Does physiological innovation change the fundamental relationships between growth and survival?

"Live fast, die young" famously describes the wild excesses of rock stars and Hollywood actors, but also encapsulates an important biological principle. Animals and plants that grow and reproduce quickly are more likely to be killed by natural enemies or environmental extremes. We usually explain this biological trade-off in terms of energy: more energy spent on growth means less energy invested in defence against enemies, the capture of essential resources, or into stores for surviving adverse conditions. A logical extension of this explanation is that, if the same growth could be achieved using less energy, more would be available for defence, resource capture and storage, thereby increasing survival. However, this prediction remains untested, despite its central importance for biology.

The evolution of C4 photosynthesis in more than seventy plant lineages has increased the efficiency of photosynthetic energy conversion at high light and hot temperatures, in comparison with the ancestral C3 type of photosynthesis. To understand how this increase in photosynthetic efficiency influences growth, we have developed an experimental approach capable of comparing growth among hundreds of plant species in the same environmental conditions. We have discovered that, as well as a direct physiological effect of C4 photosynthesis in promoting faster growth, C4 leaves are unexpectedly less dense than C3 ones, further increasing growth efficiency. This allows C4 plants to be larger, with more growth invested in roots, which leads us to hypothesize that they may be able to accumulate greater storage, and have better access to water during drought than their C3 counterparts. Together, these hypothesized effects are expected to increase plant survival following repeated defoliation and drought events. If supported by experimental evidence, these ecological differences between C3 and C4 plants would have important global scale implications for the responses of plant communities to environmental change and land management.

We propose to test these hypothesis using three large comparative experiments, capitalizing on our recent advances in developing high-throughput experimental screening methods. We are able to measure growth, allocation to roots verses shoots, storage and survival on thousands of plants in the same experimental set-up, and have developed novel statistical methods to analyze the large resultant datasets. We are also the first group to successfully apply metabolomic methods to identify and quantify storage compounds across multiple wild plant species. Our strategy for the proposed work will be to combine these approaches, investigating survival of experimentally imposed drought or repeated defoliation in seventy ecologically important grass species, representing seven independent evolutionary origins of C4 photosynthesis and their C3 sister taxa. Alternative hypothesized survival mechanisms will be tested by using plants of different ages to manipulate size. Since C4 photosynthesis also has a direct physiological effect on plant water use, by reducing stomatal aperture, we will make detailed measurements of plant hydraulics during the drought experiment.

Findings from the three experiments will allow us to test the relative importance to survival of greater storage, deeper rooting, lower plant water use, and greater plant size in C4 then C4 species, and to gain a holistic understanding of the system. The work will enhance our mechanistic understanding of how a major physiological innovation changed growth-survival relationships and enabled plants to explore new phenotypic space. Throughout the project, we will work with mathematical modelers to ensure that the experiments will generate data that are useful for developing improved models of how global vegetation stores carbon and influences climate.

We expect our research to have impact in two main ways:-

1. Climate Change Policy

Our research is likely to have its greatest impact on the modeling of vegetation dynamics for the purpose of climate change projections. Such projections are currently undertaken by governmental organizations like the Met Office (UK) and NCAR (USA), to directly inform national policy on climate change and feed into the IPCC climate change assessment reports.

Climate projections are made using General Circulation Models (GCMs) to simulate the physics of the atmosphere. A crucial component of such models is the land surface, which exchanges energy and water with the atmosphere. In models of the carbon cycle and the Earth System, carbon dioxide and trace gas exchanges are also considered. Modern land surface models incorporate a representation of vegetation dynamics and, as we discuss in our Case for Support, these make explicit and implicit assumptions about the ecological behaviour of C3 and C4 grasses. These plant types account for two out of (as few as) five to seven plant functional types represented, and dominate the ground cover over a large fraction of the terrestrial surface (>25%). C4- and C3-dominated grassy vegetation has very different rates of CO2 and water vapour exchange with the atmosphere.

We expect our proposed research to bring new understanding of the ecological behaviour of C3 and C4 grass functional types, and we have devised a plan to facilitate the translation of experimental data into information that is useful for the models of vegetation dynamics used in land surface schemes of GCMs. This translation will be enabled via interactions with scientists who are directly involved in GCM model development at the Met Office and NCAR. Interaction will be facilitated via workshops, as outlined in our Pathways to Impact plan.

2. Public engagement

Both the PI (Osborne) and two of the Co-I's (Thompson, Beerling) are very active in public engagement on scientific issues, and have extremely strong track records in this area. The PI is primarily engaged in schools events, including public lectures and "science shows" at the university and local museum, plus visits to schools to run workshops about core principles in plant biology. The Co-I's are primarily engaged in writing popular science books, which are based upon findings from their current and past research. Ken Thompson regularly appears on BBC Radio and TV to talk about his books and the science of ecology. David Beerling's book "The Emerald Planet" was adapted into a three part TV series for BBC 2 "How to Grow a Planet", one programme of which was devoted to C4 grasses and their expansion into the savanna biome. There would be an inevitable, general "trickle-down" of research findings into these ongoing activities, as the grant progressed.

In addition to these dissemination activities, we propose two specific activities linked directly to the grant. First, the PI will develop schools activities centred around how trade-offs arise in biology, and the importance of life history decisions for evolution and ecology. These activities will be plant-centred, showcasing some of the exciting questions in this area, and will be made available for schools usage via the Science and Plants in Schools (SAPS) organization. Secondly, we will also communicate scientific results in a "Plain English" form as they are published, to public audiences via the PI's website and twitter.