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The aim of the present study was to test and improve the reliability of the 15 N cotton-wick method for measuring soil N derived from plant rhizodeposition, a critical value for assessing belowground nitrogen input in field-grown legumes. The effects of the concentration of the 15 N labelling solution and the feeding frequency on assessment of nitrogen rhizodeposition were studied in two greenhouse experiments using the field pea ( Pisum sativum L.). Neither the method nor the feeding frequency altered plant biomass and N partitioning, and the method appeared well adapted for assessing the belowground contribution of field-grown legumes to the soil N pool. However, nitrogen rhizodeposition assessment was strongly influenced by the feeding frequency and the concentration of labelling solution. At pod-filling and maturity, despite similar root 15 N enrichment, the fraction of plants' belowground nitrogen allocated to rhizodeposition in both Frisson pea and the non-nodulating isoline P2 was 20 to more than 50% higher when plants were labelled continuously than when they were labelled using fortnightly pulses. Our results suggest that when 15 N root enrichment was high, nitrogen rhizodeposition was overestimated only for plants that were 15 N-fed by fortnightly pulses, and not in plants 15 N-fed continuously. This phenomenon was especially observed for plants that rely on symbiotic N 2 fixation for N acquisition, and it may be linked to the concentration of the labelling solution. In conclusion, the assessment of nitrogen rhizodeposition was more reliable when plants were labelled continuously with a dilute solution of 15 N urea.

Complementarity in acquisition of nitrogen (N) from soil and N2-fixation within pea and barley intercrops was studied in organic field experiments across Western Europe (Denmark, United Kingdom, France, Germany and Italy). Spring pea and barley were sown either as sole crops, at the recommended plant density (P100 and B100, respectively) or in replacement (P50B50) or additive (P100B50) intercropping designs, in each of three cropping seasons (2003–2005). Irrespective of site and intercrop design, Land Equivalent Ratios (LER) between 1.4 at flowering and 1.3 at maturity showed that total N recovery was greater in the pea–barley intercrops than in the sole crops suggesting a high degree of complementarity over a wide range of growing conditions. Complementarity was partly attributed to greater soil mineral N acquisition by barley, forcing pea to rely more on N2-fixation. At all sites the proportion of total aboveground pea N that was derived from N2-fixation was greater when intercropped with barley than when grown as a sole crop. No consistent differences were found between the two intercropping designs. Simultaneously, the accumulation of phosphorous (P), potassium (K) and sulphur (S) in Danish and German experiments was 20% higher in the intercrop (P50B50) than in the respective sole crops, possibly influencing general crop yields and thereby competitive ability for other resources. Comparing all sites and seasons, the benefits of organic pea–barley intercropping for N acquisition were highly resilient. It is concluded that pea–barley intercropping is a relevant cropping strategy to adopt when trying to optimize N2-fixation inputs to the cropping system.

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699; International audience; Field emergence is crucial in determining the yield and quality of French beans for the canning industry. The aim of this study was to analyze the variation in bean emergence results, particularly to determine the relative influence of seed characteristics and sowing conditions. During 2004 and 2005, at two sites in France (North: 50. 1 degrees N, 3 degrees E; West: 47.3 degrees N, 0.3 degrees W), field experiments were carried out in order to obtain information on bean emergence under a range of sowing conditions. Four seed lots of the cultivar Booster with differing germination and seedling growth parameters determined under laboratory conditions were sown. These field results were also used to compare observed results in the field with the simulations of a crop emergence model. Subsequently, the effects on crop emergence of a greater diversity of seed lots in the three main French cropping areas were simulated using the model. In the field experiment, germination rates were always high (80-100%). In contrast, final emergence rates varied from 56 to 90%, emergence lasting from 9 to 28 days. These differences were large between sowing situations but small between seed lots. Emergence was mainly delayed by low temperatures (

