Net CO₂ uptake rates for Hylocereus undatus and Selenicereus megalanthus under field conditions: Drought influence and a novel method for analyzing temperature dependence

Net CO₂ uptake rates (P_N) were measured for the vine cacti Hylocereus undatus and Selenicereus megalanthus under relatively extreme climatic conditions in Israel. Withholding water decreased rates and the daily amount of CO₂ uptake by about 10 % per day. Compared with more moderate climates within environmental chambers, the higher temperatures and lower relative humidity in the field led to a more rapid response to drought. The upper envelopes of scatter diagrams for P_N versus temperature for these Crassulacean acid metabolism species, which indicate the maximal rates at a particular temperature, were determined for both night time CO₂ uptake in Phase I (mediated by phosphoenolpyruvate carboxylase, PEPC) and early morning uptake in Phase II (mediated by ribulose-1,5-bisphosphate carboxylase/ oxygenase, RuBPCO). As stem temperature increased above 13°C, the maximal P_N increased exponentially, reaching maxima near 27°C of 12 and 8 μmol m^-2 s^-1 for Phases I and II, respectively, for H. undatus and 6 and 4 μmol m^-2 s^-1, respectively, for S. megalanthus. Based on the Arrhenius equation, the apparent activation energies of PEPC and RuBPCO were 103 and 86 kJ mol^-1, respectively, for H. undatus and 77 and 49 kJ mol^-1, respectively, for S. megalanthus, within the range determined for a diverse group of species using different methodologies. Above 28°C, P_N decreased an average of 58 % per °C in Phase I and 30 % per °C in Phase II for the two species; such steep declines with temperature indicate that irrigation then may lead to only small enhancements in net CO₂ uptake ability.