von Willert Dieter J, Armbrüster Nicole, Drees Tobias, Zaborowski Maik
Institute for Plant Ecology, Westfälische Wilhelms-University, Hindenburgplatz 55, D-48143 Münster, Germany.
Funct Plant Biol. 2005 Jul;32(5):389-395. doi: 10.1071/FP01241.
After more than 20 years of extensive study we found clear evidence that Welwitschia mirabilis Hook.f. is able to take up CO 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 CO uptake was found only in December and January when the nights were shortest and plants were flowering. CO uptake during the night was not pronounced and never accounted for more than 4% of the total CO uptake over 24 h. Maximum rates of nocturnal CO uptake never exceeded 0.1 µmol m s for the slowest and 0.2 µmol m s for the fastest growing plant. Neither water availability in the soil nor night temperature was found to determine nocturnal CO 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 C 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.
经过20多年的广泛研究，我们发现了明确的证据，即百岁兰（Welwitschia mirabilis Hook.f.）在其两个自然生态系统——纳米布沙漠和莫帕尼稀树草原中，都能够在夜间吸收二氧化碳，因此应被归类为景天酸代谢（CAM）植物。在六个不同地点，标记了85株百岁兰植株，并在2.5年的时间里测量了它们叶子和叶带的生长速率。这85株植株中生长最慢和最快的分别来自莫帕尼稀树草原和布兰德山地块的西北部。在本研究的气体交换测量中选择了这两株植株。在一年的时间里，仅在12月和1月发现了夜间二氧化碳吸收，此时夜晚最短且植株正在开花。夜间的二氧化碳吸收并不显著，从未超过24小时总二氧化碳吸收量的4%。生长最慢的植株夜间二氧化碳吸收的最大速率从未超过0.1 μmol m⁻² s⁻¹，生长最快的植株则从未超过0.2 μmol m⁻² s⁻¹。无论是土壤中的水分供应还是夜间温度，都未发现能像已知的CAM植物那样决定夜间二氧化碳吸收。无论生长地点如何，百岁兰的所有叶子都含有大量的苹果酸和柠檬酸。根据气体交换测量计算出的夜间酸含量的小幅增加，被这些酸在叶子中极其不均匀的分布所掩盖，使得夜间苹果酸或柠檬酸积累这一特征不适用于百岁兰的CAM测试。随着离海岸距离的增加，碳同位素分馏增加得到了证实。光呼吸极高，且随叶片周围的气温变化。尽管关于百岁兰是否为CAM植物的争论可以结束，但对于百岁兰为何很少利用CAM却无法给出答案。
