Functional diversity among grass species: the role of photosynthetic pathway

Lead Research Organisation: University of Sheffield
Department Name: Animal and Plant Sciences

Abstract

One of the major unresolved problems in contemporary ecology is to understand how diversity arises in plant function. This has important implications for interpreting global patterns of biodiversity and predicting the impacts of climate change. In this application, we propose to investigate the factors responsible for the diversity of growth traits among the world's grass species, addressing the question of why different species grow at different rates, and allocate different amounts of growth to roots vs leaves. We are particularly interested in the role that has been played by evolutionary transitions between the C3 and C4 photosynthetic pathways. The C4 pathway is classically thought to boost the growth of plants under hot conditions, in comparison with the C3 type. Photosynthetic pathway is a major axis of variation in plant function that has been invoked to explain significant changes in ecosystem structure in response to past episodes of climate change, the turnover of grass species composition along global climatic gradients, and the ecological sorting of species within regional floras. However, recent work has challenged these long-standing interpretations by demonstrating that ecological adaptation to temperature and drought, and evolutionary history may each play an equally important role in explaining large-scale biogeographical and ecological patterns. In combination with intriguing recent experiments, these new findings raise questions about the extent to which C4 photosynthesis interacts with other plant traits, ecology and evolutionary history to influence plant growth. We will address these major issues by taking a comparative screening approach to investigate the growth traits of ~400 species under a range of environmental conditions. The use of large species samples will allow us, for the first time, to unravel the interacting roles of multiple factors within a unified framework. This approach has been opened up through significant recent conceptual and methodological advances in a number of areas, and represents a major step in the fusion of the fields of ecological and evolutionary informatics.
 
Description In this project we are investigating the basic question of why some plant species grow faster than others, using an ambitious programme of large-scale screening experiments, each taking in hundreds of species. How do photosynthetic pathway, ecological niche, evolutionary history, life history and growth form influence plant growth rate? Which are the most important factor(s)? Our work focuses on wild plants, but we have included crops to ask how growth rate in these species compares with that in wild species.

Three results stand out as particularly important. Firstly, C4 plants grow faster than their C3 counterparts. This result has assumed by the research community for several decades but never demonstrated. Secondly, this boost to growth comes from structural rearrangements as well as physiology, via changes to leaf anatomy. Thirdly, it is accompanied by increases in the allocation of growth to roots, an important determinant of plant survival under drying soil conditions. Finally, we have found that faster growth is achieved through the more rapid enlargement of leaf and root modules, without change in the initiation or branching rates of these organs.

In related work, we have found that the rapid growth induced by C4 photosynthesis acts independently of other growth determinants, especially leaf and root functional traits. We have also found that the association previously observed between C4 photosynthesis and high leaf silicon concentrations does not arise through a direct linkage, but rather seems to be associated with environment, whereby C4 species are more likely to occupy warm, dry habitats which favour silicon accumulation.

Hypothesis-testing using our data and paper-writing are ongoing.
Exploitation Route The impact of our work is likely to come mostly within the vegetation modelling community, who require information about the basic differences between plant functional groups. These model simulations in tern feed back on climate model simulations which are crucial for our understanding of how global climate change will progress over the coming century.
Sectors Environment
 
Description The impact of our work is likely to come mostly within the vegetation modelling community, who require information about the basic differences between plant functional groups. These model simulations in tern feed back on climate model simulations which are crucial for our understanding of how global climate change will progress over the coming century.
 
Title Data from: C4 photosynthesis and the economic spectra of leaf and root traits independently influence growth rates in grasses 
Description Photosynthetic pathway is an important cause of growth rate variation between species, such that the enhanced carbon uptake of C4 species leads to faster growth than their C3 counterparts. Leaf traits that promote rapid resource acquisition may further enhance the growth capacity of C4 species. However, how root economic traits interact with leaf traits, and the different growth strategies adopted by plants with C3 and C4 photosynthetic pathways is unclear. Plant economic traits could interact with, or act independently of, photosynthetic pathway in influencing growth rate, or C3 and C4 species could segregate out along a common growth rate-trait relationship. We measured leaf and root traits on 100+ grass species grown from seeds in a controlled, common environment to compare with relative growth rates (RGR) during the initial phase of rapid growth, controlling for phylogeny and allometric effects. Photosynthetic pathway acts independently to leaf and root functional traits in causing fast growth. Using C4 photosynthesis, plants can achieve faster growth than their C3 counterparts (by an average 0.04 g g-1 day-1) for a given suite of functional trait values, with lower investments of leaf and root nitrogen. Leaf and root traits had an additive effect on RGR, with plants achieving fast growth by possessing resource-acquisitive leaf traits (high specific leaf area and low leaf dry matter content) or root traits (high specific root length and area, and low root diameter), but having both leads to an even faster growth rate (by up to 0.06 g g-1 day-1). C4 photosynthesis can provide a greater relative increase in RGR for plants with a 'slow' ecological strategy than in those with fast growth. However, aboveground and belowground strategies are not coordinated, so that species can have any combination of 'slow' or 'fast' leaf and root traits. Synthesis: C4 photosynthesis increases growth rate for a given combination of economic traits, and significantly alters plant nitrogen economy in the leaves and roots. However, leaf and root economic traits act independently to further enhance growth. The fast growth of C4 grasses promotes a competitive advantage under hot, sunny conditions. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL http://datadryad.org/stash/dataset/doi:10.5061/dryad.xwdbrv1b1
 
Title Large seeds provide an intrinsic growth advantage that depends on leaf traits and root allocation 
Description Seed mass and growth rate are important dimensions of plant ecological diversity, but their relationship remains unresolved. Negative relationships between relative growth rate (RGR) and seed mass are well established. However, RGR is size-dependent, so small-seeded species might achieve fast growth simply because they are initially small. Using a dataset of unprecedented size, sampling 382 grass species, we investigated seed mass and growth rate using both RGR and SGR (RGR at a specific size), accounting for diversity in phylogeny, ecology (e.g. life history, photosynthetic pathway) and environment (mean annual temperature and precipitation). RGR and SGR showed contrasting relationships with seed mass, such that large-seeded species had lower RGR but higher SGR than small-seeded species. However, the relationship between SGR and seed mass depended on leaf dry matter content (LDMC), and was only positive in high-LDMC species. When compared at a common size, the fast growth of large-seeded and low-LDMC species was associated with greater biomass allocation to roots in the hot, high-light environment used for our experiment. Photosynthetic pathway and life history contributed to variation in SGR, with C4 annuals having higher SGRs than C3 perennials regardless of seed size. Large seeds therefore afford an intrinsic growth advantage in species with resource-conserving leaf traits, and may provide a competitive edge in resource-poor environments. This work advances understanding of how seed mass and growth rate coevolve with other ecological factors. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL http://datadryad.org/stash/dataset/doi:10.5061/dryad.ksn02v74j