A. Guckert

Impact of growth stage on the bacterial community structure along maize roots, as determined by metabolic and genetic fingerprinting

Metabolic and genetic profiles were used to determine whether spatial and temporal variations in rhizodeposition along maize roots coincide with different bacterial community structures. Bacterial communities were extracted from the bulk soil and adhering soil of three maize rhizosphere zones (ramification, root hair-elongation, apex) 2 and 4 weeks after planting. Biolog® substrate utilization profiles, and rDNA internal spacer analysis of bacterial communities, were compared. Biolog® data showed that the functional abilities of bacterial communities from bulk and adhering soils were distinct after 2 weeks (Day 15). Moreover, these abilities were dissimilar between ramification zone on the one hand, and the root hair-elongation zone and apex on the other hand. The differences between bulk and rhizosphere soil responses were more pronounced after 4 weeks (Day 30), but rhizosphere samples were clearly aggregated. These results argue in favor of a greater influence of the maize rhizosphere environment on bacterial metabolic potentialities, mainly based on the developmental state of the plant. Different genetic fingerprints, and thus different genetic structures, were observed between bacterial communities at distinct sampling dates and, to a lesser extent, between rhizospheric and non-rhizospheric samples at both sampling dates. The latter difference was reinforced at Day 30. However, no clear groupings of samples could be identified on the basis of root-zone origin (ramification, root hair, apex). These results suggest a marked influence of time on microbial pools, irrespective of their root zone origins.

Measurement of Pb2+, Cu2+ and Cd2+ binding with mucilage exudates from maize (Zea mays L.) roots

SummaryThe 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.

INFLUENCE OF MAIZE ROOT MUCILAGE ON SOIL AGGREGATE STABILITY

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.

Influence of mechanical impedance on root exudation of maize seedlings at two development stages

Studies were undertaken to evaluate the effects of mechanical impedance on root exudation by maize (Zea mays L., var Dea) and to examine the importance of these effects in relation to the stage of plant development. Plants were grown under sterile and hydroponic conditions. Mechanical impedance was simulated using glass beads of 1 mm diameter. This treatment was compared with a control without beads. Results demonstrated that plant growth was influenced by mechanical impedance. Mechanical impedance markedly affected the growth of the shoot, whether this was measured as leaf area or total dry matter. Besides increasing root/shoot biomass ratios, mechanical impedances also stimulated root exudation of organic and inorganic compounds. Stressed plants lost more nitrogenous compounds than control plants. Otherwise, the percentage of released carbon decreased. Depending on the developmental stage of the plant, there was a large variation in the magnitude and time course on mechanical impedance effects. The effects of mechanical impedance persist and accentuate with time.

Metabolic fingerprint of microbial communities from distinct maize rhizosphere compartments

The metabolic abilities of microbial communities extracted from maize rhizosphere and non-rhizosphere compartments were compared by using the Biolog® system. A double sampling protocol (from bulk soil to root surface and along the root) was used for testing the hypothesis that nature and quantity of rhizodeposits could be key factors governing microbial community structure. Rhizoplane microbes are closer to rhizodeposition sites than microbes living in the adhering soil. This proximity generates distinct pools of rhizodeposits between adhering soil and rhizoplane; the carbon pool available for adhering soil microbes is qualitatively and quantitatively different from the original one because of rhizoplane microbial activity. The first sampling was designed to extract microbes from these two compartments. Moreover the different parts of a root are known to release distinct pools of rhizodeposits; the second sampling along the root was designed to extract microbes from the main morphological parts of a root (ramification zone, root hair-elongation zone and apex). Bulk soil without plant (control soil) was also investigated to obtain microbes deprived from rhizodeposits. The comparative metabolic profiling revealed clear shifts between the microbial populations of the rhizoplane and the control soil ones. The main discriminating substrates were carbohydrates, amino acids and amides. Microbial populations from the ramification zone and the control soil were clearly separated from those of the root hair-elongation zone. This discrimination was mainly based on some carboxylic acids, carbohydrates, amino acids and amides. The sampling procedure and the community structure differences revealed in this study confirm a possible structural effect of maize rhizodeposits on telluric microbial populations.

Influence of plant morphology on root exudation of maize subjected to mechanical impedance in hydroponic conditions

Mechanical impedance stimulates maize root exudation. The purpose of this work was to evaluate the direct effect of mechanical impedance on root exudation from the indirect effect involving root morphological modifications induced by mechanical impedance. Maize plants were grown in axenic hydroponic culture conditions for 4, 8, 12 and 16 days, and mechanical impedance was simulated by glass beads. At the end of the culture, exudation of plants in a nutrient solution was measured during 24 h. At harvest, plant growth and development parameters as well as carbon exudation were measured. The results demonstrated a major influence of mechanical impedance on root growth with a reduction in root elongation. Comparisons with previous studies in soil conditions have indicated that the glass-bead system realistically simulated mechanical impedance. The carbon exudation rate fluctuated from 0.2 to 1.2 mg C plant-1 day-1 and a fraction of this carbon (0.06 to 0.11 mg C plant-1 day-1) was recovered from glass beads in impeded conditions. The difference in exudation between both treatments for comparable plant morphologies lead to the conclusion that the mechanical impedance had a direct effect on exudation rate. Correlations between plant morphology and root exudation suggest that root morphology is probably involved in the modification of root exudation.

