Rapid CO2 changes cause oscillations in photosynthesis that implicate PSI acceptor-side limitations

Oscillations in CO2 assimilation rate and associated fluorescence parameters have been observed alongside the triose phosphate utilization (TPU) limitation of photosynthesis for nearly 50 years. However, the mechanics of these oscillations are poorly understood. Here we utilize the recently developed dynamic assimilation techniques (DATs) for measuring the rate of CO2 assimilation to increase our understanding of what physiological condition is required to cause oscillations. We found that TPU-limiting conditions alone were insufficient, and that plants must enter TPU limitation quickly to cause oscillations. We found that ramps of CO2 caused oscillations proportional in strength to the speed of the ramp, and that ramps induce oscillations with worse outcomes than oscillations induced by step change of CO2 concentration. An initial overshoot is caused by a temporary excess of available phosphate. During the overshoot, the plant outperforms steady-state TPU and ribulose 1,5-bisphosphate regeneration limitations of photosynthesis, but cannot exceed the rubisco limitation. We performed additional optical measurements which support the role of PSI reduction and oscillations in availability of NADP(+) and ATP in supporting oscillations. Photosynthetic CO(2)uptake at high rates sometimes oscillates, especially when CO(2)changes rapidly. We present evidence that this results from unbalanced interactions between electron transport metabolism and carbon metabolism.

Automated summary

Oscillations in CO2 assimilation rate and associated fluorescence parameters have been observed for nearly 50 years, but the mechanics of these oscillations are poorly understood. In this study, the researchers used dynamic assimilation techniques (DATs) to measure the rate of CO2 assimilation and found that TPU-limiting conditions alone were insufficient to cause oscillations. Ramps of CO2 caused oscillations proportional in strength to the speed of the ramp, and ramps induced oscillations with worse outcomes than step changes in CO2 concentration.