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This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg−1 dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO2 was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO2 evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO2 up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.

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International audience; Since several years, accelerated decline of Aleppo pine (Pinus halepensis) forests has been observed in mediterranean areas. In fact, the combination of various environmental factors (photochemical oxidants, drought, high light,...) was suspected to cause this decline. In this study, three year-old Aleppo pines were exposed during 3 months to ozone fumigation (100 ppb) combined or not with mild drought to study the effects of these combined factors on some sequences linked to carbon partitioning and primary carbon metabolism within the tree. After a cumulative ozone exposure of 132 ppm . h, ozone induced a significant decrease in specific activity of the whole-plant (-38%) combined with a disequilibrium of the carbon transfer between root and shoot in favour of the shoots (non significant). Moreover, while the same cumulative dose of ozone had no effect on total Rubisco activity in one year-old needles, mitochondrial NAD malic enzyme activity increased significantly (+32%). By combining ozone with mild drought, the ozone-induced responses of all the parameters were significantly amplified and Rubisco activity was significantly decreased (by 44%). These results allowed us to conclude that at 132 ppm . h, ozone alone led to an increase in dark respiration. Moreover, by the combination of ozone and mild drought, a decrease carbon fixation capacity was associated to a decrease of the carbon transfered to the roots, leading to a reduced root growth. Thus, there are indications that high levels of ozone during the summer months may impair the ability of Pinus halepensis to withstand severe water stress in its natural environment.

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International audience

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Le comportement du trèfle blanc (Trifolium repens L cv Crau) est étudié en situation de contrainte hydrique et après réhydratation en liaison avec la fertilisation potassique. Le déficit en eau occasionne une décroissance progressive importante du potentiel hydrique des feuilles et une fermeture rapide des stomates. Ces manifestations sont accompagnées d’une chute de l’activité photosynthétique et de la fixation symbiotique de l’azote. En présence de potassium, la diminution de l’activité fixatrice est significativement ralentie. L’action favorable du potassium est également observée lors de la réhydratation des plantes soumises au déficit. Le rétablissement du potentiel hydrique et de la fixation symbiotique d’azote est plus important en présence de potassium. La fertilisation potassique exerce un effet positif sur la production de matière sèche aérienne aussi bien dans le cas des plantes soumises au déficit que des plantes régulièrement arrosées. Les résultats montrent qu’un apport de potassium favorise la résistance du trèfle blanc au manque d’eau., The effects of successive water stress and irrigation on white clover (Trifolium repens L cv Crau) photosynthetic activity, nitrogen fixation, predawn leaf water potential and stomatal resistance were studied in relation to potassium supply. Water deficit induced a gradual decrease in the predawn leaf water potential and a rapid stomatal closure (figs 1 and 2). Nitrogen fixation and photosynthetic activity were also reduced (fig 4 and table I). The decrease in nitrogen fixation activity was significantly lower with K supply (fig 4). For the same water potential and the same stomatal resistance, nitrogen fixation activity (ARA’s method) was higher in presence of potassium fertilization (fig 5 a, b). The positive effect of K was also observed for the rehydrated plants. Water potential and symbiotic nitrogen fixation recovery was more significant with K supply. Potassium fertilization increased dry matter production in stressed plants as in controls (fig 3). These results showed that K supply increased white clover resistance to water stress.

The pectic nature of root mucilages suggests a hypothetical action of these substances on heavy metal flux into the root. In this study the existence of relations between heavy metals and root mucilages were verified and quantified. In order to obtain substantial amounts of pure root mucilages, two methods of collection were developed, using: (1) maize plants grown in the field and (2) hydroponic axenic cultures. The study of mucilage-metal binding was conducted using the dialysis method, which was developed in a previous work. Results show that root mucilages are able to bind metals. The importance of the binding depends on the nature of the cation, following the order Pb > Cu > Cd. These reactions could be due to exchange processes involving mucilage cations (Ca2+, Mg2+) and heavy metals. The role of mucilages on the retention of heavy metals in the rhizosphere is also discussed.

