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International audience; In recent decades, increases in wheat yield have been achieved mainly through increases in grain number per m2 (GNM2) rather than through increases in average grain weight (AGW). Using AGW as a lever to increase yield would require avoidance of the negative relationship between GNM2 and AGW. It is usually supposed that this trade-off arises from an increase in the proportion of small grains as GNM2 rises. The proportional increase in small grains being the result either of (1) an increase in the proportion of secondary tillers in the spike population or (2) of an increase in the proportion of grains located in distal positions within each spike. Either or both of these two populational effects would tend to mask any true genotypic differences in AGW. The existence of these constitutive differences has already been proposed, but without considering the full extent of the populational confounding effects. Identification of a component of the constitutive genetic determinism of AGW - one that is truly independent of GNM2 - could contribute to cultivar developments that would lead to further increases in grain yield under future target environments. To address this question, we analysed populational effects on AGW in four, modern, well-adapted bread-wheat cultivars. The four chosen cultivars show similar grain yields but contrasting AGWs. The analysis of populational effects was carried out at three hierarchical levels (the plot, the spike and the single grain) and under two contrasting environmental conditions (well-watered vs waterdeficit conditions). Regardless of the environment, no (or only slight) differences in individual spike size were observed between cultivars. Furthermore the weak relationship between spike size and AGWdemonstrates that AGW differences between cultivars cannot be attributed to spike-level populational effects. Meanwhile, the analysis of individual grain mass distributions, showed that the differences in AGW between cultivars, originated from shifts in the whole grain-mass distribution, rather than from shape changes in the grain-mass distribution. This clearly indicates that AGW differences between cultivars cannot be attributed to populational effects at the individual grain level. The analysis carried out at both spike and individual grain levels indicates that the AGW differences between cultivars are largely constitutive, so that increases in grain yield through AGW may be considered independently of the GNM2 : AGW trade-off. Taken together, these findings offer a new perspective for the genetic improvement of wheat, and one that should lead to further increases in yield.

A new method for estimating mixing ability: application to winter wheat variety mixtures and propositions for plant breeding

International audience

Variance des tailles des grains de blé tendre : un déterminisme génétique propre ou lié aux stratégies de mise en place du rendement ?. 7. Colloque du Réseau Français de Biologie des Graines

Open Access; The NAC family is one of the largest plant-specific transcription factor families, and some of its members are known to play major roles in plant development and response to biotic and abiotic stresses. Here, we inventoried 488 NAC members in bread wheat (Triticum aestivum). Using the recent release of the wheat genome (IWGS RefSeq v1.0), we studied duplication events focusing on genomic regions from 4B-4D-5A chromosomes as an example of the family expansion and neofunctionalization of TaNAC members. Differentially expressed TaNAC genes in organs and in response to abiotic stresses were identified using publicly available RNAseq data. Expression profiling of 23 selected candidate TaNAC genes was studied in leaf and grain from two bread wheat genotypes at two developmental stages in field drought conditions and revealed insights into their specific and/or overlapping expression patterns. This study showed that, of the 23 TaNAC genes, seven have a leaf-specific expression and five have a grain-specific expression. In addition, the grain-specific genes profiles in response to drought depend on the genotype. These genes may be considered as potential candidates for further functional validation and could present an interest for crop improvement programs in response to climate change. Globally, the present study provides new insights into evolution, divergence and functional analysis of NAC gene family in bread wheat.

