Crassulacean acid metabolism species differ in the contribution of C(3) and C(4) carboxylation to end of day CO(2) fixation

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO(2) uptake, via phosphoenolpyruvate carboxylase (PEPC, C(4) carboxylation), from the diurnal refixation by Rubisco (C(3) carboxylation). At the end of the day (CAM‐Phase IV), when nocturnally stored CO(2) has depleted, stomata reopen and allow additional CO(2) uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO(2) uptake via C(3) and C(4) carboxylation in phase IV in the CAM species Phalaenopsis “Sacramento” and Kalanchoe blossfeldiana “Saja.” Short blackout periods during phase IV caused a sharp drop in CO(2) uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O(2) indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.