Agriculture is facing up to an increasing number of challenges, including the need to ensure various ecosystem services and to resolve apparent conflicts between them. One of the ways forward for agriculture currently being debated is a set of principles grouped together under the umbrella term "ecological intensification". In published studies, ecological intensification has generally been considered to be based essentially on the use of biological regulation to manage agroecosystems, at field, farm and landscape scales. We propose here five additional avenues that agronomic research could follow to strengthen the ecological intensification of current farming systems. We begin by assuming that progress in plant sciences over the last two decades provides new insight of potential use to agronomists. Potentially useful new developments in plant science include advances in the fields of energy conversion by plants, nitrogen use efficiency and defence mechanisms against pests. We then suggest that natural ecosystems may also provide sources of inspiration for cropping system design, in terms of their structure and function on the one hand, and farmers' knowledge on the other. Natural ecosystems display a number of interesting properties that could be incorporated into agroecosystems. We discuss the value and limitations of attempting to 'mimic' their structure and function, while considering the differences in objectives and constraints between these two types of system. Farmers develop extensive knowledge of the systems they manage. We discuss ways in which this knowledge could be combined with, or fed into scientific knowledge and innovation, and the extent to which this is likely to be possible. The two remaining avenues concern methods. We suggest that agronomists make more use of meta-analysis and comparative system studies, these two types of methods being commonly used in other disciplines but barely used in agronomy. Meta-analysis would make it possible to quantify variations of cropping system performances in interaction with soil and climate conditions more accurately across environments and socio-economic contexts. Comparative analysis would help to identify the structural characteristics of cropping and farming systems underlying properties of interest. Such analysis can be performed with sets of performance indicators and methods borrowed from ecology for analyses of the structure and organisation of these systems. These five approaches should make it possible to deepen our knowledge of agroecosystems for action. (Résumé d'auteur

Increasing pea (Pisum sativum) seed nutritional value and particularly seed protein content, while maintaining yield, is an important challenge for further development of this crop. Seed protein content and yield are complex and unstable traits, integrating all the processes occurring during the plant life cycle. During filling, seeds are the main sink to which assimilates are preferentially allocated at the expense of vegetative organs. Nitrogen seed demand is satisfied partly by nitrogen acquired by the roots, but also by nitrogen remobilized from vegetative organs. In this study, we evaluated the respective roles of nitrogen source capacity and sink strength in the genetic variability of seed protein content and yield. We showed in eight genotypes of diverse origins that both the maximal rate of nitrogen accumulation in the seeds and nitrogen source capacity varied among genotypes. Then, to identify the genetic factors responsible for seed protein content and yield variation, we searched for quantitative trait loci (QTL) for seed traits and for indicators of sink strength and source nitrogen capacity. We detected 261 QTL across five environments for all traits measured. Most QTL for seed and plant traits mapped in clusters, raising the possibility of common underlying processes and candidate genes. In most environments, the genes Le and Afila, which control internode length and the switch between leaflets and tendrils, respectively, determined plant nitrogen status. Depending on the environment, these genes were linked to QTL of seed protein content and yield, suggesting that source-sink adjustments depend on growing conditions. The last two decades have seen an exponential increase in the number of plant sequences in databases and the explosion of investigations on the molecular functions and physiological roles of these genes. At the same time, new concepts, such as quantitative trait loci (QTL) mapping followed by the development of statistical tools, have emerged in quantitative genetics to identify the genes involved in the genetic variability of complex traits (Lander and Botstein, 1989). The functions of thousands of genes have been identified mainly through knockout mutant analysis (Østergaard and Yanovsky, 2004), but also through QTL identification (for review, see Paran and Zamir, 2003). These tools can now be used to address the question of phenotypic plasticity-which genes control plant functioning in which environments-and to provide some clues about which forces shaped natural variation and the strategies that should be used to breed more adapted cultivars (Paran and Zamir, 2003;Reymond et al., 2003;Koornneef et al., 2004;Mitchell-Olds and Schmitt, 2006). In this respect, Tonsor et al. (2005) proposed to analyze natural genetic variation in the model species Arabidopsis (Arabidopsis thaliana) to identify the role of genes having subtle, partially redundant, and/or environment-dependent effects on phenotypes, and to better understand gene interactions and pleiotropy. The study of natural genetic ...

