Plants that evolved during warm periods with high CO2 have larger but fewer leaf pores than species that first appeared during cooler times.
Leaf pores, called stomata, are microscopic structures that control the exchange of water and carbon dioxide between the plant and the atmosphere. Stomata evolved when plants colonised land about 400 million years ago and have kept the same general shape ever since. But their size and number has changed quite considerably throughout their history.
'This variation was well known from the fossil record of plants, but it was never explained,' says Dr Peter Franks, from the University of Sheffield and lead author of the paper co-authored with Professor David Beerling.
To make the connection between pore size and CO2 levels, Franks and Beerling combined data from fossils, climate models and physiology. It's an 'unusual set of resources to gain a completely new insight into when, why and how modern plants evolved to grow much faster than their ancestors,' says Franks.
Change in pore size
Franks and Beerling found that variation in pore size through time is related to changes of CO2 in the atmosphere. At times when the Earth is warm and CO2 concentrations are high, new plants species evolve with large stomata, sparsely distributed through the leaf. But when the temperature is cooling down and the CO2 in the atmosphere is relatively low, new plants emerge with numerous but tiny leaf pores.
This correlation exists because lots of small stomata promote a more efficient gas exchange between leaves and the atmosphere than few bigger pores. 'Grasses are a good example,' says Franks. With small yet plentiful pores, they are a successful and relatively recent plant group, perfectly adapted to make the most of a CO2-poor atmosphere.
On the other hand, club mosses first appeared during the Devonian, about 400 million years ago, when the Earth was going through a greenhouse climate with high CO2 concentrations in the atmosphere. Nowadays, 'club mosses are remnants of an ancient flora typical of warm climates, with large but few stomata,' says Franks.
The findings, published this week in the Proceedings of the National Academy of Sciences, show for the first time that long-term changes in atmospheric CO2 are able to steer the evolution of plants.