants. Regardless of the growing site all leaves of W. mirabilis contained high amounts of malic and citric acid. Small increases of acids over night as calculated from the gas exchange measurements are masked by the extremely uneven distribution of these acids in the leaves, making the feature of an overnight malic or citric acid accumulation an unsuited test for CAM in W. mirabilis. An increase in 13C discrimination with increasing distance from the coast was confirmed. Photorespiration was extremely high and followed air temperature around the leaf. Although the debate whether or not W. mirabilis is a CAM plant can be closed, no answer could be given why W. mirabilis makes so little use of CAM.
Keywords: citric acid, CO2 uptake, malic acid, Welwitschia mirabilis.
Acknowledgments
This investigation was partially sponsored by the Deutsche Forschungsgemeinschaft. We also thank Volkswagen (Uitenhage, RSA) for providing 4WD vehicles that allowed us to reach the plants with our heavy equipment even in rough field. AFROX (Windhoek) kindly supplied all gases needed for our measurements. We thank Frank von Willert for developing the necessary transfer, converting and calculating computer programmes. For unremitting help with the field work we thank F Bücker, U Wagner-Douglas, M Steinberg, A van der Merwe, J Olivier and M Veste. We are grateful to the Ministry of Environment and Tourism of Namibia for giving us the permission to conduct these investigations on Welwitschia mirabilis over a period of 20 years.
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