CONCLUSIONS
Photosynthesis is highly adaptive to the environment, and in TPU-limiting conditions experiences a series of regulatory steps to alleviate the stress along the electron transport chain. These steps can be organized into a timeline. At first, electrons build up along the electron transport chain, and reduction of Qa causes extra energy to be funneled into nonphotochemical quenching. This causes transients in photosynthesis, which are damped after a few minutes by accumulation of PMF , causing elevated energy-dependent quenching and photoprotection at the cytochrome b6fcomplex, accompanied by reduction in rubisco activation state. Over a longer period of time, energy-dependent quenching decreases and is supplanted by photoinhibition. The accumulation of these regulatory mechanisms causes the plant to no longer be TPU limited. Counterintuitively, the plant did not increase its TPU capacity, but instead limited the photosynthetic rate by rubisco deactivation and electron transport regulation.
The disappearance of TPU limitation over 30 h of adaptation justifies the removal of TPU limitation from global models. Plants that are TPU-limited will eventually not be TPU limited, through a combination of regulatory means. However, TPU limitation is still an important part of photosynthetic regulation and cannot be disregarded in experimental design or data analysis. The occurrence of TPU limitation in the field is probably very low due to the swift adaptation demonstrated here, but in artificial experiments is easy to provoke. In FACE experiments (Allenet al. 2020), or experiments that involve low temperature many of the effects studied may be caused by TPU limitation or the acclimation to TPU limitation. In other cases, sugar signaling may match photosynthesis to growth without explicit TPU limitations.
ACKNOWLEDGEMENTS
This research was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the United States Department of Energy (Grant DE-FG02-91ER20021). DMK and TDS received partial salary support from Michigan AgBioResearch.
CONFLICT OF INTEREST
The authors have no conflict of interest to report.
DATA AVAILABILITY
All data are available upon request to the corresponding author.
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