Relationship between rate of crop growth at date of fertiliser N application and fate of fertiliser N applied to winter wheat

To investigate the relationship between the timing of fertiliser N applications and the N use efficiency of wheat, three field experiments with 15N were set up on winter wheat, on three different soils in France. Different crop N demands on the day of fertiliser application were obtained by varying either crop densities or date of fertiliser application. Labelled 15NH415NO3 was applied at tillering and during stem elongation. The 15N recovered from plant and soil at different dates after 15N addition and at maturity of wheat was measured. The fate of fertiliser N was rapidly determined, most of the fertiliser N accumulated in the wheat at maturity having been taken up within a few days of application. 15N recovery by the crop at final harvest (%) varied greatly (19–55% N applied) according to crop density, soil type and date of application. It was linearly related to the instantaneous crop growth rate calculated at the day of 15N application. The amount of fertiliser N immobilised in the soil was constant at 20 kg N ha−1, for all soil types and crop densities. Because residual mineral 15N in the soil at harvest was negligible and immobilisation was constant, the level of total 15N measured in the different N pools (soil+plant) reflected the% 15N uptake by the plant. There was consequently a negative linear relationship between the percentage of 15N not recovered for measurement, and crop growth rate (i.e. crop N demand) at date of fertiliser application. These results suggest that crop N demand at the time of N application determines the ability of the crop to compete for N with other processes, and may be a major factor determining the division of N between soil and crop.

Influence of microorganisms on iron acquisition in maize

Abstract Graminaceous species can enhance acquisition of iron (Fe) by release of phytosiderophores mainly from apical root zones. However, phytosiderophores are readily degradable by microorganisms. To study the effect of rhizosphere microorganisms in Fe acquisition, maize plants were grown axenically or inoculated with a mixture of microorganisms in a limestone substrate supplemented with a small amount of FeIII-oxide. Axenic plants grew well without Fe deficiency symptoms and released considerable amounts of phytosiderophores. In contrast, inoculated plants showed severe symptoms of Fe deficiency chlorosis and much less phytosiderophores were detectable in the substrate. The severity of Fe deficiency chlorosis was strongly influenced by the mode of water supply either continuously by glass fibre wicks or by periodic short-term flooding. Inoculated plants became more chlorotic, when watered by flooding than plants watered by glass fibre wicks. This was suggested to be due to greater microbial degradation of phytosiderophores as a consequence of higher microbial population density in apical root zones, the sites of phytosiderophore release. To prove this hypothesis maize plants were grown in a silty loam soil. Short-term periodic flooding resulted in a uniform distribution pattern of rhizosphere microorganisms along the root axis, whereas in non-flooded plants the number of rhizosphere microorganisms was lower in apical root zones and increased sharply from 0–20 to 20–40 mm distance from the root apex. It is concluded, that in solid substrates a low population density of rhizosphere microorganisms in the apical root zone is of particular importance for efficient Fe acquisition by phytosiderophores in graminaceous species like maize.

Establishment of hairy root cultures of Psoralea species.

Eight Psoralea species (Leguminosae) were inoculated with Agrobacterium rhizogenes, strains 8196 and 9402. Hairy roots were only induced by strain 9402. Attention was focussed on Psoralea lachnostachys. Transformed roots grew very rapidly in Gamborg B5 liquid medium with a doubling time of the culture of 38 hours. Whatever the culture conditions, the two furanocoumarins usually found in roots of Psoralea plants, psoralen and angelicin, were not detected in cultured transformed and non transformed roots even when some chitosan was added to the medium. However, 669 μg.g−1 dry matter of psoralen and 215 μg.g−1 dry matter of angelicin were found in roots from soil grown plants. A possible translocation of these compounds from the aerial parts to the roots is suggested.

Effect of maize rhizodeposits on soil microbial community structure

In order to characterise the relationships between a plant and its rhizosphere microflora, we studied the effect of rhizodeposits released by roots on the phenotypic and genetic structures of soil bacterial populations. Sterile cultures of maize (cv. F66) were cultivated for 30 days in order to obtain sterile rhizodeposits. These rhizodeposits were added daily to a test soil during the last 15 days of maize cultivation. Afterwards, the structure of bacterial communities was analysed using a phenotypic test (Biolog®) and several dominant isolates were analysed by genetic test (ARDRA). Bacteria isolated from the soil watered only with plant nutrient solution used certain carboxylic acids more intensely than bacteria isolated from the soil conditioned with F66 rhizodeposits. Among the most representative ribotypes, Bacillus mycoides dominated in soil enriched with rhizodeposits, whereas Bacillus macroides was isolated from soil without carbon amendment.