ISBN: 978-2-9563873-0-5; Agronomic performance of wheat genotypes is usually evaluated by studying the mean value of pertinent agronomic variables. In particular, grain yield is analyzed through its different average numeric components (number of plants per m-2, number of spikes per plant, number of spikelets per spike, number of grains per spike, number of grains per m-2, individual grain weight). Nevertheless, the variance of these components has not be so far fully explored. Moreover, number of grains per m-2 is often considered as the most important component impacting grain yield. Yet it has been observed that the bigger the number of grains is, the more the number of small grains is, especially those in apical spikelets or in apical position within a spikelet. The variance associated to the grain size and individual grain composition (nitrogen concentration) is linked to both tillering and spike structure. Its impact on agronomic performance, especially in stressful environments (abiotic stress) is not much known.The aim of this study is to analyze the genetic and ecophysiological determinism of wheat individual grain characteristics variability and evaluate its impacts on agronomic performance in order to quantify the actual genetic variance potential. To this aim, several issues have to be investigated: (i) understand precisely the structure of the variance associated to individual grain characteristics and (ii) understand causal processes of this variance, especially spatial structure of the spike and spikelets.The present work is based on a field experiment where 228 genotypes were grown under optimal conditions or under a severe water stress. All yield components were recorded. Out of this panel, four genotypes were selected for in depth analysis of grain traits variance structure. For these genotypes, 10 spikes were randomly chosen in each plot and were individually dissected in order to record the exact position of each grain within the spike structure. Grains originating from 2 spikes among these 10 spikes, nitrogen concentrations per grain were also measured.Our results show that (i) individual grain weight is strongly affected both by its position within the spike as well as by mean spike characteristics and (ii) variance components are under both genetic and environmental control. These results suggest that studying variable components of grain traits can help defining typologies of wheat genotypes differing by their strategies to set grain number (and thus potential yield) at anthesis. Further studies will be needed in order to test whether these strategies have differential effects on wheat tolerance to abiotic stresses during grain filling.

European wheat production is stagnating while its demand is in constant evolution. Indeed, maintain or increase its yield in less favorable conditions is a major objective for the future. Grain yield is built during wheat development mainly through two components: the number of grains per square meter until anthesis and the thousand-grain weight during grain filling. Usually only the average grain weight is considered but grain weight may vary largely depending on the position along the spike and within a spikelet. As study was then carried out to explain why the individual weight grain is not homogenous along the spike. Two different varieties were grown in a greenhouse at daily temperature of 22.6°C and a photoperiod of 16 hr, with water and nutrient solution supplied twice a day. From anthesis to harvest at least 13 samplings of eight spikes of main tillers were conducted for each variety. The ear was divided into three parts (basal, central and apical) in which the two proximal grains of two spikelets were dissected. The individual grains were then dried and weighed. A three-parameter logistic model was fitted to describe grains growth depending on their position on the spike. At maturity, no significantly difference of dry matter was shown between the two proximal grains for both varieties. They were pooled in the following analyses. Grains in the central part of the spike were the heavier followed by the basal ones, the lighter ones being in the apical part. The logistic model fit well to the data. Differences in final grain weight are mainly explained by a difference of ovary’s dry matter at anthesis and grain growth while the duration of grain filling is the same for each part. This work brings new information to explain individual grain weight within the spike through different growth dynamics.

ISBN: 978-3-900932-48-0; Optimization of trial networks for genomic selection based on crop models. 13. IWGS

In addition to its role in vernalization, temperature is an important environmental stimulus in determining plant growth and development. We used factorial combinations of two photoperiods (16H, 12H) and three temperature levels (11, 18 and 25 °C) to study the temperature responses of 19 wheat cultivars with established genetic relationships. Temperature produced more significant effects on plant development than photoperiod, with strong genotypic components. Wheat genotypes with PPD‐D1 photoperiod sensitive allele were sensitive to temperature; their development was delayed by higher temperature, which intensified under non‐inductive conditions. The effect of temperature on plant development was not proportional; it influenced the stem elongation to the largest extent, and warmer temperature lengthened the lag phase between the detection of first node and the beginning of intensive stem elongation. The gene expression patterns of VRN1, VRN2 and PPD1 were also significantly modified by temperature, while VRN3 was more chronologically regulated. The associations between VRN1 and VRN3 gene expression with early apex development were significant in all treatments but were only significant for later plant developmental phases under optimal conditions (16H and 18 °C). Under 16H, the magnitude of the transient peak expression of VRN2 observed at 18 and 25 °C associated with the later developmental phases. This research was funded by the ADAPTAWHEAT EU‐FP7 and EU_BONUS_12‐1‐2012‐0024 research grants.

