Colonization of wheat roots by Gaeumannomyces graminis inhibited by specific soils,microorganisms and ammonium-nitrogen

Lineal extension of Gaeumannomyces graminis var. tritici hyphae along roots of intact wheat plants growing in soils was measured. Hyphal growth rates were lower in soils treated with NH₄⁺-N than with NO₃^?-N. In a soil that is suppressive to the take-all disease, the controlling influence of NH₄⁺-N was eliminated by soil fumigation (methyl bromide), and reintroduced to fumigated soil by additions of 1% nonsterile soil. Effects of fumigation on hyphal growth were absent in a nonsuppressive soil, and in NO₃^?-treatments of the suppressive soil. When inocula of selected groups of wheat rhizoplane microflora were reintroduced into a fumigated or a soil-reinoculated soil via a root-food base, the Pseudomonas spp. consistently appeared more suppressive in NH₄⁺-N treatments than the general bacterial flora, Bacillus spp. spores, streptomycetes, and fungi.     

Suppression of wheat take-all and ophiobolus patch by fluorescent pseudomonads from a fusarium-suppressive soil

Fluorescent pseudomonads isolated from a soil suppressing Fusarium wilt significantly reduced take-all (Gaeumannomyces graminis var. tritici) in wheat and Ophiobolus patch (G. graminis var. avenae) in Agrostis turfgrass. The bacteria were mixed into a conducive soil at a concentration of 10⁷ colony-forming units (cfu)g^?1 soil at sowing. There were significantly fewer (P ? 0.05) diseased wheat roots in the treatments with the bacteria and pathogen than in those with the pathogen alone. Dry weights of the tops of wheat and Agrostis turfgrass were significantly greater (P ? 0.01) in treatments inoculated with the bacteria in the presence of the pathogens compared to controls with the pathogens alone. Dry weights of the tops of plants from treatments inoculated with the bacteria alone were not significantly different to those of healthy wheat non-inoculated with the bacteria, showing that the fluorescent pseudomonads did not stimulate plant growth. At the end of the experiments, the bacterial isolates (genetically-marked with rifampicin resistance) were recovered from wheat roots and rhizosphere soil at concentrations of 10⁵–10⁷cfu g^?1 fresh weight of roots or oven-dried rhizosphere soil.Many of the fluorescent pseudomonads and some non-fluorescent pseudomonads showed in vitro antibiosis on quarter-strength potato dextrose agar (QPDA) against the pathogens. However, there was no correlation between in vitro antibiosis on agar plates and suppression of disease in pot experiments. Further, while some isolates of G. graminis var. tritici and var. avenae were inhibited by certain bacterial isolates, other isolates of the same fungus were not similarly inhibited by the same isolates of bacteria. Most of the fluorescent pseudomonads that produced inhibition zones (>5mm) against G. graminis var. tritici on QPDA did not do so on King's medium B, where fluorescent siderophores were formed. In vitro antibiosis is, therefore, a poor criterion for selecting effective bacterial antagonists of the wheat take-all fungus. All of the fluorescent pseudomonads tested produced siderophores in low-Fe media while a non-fluorescent pseudomonad and the fungal pathogens did not produce siderophores of comparable activity. The addition of 500 μg FeEDTA g^?1 with a lower stability constant did not. The evidence suggests that iron competition at the rhizoplane or in the rhizosphere is one mechanism of suppression.     

Wheat-rhizoplane pseudomonads as antagonists of Gaeumannomyces graminis

The role of rhizoplane-inhabiting Pseudomonas spp as inhibitors of take-all on wheat was investigated. Apparent numbers of pseudomonads in wheat rhizoplanes and numbers that were antagonistic in vitro toward Gaeumannomyces graminis var, tritici did not differ when wheat was supplied with NH⁺₄-N or NO^?₃-N. More intense antagonism was expressed by colonies selected from soil treated with NH⁺₄-N than with NO^?₃-N, and from isolation media prepared at pH 5.5 rather than at 7.0. Antagonists were not recovered from methyl bromide-treated soil. Highly antagonistic pseudomonads were recovered from a wheat-monoculture soil which is considered suppressive toward the pathogen in the field, and were not recovered from a “nonsuppressive” soil. Pseudomonad antagonism ratings were inversely correlated with take-all severity in the suppressive soil, but not in the nonsuppressive soil. Pseudomonads were considered to be antagonists of G. graminis on rhizoplanes of wheat in a soil exhibiting the “take-all decline” phenomenon, but the significance of this interaction remains to be determined.     

