Photorespiration in CAM Plants

With isolated mesophyll cells of Sedum telephium, Rouhani et al. (1973) were able to demonstrate ribulose diphosphate-dependent CO 2 fixation. Their further demonstration of P-enolpyruvate-dependent CO2 fixation led them to conclude also that light CO2 fixation could be through the PEP carboxylase sequence, or directly via ribulosediphosphate carboxylase. 

The above interpretation is supported by the finding of Osmond and Bjork-man (1975) that both dark CO2 fixation and the initial burst of light CO2 fixation were not affected by different O2 concentrations. In contrast, CO2 fixation at the end of the day was high with low oxygen concentration and vice versa (see Fig. 5.12). The conclusion is that only at the end of the light period does RudP carboxylase/oxygenase contribute substantially to fixation of exogenous CO2. 

As early as 1956, Moyse and Jolchine obtained evidence for light-dependent glycolate synthesis in Bryophyllum. Hence, the substrate for photorespiration is produced in the light by CAM plants. 

Badger et al. (1975) reported that the RudP carboxylase isolated from Kalan-choe daigremontiana is not unlike that of C3 plants (or C4 plants) in that it shows oxygenase activity and is inhibited by O2. 

Osmond and Bjorkman (1975) found that light-dependent CO2 fixation in K. daigremontiana was inhibited by O2 in the same manner as C3 plants, whereas dark CO2 fixation was independent of 4%, 26%, or 36% O2. Further, O2 inhibition of photosynthetic CO2 fixation is accompanied by a high CO2 compensation point (Allaway et al., 1974a). Unlike C3 plant photorespiration, however, the O2 inhibition of photosynthetic CO2 fixation is not eliminated by high CO2 tension, and the postillumination CO2 burst is present at low O2 (Osmond and Bjorkman, 1975 see Fig. 5.13). 

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