Three new methods indicate that CO2 concentration affects plant respiration in the range relevant to global change

Dependence of cell's membrane potential on extracellular voltage observed in Chara globularis

The membrane potential (Vm) of a cell results from the selective movement of ions across the cell membrane. Recent studies have revealed the presence of a gradient of voltage within a few nanometers adjacent to erythrocytes. Very notably this voltage is modified in response to changes in cell's membrane potential thus effectively extending the potential beyond the membrane and into the solution. In this study, using the microelectrode technique, we provide experimental evidence for the existence of a gradient of negative extracellular voltage (Vz) in a wide zone close to the cell wall of algal cells, extending over several micrometers. Modulating the ionic concentration of the extracellular solution with CO2 alters the extracellular voltage and causes an immediate change in Vm. Elevated extracellular CO2 levels depolarize the cell and hyperpolarize the zone of extracellular voltage (ZEV) by the same magnitude. This observation strongly suggests a coupling effect between Vz and Vm. An increase in the level of intracellular CO2 (dark respiration) leads to hyperpolarization of the cell without any immediate effect on the extracellular voltage. Therefore, the metabolic activity of a cell can proceed without inducing changes in Vz. Conversely, Vz can be modified by external stimulation without metabolic input from the cell. The evolution of the ZEV, particularly around spines and wounded cells, where ion exchange is enhanced, suggests that the formation of the ZEV may be attributed to the exchange of ions across the cell wall and cell membrane. By comparing the changes in Vm in response to external stimuli, as measured by electrodes and observed using a potential-sensitive dye, we provide experimental evidence demonstrating the significance of extracellular voltage in determining the cell's membrane potential. This may have implications for our understanding of cell membrane potential generation beyond the activities of ion channels.

Carboxylation Capacity Can Limit C3 Photosynthesis at Elevated CO2 throughout Diurnal Cycles

The response of carbon fixation in C₃ plants to elevated CO₂ is relatively larger when photosynthesis is limited by carboxylation capacity (V_C) than when limited by electron transport (J). Recent experiments under controlled, steady-state conditions have shown that photosynthesis at elevated CO₂ may be limited by V_C even at limiting PPFD. These experiments were designed to test whether this also occurs in dynamic field environments. Leaf gas exchange was recorded every 5 min using two identical instruments both attached to the same leaf. The CO₂ concentration in one instrument was controlled at 400 μmol mol⁻¹ and one at 600 μmol mol⁻¹. Leaves were exposed to ambient sunlight outdoors, and cuvette air temperatures tracked ambient outside air temperature. The water content of air in the leaf cuvettes was kept close to that of the ambient air. These measurements were conducted on multiple, mostly clear days for each of three species, Glycine max, Lablab purpureus, and Hemerocallis fulva. The results indicated that in all species, photosynthesis was limited by V_C rather than J at both ambient and elevated CO₂ both at high midday PPFDs and also at limiting PPFDs in the early morning and late afternoon. During brief reductions in PPFD due to midday clouds, photosynthesis became limited by J. The net result of the apparent deactivation of Rubisco at low PPFD was that the relative stimulation of diurnal carbon fixation at elevated CO₂ was larger than would be predicted when assuming limitation of photosynthesis by J at low PPFD.

Dark Respiration Measurement from Arabidopsis Shoots

Dark respiration refers to experimental measures of leaf respiration in the absence of light, done to distinguish it from the photorespiration that occurs during photosynthesis. Dark aerobic respiration reactions occur solely in the mitochondria and convert glucose molecules from cytoplasmatic glycolysis and oxygen into carbon dioxide and water, with the generation of ATP molecules. Previous methods typically use oxygen sensors to measure oxygen depletion or complicated and expensive photosynthesis instruments to measure CO₂ accumulation. Here, we provide a detailed, step-by-step approach to measure dark respiration in plants by recording CO₂ fluxes of Arabidopsis shoot and root tissues. Briefly, plants are dark acclimated for 1 hour, leaves and roots are excised and placed separately in airtight chambers, and CO₂ accumulation is measured over time with standard infrared gas analyzers. The time-series data is processed with R scripts to produce dark respiration rates, which can be standardized by fresh or dry tissue mass. The current method requires inexpensive infrared gas analyzers, off-the-shelf parts for chambers, and publicly available data analysis scripts.