of having to increase food production by about 50% by 2050 to cater for an additional three billion inhabitants, in a context of arable land shrinking and degradation, nutrient deficiencies, increased water scarcity, and uncertainty due to predicted climatic changes. Already today, water scarcity is probably the most important challenge, and the consensual prediction of a 2-4°C degree increase in temperature over the next 100 years will add new complexity to drought research and legume crop management. This will be especially true in the semi-arid tropic areas, where the evaporative demand is high and where the increased temperature may further strain plant-water relations. Hence, research on how plants manage water use, in particular, on leaf/root resistance to water flow will be increasingly important. Temperature increase will variably accelerate the onset of flowering by increasing thermal time accumulation in our varieties, depending on their relative responses to day length, ambient, and vernalizing temperature, while reducing the length of the growing period by increasing evapotranspiration. While the timeframe for these changes (>10-20 years) may be well in the realm of plant adaptation within breeding programs, there is a need for today's breeding to understand the key mechanisms underlying crop phenology at a genotype level to better balance crop duration with available soil water and maximize light capture. This will then be used to re-fit phenology to new growing seasons under climate change conditions. The low water use efficiency, i.e., the amount of biomass or grain produced per unit of water used, under high vapor pressure deficit, although partly offset by an increased atmospheric CO 2 concentration, would also require the search of germplasm capable of maintaining high water use efficiency under such conditions. Recent research has shown an interdependence of C and N nutrition in the N performance of legumes, a balance that may be altered under climate change. Ecophysiological models will be crucial in identifying genotypes adapted to these new growing conditions. An increased frequency of heat waves, which already happen today, will require the development of varieties capable of setting and filling seeds at high temperature. Finally, increases in temperature and CO 2 will affect the geographical distribution of pests, diseases, and weeds, presenting new challenges to crop management and breeding programs.Abstract Humanity is heading toward the major challenge

The fluxes of (1) exogenous nitrogen (N) assimilation and (2) remobilization of endogenous N from vegetative plant compartments were measured by 15 N labeling during the seed-filling period in pea (Pisum sativum L. cv Caméor), to better understand the mechanism of N remobilization. While the majority (86%) of exogenous N was allocated to the vegetative organs before the beginning of seed filling, this fraction decreased to 45% at the onset of seed filling, the remainder being directed to seeds. Nitrogen remobilization from vegetative parts contributed to 71% of the total N in mature seeds borne on the first two nodes (first stratum). The contribution of remobilized N to total seed N varied, with the highest proportion at the beginning of filling; it was independent of the developmental stage of each stratum of seeds, suggesting that remobilized N forms a unique pool, managed at the whole-plant level and supplied to all filling seeds whatever their position on the plant. Once seed filling starts, N is remobilized from all vegetative organs: 30% of the total N accumulated in seeds was remobilized from leaves, 20% from pod walls, 11% from roots, and 10% from stems. The rate of N remobilization was maximal when seeds of all the different strata were filling, consistent with regulation according to the N demand of seeds. At later stages of seed filling, the rate of remobilization decreases and may become controlled by the amount of residual N in vegetative tissues.Pea (Pisum sativum) is an important agricultural crop grown primarily for its high seed protein content. However, the protein yield of the pea crop remains too low and variable between cropping area and years (http://apps.fao/faostat/, http://www.prolea.com/ unip/) to sustain the needs in plant protein of European countries. To extend the pea crop in Europe and to increase use of pea seed in the feed industry, breeders have to develop varieties with better harvest and nitrogen (N) indices. Toward this aim, a better understanding of the processes involved in the elaboration of seed protein content is needed. The final protein yield of seeds depends both upon the genotype and on environmental factors during seed filling (Lhuillier-Soundélé et al., 1999a). Nitrogen accumulation by seeds during filling depends upon the external N supply: soil mineral N assimilation and/or symbiotic fixation of atmospheric N 2 . However, exogenous N cannot generally sustain the high N demand of filling seeds, so endogenous N previously accumulated in vegetative parts is largely remobilized to fulfill this demand (Sinclair and de Wit, 1976;Salon et al., 2001). Seed N concentration is correlated to N availability within plants, and the extent of the contribution of N remobilization to seed N yield varies among legumes: 70% in field-grown pea (Atta et al., 2004), 80% to 90% in soybean (Glycine max; Warembourg and Fernandez, 1985;Grandgirard, 2002), 43% to 94% in rain-fed grown lentil (Lens culinaris; Kurdali et al., 1997), 84% in bean plants (Phaseolus vulgaris;Westermann et al., 1...