Two 15N labelling methods for assessing net rhizodeposition of nitrogen (N) in pea crop (Pisum sativum L.) were compared in the greenhouse and in the field: the cotton-wick (CW) and the split-root (SR) methods. Rhizodeposition is defined as the organic material lost from roots during their growth through the soil. CW is a method in which 15N urea was supplied to the plant in pulses via a wick threaded through the stem. In SR, the root system was divided between a hydroponic labelling compartment (LC) containing the labelling nutrient solution (1 or 5 mM 15NO3–15NH4) and a compartment filled with soil in which the amount of 15N rhizodeposition was assessed. The percentage of N derived from rhizodeposition (%Ndfr), was used to calculate the amount of N rhizodeposition which was obtained from the ratio of atom % 15N excess of the soil : atom % 15N excess of the roots. Above ground parts in the field accumulated markedly more dry matter and N than in the greenhouse, regardless of the labelling method. 15N enrichments of above ground parts were higher than those of roots recovered from the soil. Results indicated that amount of 15N applied to plants were lower in SR than in CW. Additionally, LC roots of SR tended to retain large amounts of 15N. As a consequence, atom % 15N excess of roots was less than 1% in SR, whereas most values varied from 1% to 4% in CW. However, relationships between enrichments of the soil and of the roots were different in SR and CW. It was not possible to compare the Ndfr:root-N ratio between the two methods, but the ratio of Ndfr:plant-N was found to be 10% higher in SR than in CW. Finally, relative to total plant-N, the total contribution of below ground parts to the N pool of the soil reached 22–25% at maturity for the two methods. From our experiments, we could not conclude that one method is better than the other for estimating either net rhizodeposition of N or the contribution of a pea plant to the soil N pool. However, CW is easier to adapt and monitor under field conditions than SR.

Field experiments were carried out during three successive years to study through a dynamic approach the competition for soil N and its interaction with N2 fixation, leaf expansion and crop growth in pea–barley intercrops. The intensity of competition for soil N varied between experiments according to soil N supply and plant densities. This study demonstrates the key role of competition for soil N which occurs early in the crop cycle and greatly influences the subsequent growth and final performance of both species. Relative yield values for grain yield and N accumulation increased with the intensity of competition for soil N. Barley competed strongly for soil N in the intercrop. Its competitive ability increased steadily during the vegetative phase and remained constant after the beginning of pea flowering. The period of strong competition for soil N (500–800 degree-days after sowing) also corresponded to the period of rapid growth in leaf area for both species and therefore an increasing N demand. For each species, the leaf area per plant at the beginning of pea flowering was well correlated with crop nitrogen status. Barley may meet its N needs more easily in intercrops (IC) and has greater leaf area per plant than in sole crops (SC). Barley having a greater soil N supply results in an even higher crop N status and greater competitive ability relative to pea in intercrop. Competition by barley for soil N increased the proportion of pea N derived from fixation. The nitrogen nutrition index (NNI) values of pea were close to 1 whatever the soil N availability in contrast to barley. However N2 fixation started later than soil N uptake of pea and barley and was low when barley was very competitive for soil N. Due to the time necessary for the progressive development and activity of nodules, N2 fixation could not completely satisfy N demand at the beginning of the crop cycle. The amount of N2 fixed per plant in intercrops was not only a response to soil N availability but was largely determined by pea growth and was greatly affected when barley was too competitive.

Two methods were developed and used to study the root system dynamics of two species grown together or separately under field conditions. The first method, based on herbicide injection at different soil depths, was used to determine the rooting depth penetration rate of each species in pea–barley and pea–mustard mixtures. The roots absorbed the herbicide when they reached the treated zone leading to visible symptoms on the leaves which could be readily monitored. The second method used differences in 15N natural abundance and N concentration between legume and non-legume species to quantify the contribution of each species to root biomass of a pea–barley mixture. Each contribution was calculated using 15N abundance and N concentration of root mixtures and of subsamples of roots of individual species within mixtures. Both methods can indeed be used to distinguish roots of species in mixtures and thus to study belowground competition between associated species. The use of these methods demonstrated species differences in root system dynamics between species but also significant effects of interactions between species in mixtures. The rooting depth penetration rate was mainly species specific whereas root biomass was dependant on plant growth, allocation of dry matter between shoot and root components and growth factors such as N fertilization. Root biomass of each species may vary therefore with the level of competition between species.

