Four temperature treatments were studied in the climate controlled growth chambers of the Georgia Envirotron: 25/20, 30/25, 35/30, and 40/35 °C during 14/10 h light/dark cycle. For the first growth stage (V3-5), the highest net photosynthetic rate (PN) of sweet corn was found for the lowest temperature of 28-34 µmol m-2 s-1 while the PN for the highest temperature treatment was 50-60 % lower. We detected a gradual decline of about 1 P N unit per 1 °C increase in temperature. Maximum transpiration rate (E) fluctuated between 0.36 and 0.54 mm h-1 (≈5.0-6.5 mm d-1) for the high temperature treatment and the minimum E fluctuated between 0.25 and 0.36 mm h-1 (≈3.5-5.0 mm d-1) for the low temperature treatment. Cumulative CO2 fixation of the 40/35 °C treatment was 33.7 g m-2 d-1 and it increased by about 50 % as temperature declined. The corresponding water use efficiency (WUE) decreased from 14 to 5 g(CO2) kg-1(H2O) for the lowest and highest temperature treatments, respectively. Three main factors affected WUE, PN, and E of Zea: the high temperature which reduced PN, vapor pressure deficit (VPD) that was directly related to E but did not affect PN, and quasi stem conductance (QC) that was directly related to PN but did not affect E. As a result, WUE of the 25/20 °C temperature treatment was almost three times larger than that of 40/35 °C temperature treatment. and J. Ben-Asher, A. Garcia y Garcia, G. Hoogenboom.
Net CO2 uptake rates (PN) 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 PN 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 PN 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, PN 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. and J. Ben-Asher ... [et al.].