When reciprocal crosses are made between different pea genotypes, there is a strong maternal influence on mature seed size of the reciprocal hybrids, i.e. their dry weights are similar to that of seeds obtained from their maternal parents. Reciprocal crosses between pea varieties having very different mature seed sizes were used to investigate how the maternal genotype controls seed development and mature seed size. The differences in dry seed weight between genotypes and reciprocal hybrids reflected differences in both cotyledon cell number and mean cell volume, and the maternal control on the establishment of these two traits was investigated. Using flow cytometry, data relative to endoreduplication kinetics in cotyledons during the transition between the cell division phase and maturation were obtained. The appearance of nuclei having an 8C DNA content indicates the initiation of the endoreduplication phenomenon and thus the end of the cell division phase. It was shown that the duration of the cell division phase was the same in the reciprocal hybrids, its value being intermediate between those recorded for their maternal parents. This result indicates that the timing of development of the embryo is not under maternal control, but depends on its own genotype. Consequently, maternal genotype must influence the mitotic rate during the cell division phase to achieve differences in cell number found in the cotyledons of mature F1-reciprocal hybrids. The final level of endoreduplication in cotyledons of mature seeds was also investigated. This study showed that there is a close relationship (r2 = 0.919) between the endoreduplication level in mature cotyledons and seed dry weight or mean volume of cotyledon cells, suggesting that both maternal and non-maternal factors could control the number of endoreduplicating cycles in the cotyledons and, hypothetically, the cotyledon cell size.

Integrating principles of ecological intensification into weed management strategies requires an understanding of the many relationships among weeds, crops and other organisms of agro-ecosystems in a changing context. Extensively used during the last two decades in weed science, trait-based approaches have provided general insights into weed community response to agricultural practices, and recently to understanding the effect of weeds on agro-ecosystem functioning. In this review, we provide a holistic synthesis of the current knowledge on weed response and effect functional traits. Based on the literature and recent advances in weed science, we review current knowledge on (i) weed functional groups and ecological strategies, (ii) weed functional response traits to cropping systems and (iii) weed functional effect traits affecting agro-ecosystem functioning. For each functional trait, we explicitly present the assumptions and evidence on the linkage between trait values and ecological functions, in response to either management practices, for example tillage, sowing and herbicides, or biotic interactions, for example crop-weed competition and pollination. Finally, we address and discuss major research avenues that may significantly improve the use of traits and the knowledge of functional diversity in weed science for the future, especially to design and implement more environmentally sustainable weed management strategies.

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A better knowledge of the nitrogen nutrition of Medicago truncatula at the whole plant level and its modulation by environmental factors is a crucial step to reach a complete understanding of legume nitrogen nutrition. This study was based on the symbiotic system that is the most commonly used by the research community (M. truncatula cv. Jemalong A17 x Rhizobium meliloti strain 2011). Plant nitrogen nutrition was analysed in relation to carbon nutrition, under a range of nitrate concentrations in the nutrient solution and different light conditions. This study shows that this 'model symbiotic association' does not allow the plant to meet its nitrogen requirements, when dinitrogen fixation is the main nitrogen source for plant growth. A strong interaction between nitrogen and carbon nutrition was shown: when plant nitrogen requirements were not sustained, plant leaf area was much affected whereas photosynthesis per unit leaf area remained relatively stable. Both total nitrogen uptake and leaf area increased with increasing nitrate concentration in the nutrient solution; the magnitude of these responses varied according to the light conditions. Interestingly, the plant nitrogen nutrition level remained nearly unaffected by the light conditions. The observed nitrogen-limitation in this 'model symbiotic association' is an important finding for the research community. Based on practical recommendations regarding both the experimental conditions and the phenotypic traits to consider, a methodological framework was proposed to (i) help genomicists to assess plant nitrogen nutrition better, and (ii) assist in the detection of new genetic variants affected for nitrogen uptake in large-scale phenotyping studies.