The Hypothesis Ecological Advantage of CAM

Pfeffer s interpretation of CAM is an excellent example of scientific intuition. The biochemical mechanism of CAM, and in particular the occurrence of carbon dioxide fixation in darkness and the pathway of malic acid conversion to carbohydrates were unknown when Pfeffer wrote his Pflanzenphysiologie. 

Nevertheless, some of the basic points of Pfeffer s interpretation of CAM can still be accepted. The rediscovery of dark CO2 fixation gave no reason to change the basic concept that CAM is a mechanism to recover respiratory CO2 during the night (cf. the reviews of Wolf, 1960 Ranson and Thomas, 1960). It will be shown later that in fact in special cases, CAM photosynthesis can function nearly exclusively on recycled respiratory CO2 (see Chap. 6.2.2). 

However, as indicated earlier, Nuernbergk (1959, 1961, 1962) was the first to recognize that the nocturnal fixation of external CO2 may be at least as important for the final carbon balance of a CAM plant as the refixation of respiratory CO2. It will be shown later that this proposal has been fully verified by the recent studies of CAM plants in their natural habitats. 

Thirdly, CAM plants can respond to drought by hermetically sealing the stomata, virtually eliminating water loss. In the case of cacti at least, what little water is lost from young stems is replaced by transfer from older tissue. An experiment illustrating this phenomenon was conducted at the Boyd Desert Research Center by Stan Szarek. In June, 1971, Opuntia bigelovii plants with about 40 stem sections 

Transfer of water can be observed also in other succulents. Such an example has been described by Nobel (1977 a). An individual of Agave deserti, during the 159 days between the occurrence of the flower and the ripening of the seeds, lost 24.9 kg fresh weight of the leaves. Only 7.1 kg were due to water loss by leaf transpiration. The bulk of water lost from the leaves appeared in the inflorescence where it was stored (3.1 kg), or was lost by transpiration (4.3 kg), or remained in the fruits (10.8 kg), respectively. 

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