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CONSTRAINTS Report Summary

Project ID: 639706
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - CONSTRAINTS (Ecophysiological and biophysical constraints on domestication in crop plants)

Reporting period: 2015-06-01 to 2016-11-30

Summary of the context and overall objectives of the project

A fundamental question in biology is how constraints drive phenotypic changes and the diversification of life. While domesticated species are extraordinary models for tracking phenotypic changes in response to directional selection, we know little about the role of these constraints on crop domestication, nor how artificial selection can escape them.

Crop yields experienced spectacular growth during the Green Revolution thanks to plant breeding and agronomic development. Yet, crop improvement has not seen comparable advances for increased productivity in the last decades. Artificial selection over preexisting natural variation might have reached a dead end, and the environmental impacts of intensified agronomy are unsustainable in the long term. Thus, plant breeding may have to look for novel strategies to ensure food security and environmental sustainability for a steadily increasing human world population, including the creation of ideotypes adapted to low-input agricultural management. While new biotechnological tools producing ‘super-domesticates’ may offer qualitative advancement in breeding for higher productivity, plant breeding approaches rarely account for ecophysiological and biophysical constraints - linked to resource capture, use and partitioning - that can strongly limit the improvement of the basic functions of plants: growth, reproduction and survival. These constraints arise from trade-offs (i.e. the impossibility to simultaneously optimize two conflicting functions) that take place at different organizational levels: cell, organ, organism and crop (monoculture and mixture). These constraints are considered as obstacles for the improvement of target crop traits by plant breeders but their quantification remains scarce. Interestingly, several generic, cross-species, ecophysiological and biophysical constraints have been identified in theoretical and empirical ecology, with an acceleration in the last two decades with the rising of trait-based ecology. The CONSTRAINTS project aims at examining the persistence of these constraints, found across wild species, within and between domesticated species and the possibility to overcome them in future tradeoff-free improvement approaches. We primarily focus on ecophysiological and biophysical constraints at two organization levels: at the organ level (in particular leaves and roots) and at the whole-plant level. We will investigate other organs (including grains and stems) and other organizational levels: cell and crop.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Work related to WP1 and WP2:
We used a collection of 39 genotypes representative of key steps during tetraploid wheat domestication, and grew them in a common garden experiment. We quantified the vegetative phenotype of each genotype through the measurements of 13 functional traits related to root, leaf and whole-plant dimensions. A characterization of the crop phenotype based on vegetative traits allowed to discriminate the different forms of domestication previously identified, suggesting a much more complete domestication syndrome. In modern cultivars, compared to ancestral forms, leaf longevity was shorter while net photosynthetic rate, leaf production rate and nitrogen content were higher. Modern cultivars had a shallower root system and exhibited a larger proportion of fine roots, preferring to invest biomass above- rather than below-ground. Finally, we found ancestral forms to be integrated phenotypes characterized by a coordination between above- and below-ground functioning. Conversely, in modern forms, human selection appeared to have broken this coordination and to have generated a new type of network of trait covariations. The examination of leaf, root, and whole-plant traits of wheat accessions indicated a strong shift in plant functional strategies over the course of domestication, so that elite genotypes tended to better optimize resource-use acquisition strategies than ancestral ones. Our findings highlight the benefits of using a functional trait-based characterization of crop phenotypes to document the extent of domestication syndrome and to further advance the emerging field of agroecology.

We also used 50 ecotypes and 35 cultivars from five grassland species to explore how selection drives functional trait coordination and genetic differentiation. We quantified the extent of genetic diversity among different sets of functional traits and determined how much genetic diversity has been generated within populations of natural ecotypes and selected cultivars. In general, the cultivars were larger (e.g. greater height, faster growth rates) and had larger and thinner leaves (greater SLA). We found large (average 63%) and trait-dependent (ranging from 14% for LNC to 95.8% for growth rate) genetic variability. The relative extent of genetic variability was greater for whole-plant than for organ-level traits. This pattern was consistent within ecotypes and within cultivars. However, ecotypes presented greater ITV variability. The results indicated that genetic diversity is large in domesticated species with contrasting levels of heritability among functional traits and that selection for high yield has led to indirect selection of some associated leaf traits. These findings open the way to define which target traits should be the focus in selection programs, especially in the context of community-level.

