Figure 6: Oscillations are not seen following a spike in CO2 in plants that have acclimated to elevated CO2 for 30 h. The hallmark reduction of Qa, measured here as qL , is not seen, and so more energy is not diverted into non-photochemical quenching (φNPQ ).
Figure Legends
Figure 1: Plants were exposed to elevated (1500 ppm) ambient (400 ppm) or low (150 ppm) CO2 for 30 h, including an 8-hour dark period during the typical night hours, with A/Cicurves performed every 2.5 hours. The A/Ci curves were fit according to Gregory et al. , (2021) and the three primary fit parameters, Vcmax , J , andTPU relative to an A/Ci curve run before treatment began are plotted.
Figure 2: CO2 assimilation and optical measurements from an A/Ci curve before and after a 30 h elevated CO2 treatment. After 30 h in elevated CO2, parameters show acclimation to TPU-limiting conditions, including reduced response of assimilation,φII , NPQt , andECSt to increasing CO2. The clouds are LOESS fitting (LOcal Estimation of Scatterplot Smoothing) 95% CI n=5.
Figure 3: TPU limitation causes reduced rubisco activation state that persists for an extended period. Rubisco activation state remains low over the course of adaptation (a), and the total rubisco activity declines (b). Rubisco activation state decreases quickly after adding CO2 initially (c) and remains recoverable over 10 min for at least the first several hours (d).
Figure 4: Plants are given a spike in CO2, which induces oscillations in electron transport. In the first phase, photosynthesis is unlimited by TPU and PSI becomes more oxidized by the addition of extra CO2, while proton conductivity remains high. The second phase (blue) is the earliest effect of TPU limitation and is primarily described by PSI and Qa quickly become reduced (measured as PSI ox and qL ), along with constriction of proton flow across the thylakoid membrane (gH+ ) and electron flow to PSI (ket ) blue-shaded region). The third phase (green) begins when slower regulations, which depend on proton-motive force (measured as ECSt ), energy dependent quenching (NPQt ) and photoprotection at cytochrome b6f complex, relieve reduction of the electron transport chain. Finally, the electron transport chain enters a new steady-state (red).
Figure 5: Three traces of PSI measurements from oscillations in PSI reduction induced by spike in CO2, which demonstrate varying levels of re-reduction during saturating flashes. Typically, a saturating flash should fully oxidize PSI, but kinetics in electron transport can change this. (a) Extreme re-reduction of PSI can be seen during a saturation flash when PSI is most reduced, 40 s after beginning an elevated CO2 pulse. This demonstrates a high level of PSI-acceptor side limitation. (b) Less re-reduction of PSI during a saturation flash is seen when PSI is less limited by electron acceptors 60 s after beginning a CO2 pulse. (c) After returning to a new steady-state 100 s after beginning an elevated CO2pulse, PSI acceptor-side limitation is much diminished, and PSI re-reduction is minimal.
Figure 6: Oscillations are not seen following a spike in CO2 in plants that have acclimated to elevated CO2 for 30 h. The hallmark reduction of Qa, measured here as qL , is not seen, and so more energy is not diverted into non-photochemical quenching (φNPQ ).