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

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"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.