Gas Exchange and Acid Fluctuation

The air and leaf temperature of Dudleya is remarkably constant during the day/night cycle and during the seasons because of the vicinity of the ocean. In spite of this, CAM is very pronounced. Thus, at least in Dudleya, CAM is not ultimately controlled by temperature. 

Nobel s observations of A. deserti are in close agreement with the results obtained by Szarek et al. (1973), Ting and Szarek (1975), and Hanscom and Ting (1977), who provided observations of CAM behavior in Opuntia basilaris in its natural habitat at the Boyd Deep Canyon in California. Gas exchange parameters of CAM and acid oscillation were estimated (Fig. 6.7). Similarly to A. deserti, the plant water potential was of prime importance for CAM metabolism in the field. 

The transpiration ratio of O. basilaris varied significantly during the year (see Fig. 6.7), although mean TR values remained within the values reported for the three optional types of photosynthesizing plants, i.e., 450-600 for C3-plants, 250-350 for C4-plants, and 25-150 for CAM plants. 

Because of a linear relationship between plant water potential and the acidification rate (Szarek and Ting, 1974 see Fig.6.5), the highest amplitude of acid rhythm in 0. basilaris occurred when the plant p was high, and the lowest amplitudes could be observed when plant xp were low. Nevertheless, the nocturnal acidification persists throughout the year, even during the drought period when the stomata are sealed. 

It is reasonable to conclude that this persistence of CAM activity during the drought period is the result of day/night recycling of endogenous CO2. Here, malic acid synthesis depends exclusively on CO2 produced in respiration, and photosynthesis uses only endogenously produced CO2 from malate decarboxylation and photorespiration. 

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Finally, it should be made clear that the Cactaceae type of CAM shift is quite different from the shift reported in the Aizoaceae, Portulacaceae, and certain Crassulaceae. In the latter, the response to photoperiod and water stress appear to be a real shift in metabolic pathway. Not only do acids begin to fluctuate concomitant with gas exchange at night, but enzymes change quantitatively. Hence, the evidence suggests a reversible shift from C3 to CAM. 

The question of CAM plant physiology in situ in natural habitats has recently been studied and increasingly attracts attention. Typical features of CAM plants, such as gas exchange, acid fluctuation, and stomatal behavior, have been measured continuously in the field during day/night cycles and seasonally. Also, isotope composition (5 C value) (see Chap. 3.3) has been determined in samples... 

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