Grain legumes, especially peas, could play a key role in organic cropping systems. They could provide nitrogen (N) to the system via N 2 fixation and produce grain rich in protein while improving soil N for the succeeding crop. Thus, maximising N 2 fixation and optimising grain N production together with N contribution to soil is a challenging issue for organic pea crops. However, pest, disease and weed infestation are less easy to control in organic systems than in conventional systems. Therefore, the effects of weed infestation and pea weevil ( Sitona lineatus L.) attacks on N nutrition and N 2 fixation of organic pea crops were examined by on-farm monitoring over two years. The magnitude of the net contribution of the crops to the soil N balance in relation to their productivity was also assessed. In many situations, weed infestation together with pea weevil damage severely limited the nitrogen nutrition and grain yield. Percentage of N derived from fixation (%Ndfa) increased with weed biomass because weeds appeared more competitive than peas for soil N. But %Ndfa decreased with pea weevil leaf damage score. The interaction between these two biotic factors affected N yields and the net contribution of the crops to soil N. This latter ranged from −133 kg N ha −1 to 69 kg N ha −1 depending on %Ndfa and nitrogen harvest index (NHI). Optimising both grain N and net balance would require a reduction in root nodule damage by weevil larvae in order to maximise %Ndfa and a reduction in the NHI through the choice of cultivar and/or suitable crop management.

This work studied the effects of P fertilization on nodulation of field-grown soybean by two Bradyrhizobium strains (SMGS1 and THA7), and checked if differences between strains were consistent with bacterial growth and growth pouch nodulation ability in response to P availability. In the field, nodule dry weight and nitrogen fixation activity of inoculated soybean were studied on typical acid soils of Thailand at the flowering (R1) stage and at the end of grain filling. Grain yield, growth and phosphorus content were recorded. The bradyrhizobial strains were cultivated in culture medium, and growth parameters recorded. Nodulation patterns were observed during growth pouch experiments: infective root cells were inoculated with strains cultivated at two P concentrations in their culture media, namely 1 μM and 1 mM. Ten days after inoculation, the position of each nodule was measured relative to the root tip (RT) mark, expressed relative to the smallest emerging root hairs-RT distance in the nodulation frequency profile, and the consistency of responses was tested. In the field, on P deficient soils, dry weight of nodules was higher with Bradyrhizobium japonicum strain SMGS1 than with strain THA7. P supply increased the number and dry weight of nodules for both strains, with a higher dry weight response for THA7 than for SMGS1. It also had a positive effect on tissue phosphorus status and grain yield at R8 stage. In growth media, significant differences were recorded between strains under P-limiting conditions: The growth rate was higher for strain SMGS1, as well as the maximal number of bacterial cells supported. With growth pouch, inoculating plants with bacteria grown in P-deficient medium resulted in a less intense nodulation of roots by THA7, and with nodules appearing earlier on roots than in the case of SMGS1. At 1 mM P, there was no significant difference between strains. Thus, strain THA7 is more affected by P deficiency than strain SMGS1. Although P was not supplied in the same way in the soil and in the growth pouch experiments, this consistency of behaviour between work scales indicates that phosphorus availability is a key component for a successful inoculation. Furthermore, the study of bacterial growth rates and nodulation profile represents an interesting step for bacterial screening for low P soils. [-11pt]

Development of the root system, appearance of nodules, and relationships between these two processes were studied on pea (Pisum sativum L., cv. Solara). Plants were grown in growth cabinets for 4 weeks on a nitrogen-free nutrient solution inoculated with Rhizobium leguminosarum. Plant stages, primary root length, distance from the primary root base to the most distal first-order lateral root, and distance from the root base to the most distal nodule, were recorded daily. Distribution of nodules along the primary root and distribution of laterals were recorded by sampling root systems at two plant stages. Primary root elongation rate was variable, and declined roughly in conjunction with the exhaustion of seed reserves. First-order laterals appeared acropetally on the primary root. A linear relationship was found between the length of the apical unbranched zone and root elongation rate, supporting the hypothesis of a constant time lag between the differentiation of first-order lateral's primordia and their emergence. Decline of the primary root elongation rate was preceded by a reduction in density and length of first-order laterals. Nodules appeared not strictly but roughly acropetally on the primary root. A linear relationship was found between the length of the apical zone without nodule and root elongation rate, supporting the hypothesis of a constant time lag between infection and appearance of a visible nodule. A relationship was found between the presence/absence of nodules on a root segment and the root elongation rate between infection and appearance of nodules on the considered root segment. Regulation of both processes by carbohydrate availability, as a causal mechanism, is proposed.