This chapter considers the genetic basis of yield potential and the implications for breeding, identifies barriers to future gains in yield potential, and assesses the scope for the use of physiological tools to select for high yield potential. Yield potential is the yield of cultivars when grown in environments to which they are adapted with nutrients and water nonlimiting and with pests, diseases, weeds, and other stresses effectively controlled. It is somewhat theoretical since “optimal” radiation and temperatures, for example, most likely interact with genotype and growth stage. This differs from the attainable yield, which corresponds to the best yield achieved through skillful use of the best available technology. On-farm yields normally realize from 60 to 80% of attainable yield; the gap relates to the physical environment being suboptimal on most farms and to moderate use of fertilizer and crop protection measures in environmentally sensitive farming. The yield gap may therefore be quite large, especially in the more marginal environments.

We present 9 years of eddy covariance measurements made over an evergreen Mediterranean forest in southern France. The goal of this study was to quantify the different components of the carbon (C) cycle, gross primary production (GPP) and ecosystem respiration (R-eco), and to assess the effects of climatic variables on these fluxes and on the net ecosystem exchange of carbon dioxide. The Puechabon forest acted as a net C sink of -254 g C m(-2) yr(-1), with a GPP of 1275 g C m(-2) yr(-1) and a R-eco of 1021 g C m(-2) yr(-1). On average, 83% of the net annual C sink occurred between March and June. The effects of exceptional events such the insect-induced partial canopy defoliation that occurred in spring 2005, and the spring droughts of 2005 and 2006 are discussed. A high interannual variability of ecosystem C fluxes during summer and autumn was observed but the resulting effect on the annual net C budget was moderate. Increased severity and/or duration of summer drought under climate change do not appear to have the potential to negatively impact the average C budget of this ecosystem. On the contrary, factors affecting ecosystem functioning (drought and/or defoliation) during March-June period may reduce dramatically the annual C balance of evergreen Mediterranean forests.

Les cultures et les prairies sont le lieu de production et d’absorption de gaz à effet de serre (GES). Les plus connus sont le CO2 et le N2O, mais il ne faut pas oublier le CH4, surtout sur les prairies pâturées, ainsi que l’ozone et ses précurseurs (NOx, composés organiques volatils), voire les particules. Cette synthèse présente les principaux facteurs connus qui modifient les flux de GES ou de stockage de carbone. Les flux de GES sur une culture ou une prairie peuvent être estimés par différentes méthodes, rapidement présentées. Les résultats expérimentaux ont mis en évidence une grande variabilité spatiale et temporelle de ces flux ainsi que des influences multiples des facteurs de l’environnement (sol, climat) et des pratiques agricoles (fertilisation, travail du sol, changement d’occupation du sol…). Seuls des suivis à moyen terme (quelques années) et dans des conditions variées permettent de proposer des facteurs d’émissions fiables (N2O, NOx, CH4). Pour faire une évaluation environnementale pertinente des systèmes agricoles, il apparaît aujourd’hui indispensable de pouvoir estimer les flux de l’ensemble des gaz à effet de serre non seulement à l’échelle d’une parcelle, mais également à des échelles supérieures (exploitation, petite région). Il faudrait également s’efforcer dans un proche avenir d’évaluer la contribution d’autres composés contribuant directement ou indirectement à l’effet de serre tels que l’ozone, les NOx, les composés organiques volatils et les particules. Enfin, la généralisation des résultats obtenus et leur extension à d’autres situations ou à d’autres échelles demanderont un investissement plus approfondi dans la modélisation des émissions et des dépôts de gaz à effet de serre dans les agrosystèmes., Grasslands and crops are both producers and absorbers of greenhouse-effect gases : CO2 and N2O, but CH4 should not be overlooked, especially on grazed pastures, nor should ozone and its precursors. This is a synthesis of the main known factors that affect the flows of greenhouse-effect gases and the storage of carbon. There exist different methods for the measurement of the gas flows from and to crops and grasslands ; they are summarily presented here. Experimental results have shown that these flows vary much in space and time and that they are influenced by many environmental factors (soil, climate) and agricultural practices (fertilizer dressings, tillage, changes in the occupation of the soil, etc.). In order to achieve a pertinent environmental assessment of the agricultural systems, it appears necessary to evaluate the flows of all the greenhouse-effect gases not only at the scale of individual fields, but also at larger scales (farm, small region).