Work related to WP3:
While plant species diversity in natural ecosystems has been demonstrated to regulate their productivity, stability and response to disturbance, its role in multi-species cropping systems, notably sown grasslands, remains largely less unknown. Similarly, a pivotal role of genetic diversity is suspected even if empirical evidence is scarce. Genetic diversity is potentially the most important leverage in cultivated systems where a large range of genotypes per species can be generated through artificial selection. We concomitantly tested the effects of both facets of biodiversity on fodder production and stability of biomass production of sown grasslands by jointly manipulating the number of species and genotypes of experimental assemblages with or without an intense drought event. The multi-species assemblages were more productive than monocultures, notably in the drought treatment, independently of the number of genotypes added per species. Conversely, the temporal stability of yield production increased only with the number of genotypes and was lower in the drought treatment. Together, these results highlight complementary roles of taxonomic and genetic diversity for optimizing the production, resistance and sustainable supply of livestock fodder in managed grasslands. The incorporation of multifaceted biodiversity in cropping systems remains the great challenge of grassland agriculture in the face of increasing environmental hazards.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Highly diverse grasslands, forests and aquatic ecosystems are more productive and stable than the sum of their constituent species, due to the complementary traits and interactions of different species. Ecologists have primarily focused on the negative implications of this phenomenon: plant species loss will lead to a reduction in the function and stability of natural ecosystems. We illustrated the positive implications of this relationship for agricultural systems, by showing that the addition of complementary plant species can promote forage crop yields in managed grasslands, and that further addition of genotypes within species can promote the stability of forage yield across harvests.
Many of Earth’s grasslands, savannahs and forests have been converted to agricultural land. Worldwide, pastures and rangelands cover an area larger than Africa, and croplands cover an area nearly as large as South America. These land-use changes have delivered benefits, namely an increase in food production. But they have also come at a cost to the environment, due to concomitant increases in carbon emissions and reductions in water and air quality. Some of the costs of land conversion are the direct result of biodiversity loss; diversity is kept to a minimum in most agricultural systems, rendering them leaky — that is, less able to capture and retain carbon and nutrients.
In natural and extensively managed grasslands, productivity depends on plant diversity. Adding back some of the plant diversity lost during land conversion could therefore reduce the environmental costs and increase the benefits of agriculture. The idea of diversifying agricultural systems to enhance yields is not new. More than a century ago Darwin wrote “It has been experimentally proved that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can thus be raised.” But this understanding was temporarily lost during the twentieth century, due to inadequately designed forage diversity experiments that confounded changes in species diversity with changes in species composition and biased data analyses.
Some combinations of species and genotypes are more complementary, and thus valuable, than others. Plants can negatively impact one another by reducing the abundance of shared resources, such as water and nutrients, or by increasing the abundance of shared natural enemies, such as herbivores and pathogens. Species or genotypes can coexist in a complementary manner when they consume different resources and have specialized natural enemies, or when there is temporal and/or spatial variability in the abundances of resources or enemies). The identification of complementary combinations of species remains challenging.
It is surprising that we found such complementarity among forage crop species in a short-term local-scale experiment. In natural ecosystems, the identity of the most productive plant species varies substantially across years, places, and environmental conditions. Furthermore, species interactions limit the similarity between species and reinforce differences. As such, it’s no surprise that diversity promotes productivity in these systems. In sharp contrast to native plants, however, crops have been developed to thrive in monocultures where conditions are kept as favourable and constant as possible. Over time, any initial complementary differences between crops are likely to be eroded by lack of both species interactions and spatiotemporal variability in limiting factors. It is therefore impressive that crop species can be strategically combined to complement one another. Given that increased complementarity can be naturally selected for relatively rapidly, it should be possible for agronomists to design and improve crop species mixtures to enhance yields by maximizing the extent to which species complement one another. The same tools and technologies that have been developed and employed to optimize monoculture production could be readily applied to optimize polyculture production.
There are, however, many challenges that stand in the way of the diversification of agricultural ecosystems. An agricultural diversification strategy would, in many ways, be a reversal of the current trajectory of agricultural simplification. To date, agronomists have developed extremely low-diversity but high-yielding cropping systems, that are supported by equipment for planting and harvesting monocultures, and massive inputs of water, nutrients, herbicides and pesticides, supplied to reduce spatiotemporal variability in the factors that limit plant growth. In stark contrast, a diversification strategy would require designing and developing complementary suites of crop species and genotypes that could perform well across spatially heterogeneous and temporally-fluctuating environments.
Despite these challenges, the diversification of agricultural systems could well become an increasingly profitable strategy for enhancing agricultural productivity as the climate changes and extreme events become increasingly common, and as increasingly marginal lands are adopted or intensified for agricultural purposes. In these cases, environmental variability cannot be so easily avoided or anticipated. As illustrated by our results, diversity can provide insurance against such unavoidable environmental variability.
Mixing existing crop genotypes would be an easy place to begin diversifying agroecosystems. We showed even this moderate level of diversification could stabilize yields across harvests. Designing and developing diverse crop mixtures that include many complementary species and genotypes will be more challenging, but will likely become an increasingly necessary and profitable strategy for maintaining yields over the next few decades.

Related information

Record Number: 198312 / Last updated on: 2017-05-18