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Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699; The design of more sustainable cropping systems requires increasing N-input from symbiotic N(2) fixation (SNF). However, SNF can be inhibited by nitrate exposure (e.g., soil N-mineralization). Although the effect of nitrate on SNF has been extensively investigated at the cell scale, few studies have highlighted the impact of nitrate exposure on nodule number and biomass, nodule activity of the SNF apparatus or its ability to recover. Pea plants were grown in greenhouse conditions in a N-free nutrient solution and exposed to nitrate (5 mM NO (3) (-) L(-1)) for one week during either early vegetative growth, flowering or seed filling. After nitrate removal, the plants were grown either under natural light or shade. Nitrate exposure reduced the rate of nodule establishment during vegetative growth, whereas it caused damage to existing nodules when applied during the reproductive stages. Nitrate decreased the specific activity of nodules regardless of the stage of the exposure. After nitrate removal, an extra wave of nodulation was observed on plants grown under natural light but only when nitrate exposure occurred before the seed filling stage. The recovery of SNF activity after nitrate removal depended on the amount of carbon available to nodules.

Grain legumes, such as peas (Pisum sativum L.), are known to be weak competitors against weeds when grown as the sole crop. In this study, the weed-suppression effect of pea-barley (Hordeum vulgare L.) intercropping compared to the respective sole crops was examined in organic field experiments across Western Europe (i.e., Denmark, the United Kingdom, France, Germany and Italy). Spring pea (P) and barley (B) were sown either as the sole crop, at the recommended plant density (P100 and B100, respectively), or in replacement (P50B50) or additive (P100B50) intercropping designs for three seasons (2003–2005). The weed biomass was three times higher under the pea sole crops than under both the intercrops and barley sole crops at maturity. The inclusion of joint experiments in several countries and various growing conditions showed that intercrops maintain a highly asymmetric competition over weeds, regardless of the particular weed infestation (species and productivity), the crop biomass or the soil nitrogen availability. The intercropping weed suppression was highly resilient, whereas the weed suppression in pea sole crops was lower and more variable. The pea-barley intercrops exhibited high levels of weed suppression, even with a low percentage of barley in the total biomass. Despite a reduced leaf area in the case of a low soil N availability, the barley sole crops and intercrops displayed high weed suppression, probably because of their strong competitive capability to absorb soil N. Higher soil N availabilities entailed increased leaf areas and competitive ability for light, which contributed to the overall competitive ability against weeds for all of the treatments. The contribution of the weeds in the total dry matter and soil N acquisition was higher in the pea sole crop than in the other treatments, in spite of the higher leaf areas in the pea crops.