L’étude de l’assimilation photosynthétique et du transport des assimilats chez le trèfle blanc a pour objet de préciser le rôle de la feuille dans l’établissement des relations source-puits. Des marquages courts au 14CO2 sont effectués sur les 8 limbes initiés par le stolon obtenu en conditions contrôlées par bouturage d’apex. Le profil photosynthétique de l’ensemble des limbes du stolon met en évidence un effet « âge du limbe source» pour l’assimilation du 14CO2. Les limbes 2, 3 et 4 sont les plus efficients, les limbes plus âgés sont le siège d’une perte d’activité assimilatrice. Le plus jeune limbe montre un niveau photosynthétique faible et le taux de distribution de carbone le plus bas. Il est également un puits majeur d’assimilats issus des autres limbes. Il s’établit des relations source-puits privilégiées entre les jeunes feuilles et l’apex en croissance. L’allocation de carbone aux racines nodulées est le fait des feuilles plus âgées. Quelle que soit la position de la feuille marquée, le stolon est l’organe qui accumule le plus de carbone néosynthétisé ; il régit les flux vers l’apex et vers les nodosités. Il n’a pas été observé de corrélation positive nette entre assimilation et transport du carbone récent par les limbes source., The authors have studied photosynthetic assimilation and assimilate transport in white clover to try to determine the role of leaf position on the establishment of source-sink relations. Short pulse labelling with 14CO2 was applied to the 8 leaves formed on stolons obtained by suckering of a stem apex. An effect of "age of the source" was shown on 14CO2 assimilation. Leaf numbers 2, 3 and 4 were the most efficient, the youngest leaf showing a low photosynthetic level and the lowest carbon assimilation rate. It was also a major sink for assimilate coming from the other leaves. Preferential source-sink relations were established between the growing apex and the young leaves. Carbon allocation to nodulated roots was due to the oldest leaves. Whatever the position of the labelled leaf, the stolon was the organ which accumulated the most newly synthesized carbon. The stolon governed carbon flow towards the apex and the nodules. We did not observed a clear positive correlation between assimilation and the transport of new carbon by the source leaf.

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International audience; The response to water stress was studied on white clover grown hydroponically. Two varieties (Crau and Huia) were both subjected to a moderate and a more-severe stress, induced by polyethylene glycol (10 and 20% respectively), in the presence of a nutrient solution poor in potassium (K1=0.005 mM), or abundantly supplied (K2=5mM). Dawn water potential and nitrogen fixation (acetylene reduction activity) decreased with the increasing stress. Conversely, the stomatal resistance increased whenosmoticum was added. Crau had a lower stomatal resistance to the deficit, than did Huia. In relation with the K supply, treatment K2 confirmed the superiority of Crau. Crau also showed greater nodule mass and number than Huia. The data show relationships between dawn water potential, stomatal resistance and nitrogen fixation activity.

White clover has a plagiotropic shoot system : it is a typical 'shade avoider' species. In this environment, white clover often experiences competition for light and nutrients from neighbouring plants. This paper reports a study in controlled environments on the effect of light quality on assimilate partitioning between source and sinks and the consequences on plant morphogenesis. The results demonstrated that the source-sink relationships were modified ; far-red plants partitioned more 14 C-assimilates to petioles and leaves than did control plants, at the expense of stolons and roots. These modifications in the transport of 14 C-labelled assimilates were evident less than 4 days after the commencement of the light treatments. Adding far-red at the end of each photosynthetic period modified plant morphogenesis ; far-red radiation decreased stolon branching and promoted the extension of petioles and internodes. Thus, the strategy of white clover in response to far-red is to redirect its development through a modification of its source-sink relationships in order to allow young leaves to be kept out of the shade. The shoot-root ratio was increased by far-red radiation ; thus phytochrome may act indirectly on nitrogen fixation and nutrient uptake by reducing carbohydrate allocation to the roots.

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