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Predicting the response of grazed grasslands to elevated C02 is of central importance in global change research as grasslands represent 20% of the worlds land area and grassland soils are a major sink for carbon (C). Grasslands responses to elevated C02 are strongly controlled by the availability of other nutrients and nitrogen (N) in particular. There have been many previous studies of N cycling in grasslands exposed to elevated C02 but none of these experiments were grazed. In this thesis I present data on C02 effects on N cycling from an experimental system (FACE: Free Air Carbon dioxide Enrichment) that enabled grazing to be included. The thesis focuses on the effects of elevated C02 on the different processes involved in organic matter (OM) returns from the plant to the soil and the consequences for N availability. In Chapter 1, it was shown that e1evated C02 modified N returns by grazing animals by altering the partitioning of N between faeces and urine creating a potential for enhanced N losses at elevated C02. Plant litter decomposition rates were, at the ecosystem scale, not affected by elevated C02 (Chapter 3), but a marked increase in the organic matter fluxes, from roots, led to an accumulation of coarse OM in the soil (Chapter 4). In Chapter 5, using 14C and 15N labelling, I compared short-term (plant mediated) and long-term (soil mediated) effects of elevated C02 on soil OM dynamics and concluded that soil OM accumulation under elevated C02 was not caused by C or N limitation but probably by the availability of other nutrients. The thesis demonstrates that the inclusion of grazing animals can strongly modify N cycling under elevated C02. As most grasslands are grazed, the prediction of grassland responses to elevated CO2 must be derived from systems in whichanimais are an integral part., Prédire la réponse des prairies pâturées à une élévation de la concentration en CO2 revêt une importance majeure dans la mesure où cet écosystème représente environ 20% de la surface terrestre non immergée mais aussi, parce que les sols prairiaux représentent un puit majeur de carbone (C). La réponse des prairies à un enrichissement en CO2 est fortement contrôlée par la disponibilité des autres nutriments et en particulier l'azote (N). De nombreuses expériences ont par le passé étudié le cycle de l'azote en prairie sous CO2 enrichi mais aucunes de ces études n'a inclus le pâturage. Dans le cadre de cette thèse, je présente des données concernant les effets du CO2 sur le cycle de l'N provenant d'un système expérimental (FACE: enrichissement en dioxyde de carbone à l'air libre) permettant d'inclure des ruminants. Cette thèse est dédiée à l'étude des effets de l'élévation en CO2 sur les différents processus impliqués dans les retours de matière organique (MO) de la plante vers le sol et leurs conséquences pour la disponibilité en N. Dans le Chapitre 1, il a été montré que le CO2 pouvait modifier les retours d'N par les ruminants en affectant la partition d'N entre l'urine et les faeces, ce qui induisait des pertes d'N potentiellement accrues. La décomposition de la litière végétale, considérée à l'échelle de l'écosystème, n'a pas été affectée par le CO2 (Chapitre 3) mais une forte augmentation du volume de MO retournant au sol depuis les racines a induit une accumulation de MO grossière dans le sol (Chapitre 4). Au cours du Chapitre 5, à l'aide d'un double marquage isotopique 14C et 15N, nous avons comparé les effets court terme (transmis par la plante) et long terme (transmis par le sol) du CO2 sur la dynamique de la MO du sol et il a été conclu que l'accumulation de MO n'était pas causée par une limitation en C ou en N mais probablement par la disponibilité en autres nutriments. Cette thèse démontre que l'inclusion des ruminants peut fortement modifier la réponse des prairies au CO2. Dans la mesure où ce mode d'utilisation des pâtures est largement majoritaire, prédire les réponses des pâtures à un enrichissement en CO2 doit provenir de systèmes où les ruminants sont partie intégrante.

International audience

International audience

*INRA Centre de Lille laboratoire Sol et Environnement (FRA) Diffusion du document : INRA Centre de Lille laboratoire Sol et Environnement (FRA)