Cereal-legume intercrops are a promising way to combine high productivity and several ecological benefits in temperate agro-ecosystems. However, the proportion of each species in the mixture at harvest is highly variable. The aim of this study was to test whether the timing of small application of N fertilizer is an effective way of influencing the dynamic interactions between species during crop growth and affecting the percentage of each species in the biomass of the mixture without greatly disturbing N-2 fixation. The influence of timing of nitrogen fertilization in pea-wheat intercrops was studied as regards (i) the dynamics of crop growth, (ii) nitrogen acquisition of each species, (iii) the inhibition and recovery of symbiotic N-2 fixation (SNF) after N application and (iv) final performance (yield, % of wheat. grain protein content). This was assessed in winter pea-wheat (Pisum sativum L-Triticum aestivum L) intercrops in 2007 and 2008 at two locations in France. Whatever the stage of application. N fertilizer tended to increase wheat growth and to decrease pea growth. N fertilization (applied once at different dates from tillering to the end of stem elongation) delayed the decrease in the contribution of wheat to total biomass and maintained the competitive ability of wheat over pea for longer than in unfertilized intercrops. N acquisition dynamics and N sharing between the two species were modified by N fertilization and its timing. Crop conditions at the time of N application (growth and phenology of each species, and their proportions in the intercrop biomass) greatly influenced intercrop response to N fertilization. Partitioning between species of soil and fertilizer N was correlated with the proportion of wheat in the total intercrop biomass observed at the date of N application. Short-term inhibition of nitrates on SNF was shown during the few days after N application, whatever its date. SNF recovery after N applications was observed only until pea flowering, but was prematurely stopped by N fertilization after this stage. The effect of N fertilization on the amount of fixed N-2 at harvest was correlated with pea biomass. N fertilization affects N-2 fixation mainly by affecting crop growth rather than %Ndfa in pea-wheat intercrops. In conclusion, N fertilization could be used as a tool to enhance the contribution of wheat in the intercrop biomass but may reduce the amount of fixed N-2 in the intercrop by decreasing pea biomass. (C) 2010 Elsevier B.V. All rights reserved.

Correspondance: malezieux@cirad.fr; International audience; The evolution of natural ecosystems is controled by a high level of biodiversity, In sharp contrast, intensive agricultural systems involve monocultures associated with high input of chemical fertilisers and pesticides. Intensive agricultural systems have clearly negative impacts on soil and water quality and on biodiversity conservation. Alternatively, cropping systems based on carefully designed species mixtures reveal many potential advantages under various conditions, both in temperate and tropical agriculture. This article reviews those potential advantages by addressing the reasons for mixing plant species; the concepts and tools required for understanding and designing cropping systems with mixed species; and the ways of simulating multispecies cropping systems with models. Multispecies systems are diverse and may include annual and perennial crops on a gradient of complexity from 2 to n species. A literature survey shows potential advantages such as (1) higher overall productivity, (2) better control of pests and diseases, (3) enhanced ecological services and (4) greater economic profitability. Agronomic and ecological conceptual frameworks are examined for a clearer understanding of cropping systems, including the concepts of competition and facilitation, above- and belowground interactions and the types of biological interactions between species that enable better pest management in the system. After a review of existing models, future directions in modelling plant mixtures are proposed. We conclude on the need to enhance agricultural research on these multispecies systems, combining both agronomic and ecological concepts and tools.

International audience; Cereal–legume intercrops are gaining increasing interest in Europe. Modelling, by taking into account the complexity of species interactions, can be a very useful tool to study such systems and to test new strategies in various soil and climatic conditions. The present work describes the adaptation of an intercrop model for pea–barley intercrops through the extrapolation of the STICS sole crop model and its parameterisation from experimental data recorded on sole crops. Several improvements have been added to the existing crop model to allow an inversion of dominance in height between species during the crop cycle and a trophic link between crop growth rate and the potential for N2 fixation. A 2-year dataset on pea and barley sole crops grown under non-limiting water conditions and with full crop protection was first used for calibration. The intercrop model was subsequently tested on experimental datasets of pea–barley intercrops grown under the same conditions as the sole crops. The intercrop experiments used to test the intercrop model differed in soil type, soil N supply and plant densities of each species. The development of the intercrop model through the extrapolation of the sole crop model and its parameterisation from experimental data recorded in sole crops allowed an accurate simulation of the interactions for nitrogen between intercropped species. The simulated crop growth, N accumulation and N2 fixation in intercrops were in agreement with the observations from the experimental dataset. Moreover the behaviour of the model was consistent with the general knowledge about those intercropping systems. However the simulation of competition for light needs to be improved, especially the simulation of height growth. A thorough validation on many data sets, independent of those used for parameterisation, is still required. This model needs to be tested under a wider range of situations and for other limiting factors than nitrogen occurring in low-input farming systems

