Welwitschia mirabilis: CAM or not CAM - what is the answer?

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.