Dry conditions helped C4 plants emerge during the mid-Oligocene

Wednesday, May 22nd, 2019

The period between 30 million years ago to about 5 million years ago (the mid-Oligocene) saw the emergence of a new form of photosynthesis in a subset of plants, the C4 pathway. Researchers have long believed that falling carbon dioxide levels were the cause, but a new study based on biochemical modeling by a group of University of Pennsylvania biologists and paleoclimate modeling by a group at Purdue University, led by professor Matthew Huber (Earth, Atmospheric and Planetary Sciences), indicates that water availability may have been the critical factor behind the emergence of C4 plants such as corn, sorghum, and sugar cane.

The researchers used a coupled photosynthesis–hydraulic optimal physiology model in conjunction with paleoclimate modeling to examine the primary selective pressures along the ecological trajectory of C4 photosynthesis and to determine the geographic origins and expansion of the C4 pathway. This modeling system included four factors that could either favor the C3 or the C4 lineage: carbon dioxide (CO2) concentration, light, temperature and water availability. The findings suggest that water limitation was the primary driver for the initial ecological advantage of C4 over C3 in the mid-Oligocene until CO2 became low enough to, along with light intensity, drive the global expansion of C4 in the Miocene. This work helps explain how different plant lineages came to be distributed on the planet today and gives some insight into how they might respond to future conditions.

Zhou, H., Helliker, B. R., Huber, M., Dicks, A. and Akçay, E. (2018). C4 photosynthesis and climate through the lens of optimality. Proceedings of the National Academy of Sciences, 115 (47), 12057-12062.

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