International audience; The development of sustainable cropping systems is a key priority for agronomists and crop scientists. A first step involves understanding the relationship between cropping system performance and farmers’ practices. To complete this step, a methodological framework entitled Regional Agronomic Diagnosis (RAD) has been developed. During the last ten years, the scope of the regional agronomic diagnosis has been enlarged to include several factors describing crop quality and the environmental impact of cropping systems. Regional agronomic diagnosis has led to several major advances such as (1) the assessment of the effect of preceding crop and soil structure on malting barley quality in France and (2) the assessment of the effects of ploughing, nematicide use and fertilisers on soil properties in intensive banana plantations in the West Indies. Improvements have also been gained in methodology, particularly by the selection of indicators for assessing the effects of crop management, soil and weather conditions, and data analysis. Finally, regional agronomic diagnosis has been integrated into more general approaches of agricultural development. We review here this methodological progress.

SUMMARYThe effects of intercropping wheat with faba bean (Denmark, Germany, Italy and UK) and wheat with pea (France), in additive and replacement designs on grain nitrogen and sulphur concentrations were studied in field experiments in the 2002/03, 2003/04 and 2004/05 growing seasons. Intercropping wheat with grain legumes regularly increased the nitrogen concentration of the cereal grain, irrespective of design or location. Sulphur concentration of the cereal was also increased by intercropping, but less regularly and to a lesser extent compared with effects on nitrogen concentration. Nitrogen concentration (g/kg) in wheat additively intercropped with faba bean was increased by 8% across all sites (weighted for inverse of variance), but sulphur concentration was only increased by 4%, so N:S ratio was also increased by 4%. Intercropping wheat with grain legumes increased sodium dodecyl sulphate (SDS)-sedimentation volume. The effect of intercropping on wheat nitrogen concentration was greatest when intercropping had the most deleterious effect on wheat yield and the least deleterious effect on pulse yield. Over all sites and seasons, and irrespective of whether the design was additive or replacement, increases in crude protein concentration in the wheat of 10 g/kg by intercropping with faba bean were associated with 25–30% yield reduction of the wheat, compared with sole-cropped wheat. It was concluded that the increase in protein concentration of wheat grain in intercrops could be of economic benefit when selling wheat for breadmaking, but only if the bean crop was also marketed effectively.

Competition for soil resources plays a key role in the outcome of intercropping systems. In cereal–legume intercrops, competition for soil nitrogen during the vegetative phase greatly influences the final performance of the intercropped species. However, there is a lack of knowledge on the main factors involved in interspecific soil N competitive interactions between species. The dominance of cereals over legumes is often attributed to their faster growing rooting system. Nevertheless, using only field experimental approaches makes it difficult to isolate the effect of one factor because of the strong interactions between processes and the environment. Given the complexity of intercropping systems, dynamic simulation models can be especially helpful for testing hypotheses about the key factors driving competition between species. The present work was designed to investigate, under non-limiting water conditions, through an experimental and modelling approach, whether differences in root depth penetration among pea and barley grown together determined competition for soil N and dry matter accumulation (DM) by each species during the vegetative phase. This hypothesis was tested through several simulated scenarios generated using the STICS crop model. The model was first used to compare competition for soil N according to differences in root depth penetration rates between species. This rooting depth penetration effect was then studied at three levels of soil N supply leading to different degrees of N demand and N stress. A field experiment carried out in 2003 including pea–barley intercrops grown either with 130 kg N ha � 1 or without any fertilizer was used to test the model. Experimental results of aboveground biomass, nitrogen accumulation, N2 fixation and rooting depth monitored regularly during the crop cycle were compared to simulated results. The simulated responses of the intercrops were in agreement with the observations from the experimental dataset. Using the model, it is clear that faster root growth in barley gives it access to more soil nitrogen than pea during the vegetative phase. However, this advantage, which is limited to the vegetative phase, only affects the outcome of the intercrop when soil N supply is low. With higher soil N supplies, soil N sharing is not affected by the differences in rooting depth penetration between species. It appears that with higher N supplies, the differences in N demand between species have more influence on species dominance than differences in rooting depth. # 2007 Elsevier B.V. All rights reserved.

