Abstract
Crassulacean Acid Metabolism (CAM) as an ecophysiological modification of photosynthetic carbon acquisition has been reviewed extensively before. Cell biology, enzymology and the flow of carbon along various pathways and through various cellular compartments have been well documented and discussed. The present attempt at reviewing CAM once again tries to use a different approach, considering a wide range of inputs, receivers and outputs. Input is given by a network of environmental parameters. Six major ones, CO(2), H(2)O, light, temperature, nutrients and salinity, are considered in detail, which allows discussion of the effects of these factors, and combinations thereof, at the individual plant level ('physiological aut-ecology'). Receivers of the environmental cues are the plant types genotypes and phenotypes, the latter including morphotypes and physiotypes. CAM genotypes largely remain 'black boxes', and research endeavours of genomics, producing mutants and following molecular phylogeny, are just beginning. There is no special development of CAM morphotypes except for a strong tendency for leaf or stem succulence with large cells with big vacuoles and often, but not always, special water storage tissues. Various CAM physiotypes with differing degrees of CAM expression are well characterized. Output is the shaping of habitats, ecosystems and communities by CAM. A number of systems are briefly surveyed, namely aquatic systems, deserts, salinas, savannas, restingas, various types of forests, inselbergs and paramós. While quantitative census data for CAM diversity and biomass are largely missing, intuition suggests that the larger CAM domains are those systems which are governed by a network of interacting stress factors requiring versatile responses and not systems where a single stress factor strongly prevails. CAM is noted to be a strategy for variable, flexible and plastic niche occupation rather than lush productivity. 'Physiological syn-ecology' reveals that phenotypic plasticity constitutes the ecophysiological advantage of CAM.
Highlights
The ®rst comprehensive review of Crassulacean Acid Metabolism (CAM) was published in 1960 (Wolf, 1960)
The simplest de®nition of CAM, ®rst described for species of the family Crassulaceae, is that there is (1) nocturnal uptake of CO2 via open stomata, ®xation by phosphoenolpyruvate carboxylase (PEPC) and vacuolar storage of CO2 in the form of organic acids, mainly malic acid, and (2) daytime remobilization of vacuolar organic acids, decarboxylation
Nocturnal synthesis of organic acid fed by respiratory CO2 occurs, and where stomata are open during the light period with uptake of atmospheric CO2 and direct Calvin-cycle CO2 reduction (C3-photosynthesis) in addition to assimilation of CO2 remobilized from nocturnally stored organic acid
Summary
CO2, H2O, light, temperature, nutrients and salinity, are considered in detail, which allows discussion of the effects of these factors, and combinations thereof, at the individual plant level (`physiological aut-ecology'). D Receivers Receivers of the environmental cues are the plant types genotypes and phenotypes, the latter including morphotypes and physiotypes. There is no special development of CAM morphotypes except for a strong tendency for leaf or stem succulence with large cells with big vacuoles and often, but not always, special water storage tissues. D Conclusions While quantitative census data for CAM diversity and biomass are largely missing, intuition suggests that the larger CAM domains are those systems which are governed by a network of interacting stress factors requiring versatile responses and not systems where a single stress factor strongly prevails. `Physiological syn-ecology' reveals that phenotypic plasticity constitutes the ecophysiological advantage of CAM
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