The Function of the Aerenchyma in Arborescent Lycopsids: Evidence of an Unfamiliar Metabolic Strategy
Walton A. Green
Most species of the modern family Isoetaceae (Quillworts), and some other modern hydrophytes, utilize a metabolic pathway for carbon fixation that involves uptake of sedimentary carbon and enrichment of CO2 in internal gas spaces as a carbon-concentrating mechanism. This metabolism, which is related to `aquatic CAM', is characterized by morphological, physiological, and biochemical adaptations for decreasing photorespirative loss, aerating roots, and maintaining high growth rates in anoxic, oligotrophic, stressed environments. Some of the closest relatives of the Isoetaceae were the `arborescent lycopsids', which were among the dominant taxa in the coal swamps found in lowland ecosystems during the Carboniferous and Permian periods (c. 300 Ma). Morphological, ecological and geochemical evidence supports the hypothesis that the arborescent lycopsids had an unusual metabolism similar to that of modern Isoetaceae and processed a biogeochemically significant proportion of organically fixed carbon over a period of about 100 million years in the late Palaeozoic. The temporal coincidence between the dominance of plants with this metabolism and an anomalous global atmosphere (high O2; low CO2) supports the idea that biosphere feedbacks are important in regulating global climatic homeostasis. The potential influence of this metabolism on the global carbon cycle and its specific adaptive function suggest that it should perhaps be considered a fourth major photosynthetic pathway.
Comparison of photosynthetic pathways. Transverse sections showing the different anatomies characteristic of the pathways and schematic descriptions of the route taken by carbon during photosynthesis. The path taken by a carbon molecule during fixation is given at the top: C6H12O6 (glucose) is used as a proxy for more complex carbohydrates; RuBP = ribulose-1,5-bisphosphate; PEP = phosphoenolpyruvate; conc. = concentrated. Sections are modified from illustrations in Esau (1953), Gifford and Foster (1987), and an unaccessioned anatomical slide from the botany collections of the National Museum of Natural History. Light grey shading shows gas spaces and the icons show the different locations of carbon storage, fixation, and transport via the C3 and C4 cycles in each metabolic pathway.
The parichnos system. Schematic drawings showing aerenchyma system connecting buried and subaerial organs in Equisetum, Selaginella, the arborescent lycopsid Lepidodendron, and Isoetes. Note positional homology of the aerenchyma across the lycopsids. The phylogenetic relationships are shown by a schematic tree in which the branch lengths are not significant and many taxa are omitted for clarity. Phi indicates approximate diameters of organs. Parts of the illustration are adapted from Britton and Brown (1913), Stewart (1947), Gifford and Foster (1987), Pigg (1992) and Stewart and Rothwell (1993).
CO2, O2, and numbers of described species of lycopsids through time. Note that arborescent lycopsids were diverse only in the high O2, low CO2 atmosphere of the Carboniferous and Permian. In A, CO2 values are given as RCO2; relative enrichment over modern (preindustrial) level. Redrawn from figures in Berner (2006), Bergman et al. (2004) Berner and Kothavala (2001) and Niklas et al. (1985). The curves in C are traced compilations of several figures and therefore represent approximate rather than exact values.
Green, W. A. 2010. The Function of the Aerenchyma in Arborescent Lycopsids: Evidence of an Unfamiliar Metabolic Strategy. Proceedings of the Royal Society B. (in press).
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