Cette etude analyse les effets d'un ombrage simule sur les interactions entre puits reproductifs, en utilisant des cotonniers elagues, ne presentant qu'une seule serie d'organes reproducteurs. Les plantes ont ete cultivees en milieu controle sous regime hydrique non limitant. Un ombrage, reduisant de 40% le niveau lumineux (600 micromol.m-2.s-1 chez le temoin), a ete impose a partir de la 7e anthese. Des suivis de la transpiration, de la distribution de matiere seche et de la phenologie ont ete realises. Un indice de competition apparente pour les assimilats (ICA) a ete calcule en divisant la charge en capsules par le taux de croissance de la plante. A l'echelle de l'inflorescence, l'ombrage affecte la cascade de processus determinant la capacite de puits, incluant la croissance des carpelles, le nombre de graines, l'abscission de la capsule, la masse finale de coton-graine par graine et la fraction de fibres. En plus d'une reduction de la capacite des puits (abscission, nombre de graines), l'ombrage augmente la duree de developpement de la capsule, etalant dans le temps la demande pour les assimilats. Ce phenomene provoque une reduction de la qualite des fibres. L'ombrage reduit egalement le taux d'apparition des capsules, limitant ainsi le nombres de puits en competition. Les taux d'apparition des capsules, de duree de developpement des capsules et d'abscission sont correles avec l'ICA. L'utilisation future de l'ICA dans l'etude des puits au sein de systemes de plantes complexes est discutee. (Resume d'auteur)

The ureide assay method has been found to be suitable for soybean (Glycine max) to assess nitrogen ixation in a wide range of genotypes and Bradyrhizobium strains. The relationships between the relative abundance of ureide-N in xylem-sap and the proportion of plant N derived from nitrogen fixation were studied for early maturing soybeans grown under greenhouse and field conditions. Plants grown under greenhouse conditions were either supplemented with N-free nutrients or supplied with different levels of 15N-nitrate. In the field, soybeans were grown with or without inoculation, in the presence or absence of nitrogen fertilizer. Treatments were applied in order to obtain a wide range of nitrogen nutrition levels. The effect of the source and level of nitrogen nutrition on growth and nitrogen accumulation in plants, proportion of fixed nitrogen in the xylem-sap and in the shoots, and the proportions of N-compounds in the xylem-sap was determined. The greenhouse study demonstrated that the relative...

International audience; The competitiveness of two Bradyrhizobium japonicum strains was determined under controlled conditions. The relationship between the ratio of the nodules formed by the two bradyrhizobial strains and the ratio of the two strains in the inoculum was established and used as an internal standard to evaluate the equilibrium ratio between populations of the two strains in the soil. The fluorescent antibody (FA) technique was also used for counting the bradyrhizobial populations directly in the soil. Results with the FA technique were analysed and compared with the estimate obtained through nodule occupancy. The initial ratio of the two bradyrhizobial strains in populations introduced simultaneously into the soil changed during incubation time and the two strains reached the same equilibrium levels after 53 days. Never-theless, the strain introduced into the soil as an overpopulation remained clearly dominant for the nodule occupancy. We conclude that direct counts with the FA approach are selective and these counts could be mainly of the unattached bacteria.

The existence of a balance survival rate of different B. japonicum strains in soils has been well established. Balance levels (M) were shown to be independent of inoculum size but differences between strains in a soil could not be discussed partly because of the instability of the recovery's efficiency in the area of the plateau. In this paper, a dynamic model, the Gompertz equation, is fitted to the data, to estimate the rate (A) at which equilibrium is reached. For the three strains in the three soils previously studied, this rate is also independent of inoculum size and the soil-strain interaction is highly significant. Some data from the literature are also well fitted by the model. We propose to characterize the saprophytic potential of a strain with the pair of parameters A-M. An optimal experimental procedure is proposed to obtain the most precise estimates for these parameters. Their biological significance is discussed, but the fact that they are not dependent upon inoculation level is of basic importance for agronomic practice.

