The triose phosphate utilization (TPU) limitation of photosynthesis is a paradigm in which the rate at which stromal phosphate is incorporated into the organic phosphate pool can exceed the rate at which inorganic phosphate is released by processing fixed carbon into end products such as starch or sucrose. TPU limitation is unique among the three canon biochemical limitations of photosynthesis in that the plant must regulate photosynthetic rate to a level below what is maximally possible in its current environment. I investigated the methods through which the photosynthetic rate is regulated in response to TPU limitation. For the first minute after imposition of TPU limitation by excess light and CO2, the photosynthetic rate is limited by oscillations in availability of NADP+ for photosystem I and availability of inorganic phosphate for the chloroplastic ATP synthase. These oscillations cause variance in the redox state of the electron carrier Qa which primarily controls energy flow during photosynthesis. After a few minutes, slower energy-dependent regulatory mechanisms at photosystem II and the cytochrome b6f complex reduce energy flow, relieving any excess reduction at Qa or photosystem I. After a day of acclimation, photoinhibition and rubisco deactivation prevent the appearance of TPU limitation at elevated CO2 and also prevented the occurrence of oscillations in photosynthetic electron carrier redox status. Oscillations induced by TPU limitation are temporarily able to exceed the steady-state photosynthetic rate. However, the advantage is short-lived, and overall plants assimilate less over the course of oscillations than they would during steady-state photosynthesis. The plants can temporarily exceed the limitation on photosynthesis typically imposed by TPU limitation or the RuBP regeneration limitation, but not the rubisco limitation. This is due to the availability of metabolites caused by a brief period of inactivity. Furthermore, the amplitude of the oscillations depended on how quickly the plant entered TPU limitation and how severe TPU limitation was when imposed.
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