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Choose a Project: Projects Home Page 1. Atmospheric CO2 & Leaf Evolution 2. Polar Forests - Carbon Balance 3. Polar Forests - Distribution and Ecology 4. Biogenic Trace Gas Fluxes 5. The Evolution of C4 Plants 6. Palaeoceanographic changes 7. Biodiversity Response to Climate Change 8. Evolution of C4 Photosynthesis 9. Terminal Cretaceous Climate Change 10. C4 phylogenetic history

Personnel
Principal Investigator:
Research Technician:

Colin Osborne
Emily Wythe

Funding

British Ecological Society Early Career Project Grant

Project dates:
September 2004 - September 2005

Summary

More than a decade ago, Ehleringer and co-worker’s ‘CO₂ starvation hypothesis’ proposed that episodes of low atmospheric CO₂ were the principal selection pressure for C₄ plant evolution. This is because, under warm conditions, the C₄ CO₂-concentrating mechanism overcomes the limitation imposed by low CO₂, which can lead to CO₂ starvation in C₃ plants. New data from the fields of geology and molecular genetics support this contention, placing the earliest origin of C₄ grasses at a time of rapidly declining atmospheric CO₂.

Ehleringer JR, Sage RF, Flanagan LB, Pearcy RW (1991) Climate change and the evolution of C₄ photosynthesis. Trends Ecol. Evol. 6, 95-99.

Pagani M, Zachos J, Freeman KH, Tipple B, Boharty S (2005) Marked decline in atmospheric carbon dioxide concentrations during the Paleogene. Science 309, 600-603.

Kellogg EA (2001) Evolutionary history of the grasses. Plant Physiol. 125, 1198-1205.

This project sets out to test the central prediction of the CO₂-starvation hypothesis; that, when grown in sub-ambient CO₂ concentrations, C₄ grasses will maintain significantly greater photosynthetic rates than their C₃ counterparts, and therefore higher rates of vegetative growth, tillering and seed production. Additionally it tests the idea that, in low CO₂ concentrations and warm temperatures, C₄ grasses utilize water and nitrogen more efficiently than closely related C₃ plants.

But the C₄ mechanism carries a cost, losing efficiency relative to C₃ photosynthesis at cool temperatures, and rendering leaves susceptible to chilling and freezing damage. However, most previous work on these limitations has compared C₄ species with a tropical ancestry, with C₃ species originating in cool climates. A further aim of this project is therefore to use characterise the low temperature sensitivity of C₃ and C₄ grasses from the same tropical group, testing the hypothesis that C₄ photosynthesis is inherently more sensitive to low temperature limitations, and C₄ leaves more prone to chilling and frost damage.

Controlled environment experiments with C3 and C4 subspecies of Alloteropsis semialata are being used to investigate the advantages of C4 photosynthesis in a low CO2 atmosphere (left) and its costs at low temperatures (right).