Afin d’améliorer la valeur quantitative de la technique de dénombrement des bactéries telluriques (Rhizobium japonicum) par immunofluorescence, on compare différents modes d’extraction et de récupération. L’utilisation de membranes en polycarbonates (Nucléopores) supprime les 40 p. 100 de pertes occasionnées par les membranes cellulosiques (Millipores) classiquement employées. Parmi les différents floculants testés, Ca(OH)2/MgCO3(2/5) permet d’obtenir les meilleurs résultats. Cependant pour les deux sols tropicaux étudiés, une floculation par centrifugation est nécessaire. Lorsque l’étape de la floculation n’est pas limitante, la dispersion du sol dans un extractif composé de gélatine diluée dans une solution de (NH4)2 HPO4 0,1 M améliore significativement le taux de récupération des bactéries., We have tested different procedures for releasing bacteria (Rhizobium japonicum) from soil particles and collecting them for fluorescent antibody quantification. The use of polycarbonate filters (Nucleopore) avoided the 40 % loss of bacteria usually found with classical cellulose filters (Millipore). A flocculant mixture of Ca(OH)2/MgCO3(2/5) gave the best results. However flocculation by centrifugation was needed for the two tropical soils tested. When the flocculation step was not the limiting factor, partially hydrolysed gelatin diluted in ammonium phosphate (0.1 M) could be used to suspend the soil and increase the recovery.

The ability of Bradyrhizobium strains to survive saprophytically in soil was studied by means of fluorescent antibodies (FA). It was found that the recovery rate may be considered a constant value although the limit of detection by the FA technique is approximately 103 bacteria g−1 soil. By studying the survival kinetics of B. japonicum strains introduced into soils, we observed that whichever soil-strain combination was tested in a given soil during the incubation all the different populations of a strain reached the same survival balance level, generally about 103–104Bradyrhizobium g−1 soil. When we reintroduced strains into a soil containing rhizobia of the same specificity, the new inhabitants reached the same equilibrium level as that of the initial population. The balance threshold level does not appear to be a very sensitive way to classify, strains on their saprophytic ability. We suggest that survival kinetics should be characterized by the rate at which the population reaches equilibrium.

La technique des anticorps marqués (immunofluorescence) a été utilisée pour l’étude de la persistance et de la compétition dans le sol de Rhizobium japonicum. La même technique a permis d’apprécier la multiplication de Rhizobium dans le sol rhizosphérique et dans le rhizoplan. Les populations de R. japonicum, souches G2 et G3 se maintiennent à un haut niveau (104 bactéries/g de sol) pendant tout le temps de l’expérimentation. Lorsque 2 souches sont introduites simultanément, on n’observe pas de compétition saprophytique entre celles-ci dans le sol. L’étude de R. japonicum dans la rhizosphère montre que seule la plante hôte de soja, var. « Chippewa nodulante », possède dans le sol proche de ses racines une population de Rhizobium plus importante que dans le sol normal. Dans une expérimentation portant sur le rhizoplan, 4 cultivars de soja sont testés. Les résultats montrent que R. Japonicum est 100 à 1 000 fois plus abondant dans le rhizoplan que dans le sol ne portant pas de végétation. Nous suggérons qu’au cours de la multiplication de Rhizobium dans le rhizoplan, l’équilibre entre 2 souches peut être modifié et une souche stimulée préférentiellement à l’autre., The fluorescent-antibody technique was used to study persistence and competition of Rhizobium japonicum strains in soils at different temperatures and to examine the effect of plants on this bacterium in thee rhizosphere and rhizoplane. The soil population of R. japonicum strains G2 and G3 remained the same (104 bacteria/g of soil) throughout the experiment, without any significant competition between the two strains. In the rhizosphere experiment, only the host legume (Glycine max, cv. « Chippewa » ) supported highh populations of strain G3. In another experiment with four cultivars of soybeans, the populations of R.japonicum 102,103 more numerous in the rhizoplane soil than in normal soil. We suggest that the multiplication of Rhizobium in the rhizoplane of the host plant is an important phenomenon, able to modify the equilibrium between two specific strains and thus to favour better nodulation by one strain rather than another.