Root-induced changes in the rhizosphere: Importance for the mineral nutrition of plants

Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Examples for this are changes in rhizosphere pH in response to the source of nitrogen (NH₄-N versus NO₃-N), and iron and phosphorus deficiency. These pH changes can readily be demonstrated by infiltration of the soil with agar containing a pH indicator. The rhizosphere pH may be as much as 2 units higher or lower than the pH of the bulk soil. Also along the roots distinct differences in rhizosphere pH exist. In response to iron deficiency most plant species in their apical root zones increase the rate of H⁺ net excretion (acidification), the reducing capacity, the rate of Fe^III reduction and iron uptake. Also manganese reduction and uptake is increased several-fold, leading to high manganese concentrations in iron deficient plants. Low-molecular-weight root exudates may enhance mobilization of mineral nutrients in the rhizosphere. In response to iron deficiency, roots of grass species release non-proteinogenic amino acids (?phytosiderophores”?) which dissolve inorganic iron compounds by chelation of Fe^III and also mediate the plasma membrane transport of this chelated iron into the roots. A particular mechanism of mobilization of phosphorus in the rhizosphere exists in white lupin (Lupinus albus L.). In this species, phosphorus deficiency induces the formation of so-called proteoid roots. In these root zones sparingly soluble iron and aluminium phosphates are mobilized by the exudation of chelating substances (probably citrate), net excretion of H⁺ and increase in the reducing capacity. In mixed culture with white lupin, phosphorus uptake per unit root length of wheat (Triticum aestivum L.) plants from a soil low in available P is increased, indicating that wheat can take up phosphorus mobilized in the proteoid root zones of lupin. At the rhizoplane and in the root (root homogenates) of several plant species grown in different soils, of the total number of bacteria less than 1 % are N₂-fixing (diazotrophe) bacteria, mainly Enterobacter and Klebsiella. The proportion of the diazotroph bacteria is higher in the rhizosphere soil. This discrimination of diazotroph bacteria in the rhizosphere is increased with foliar application of combined nitrogen. Inoculation with the diazotroph bacteria Azospirillum increases root length and enhances formation of lateral roots and root hairs similarly as does application of auxin (IAA). Thus rhizosphere bacteria such as Azospirillum may affect mineral nutrition and plant growth indirectly rather than by supply of nitrogen.     

MICROBIAL POPULATIONS AND NITROGEN IN SOIL GROWING CONSECUTIVE CEREAL CROPS INFECTED WITH TAKE-ALL

Annually, for the past 12 years, a consecutive cereal-cropping sequence was begun. During 1969–72 soil was collected from plots with similar fertilizer treatments, but with different sequences of crops. In 1971 and 1972 wheat seedling bioassays showed that take-all disease became miximal after two or three crops, decreased to the fifth or sixth crop and then remained fairly constant in the‘take-all decline’state. The sequence with maximum disease had most NH₄⁺ -N and least NO₃^- -N in the rhizosphere soil in spring 1972, but there were no similar relationships in bulk soil in either of 2 years. Soil and rhizoplane populations of ammonifying and denitrifying bacteria were notably smaller in autumn 1970 and 1971 in short cereal sequences (2-5 yr.) than in the longer sequences. After γ-irradiating or autoclaving irradiated soils, all sequences contained much NH₄⁺ -N and although disease developing from added inoculum differed among soils, it did not follow the take-all decline pattern. Diffusate from sterilized soil favoured growth of the pathogen (Gaeumannomyces graminis var. tritici) least when it originated from the sequence that supported maximum disease.     

Micromycetes in the rhizosphere, the rhizoplane and the roots of wheat under conventional and zero tillage

The effects of different soil cultivation on the occurrence of micromycetes in the rhizosphere, at the root surface (rhizoplane) and in the root interior were investigated in a long-term field experiment on a chernozem soil at Hru?ovany u Brna, in an area of the corn production type. Cereals were grown as a monoculture with conventional and zero tillage.Soil cultivation had no significant effect on the total number of species of micromycetes. Zero tillage resulted in an increased number of colonies isolated from the rhizosphere and the rhizoplane. On the contrary, conventional tillage resulted in an increased number of isolates from the roots interior. The abundance of individual species of micromycetes varied somewhat in individual years, larger differences being observed with the saprophytic rather than with the parasitic (Gaeumannomyces graminis, Fusarium) species. In wheat roots, Gaeumannomyces graminis was more common under conventional tillage that under zero tillage, while the reverse was true of Fusarium.     

Ammonium and nitrate in the rhizosphere of spring barley, hordeum vulgare L., and take-all disease

The incidence and severity of take-all disease, due to Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker, was observed on spring barley plants growing in soil in two glasshouse experiments. Soil amendments of NH⁺₄-N significantly increased the number of diseased plants and roots during the first month after germination in comparison with controls unamended with N (P < 0.05). No significant difference in the incidence of take-all disease was detected between more mature barley plants growing in soil amended with either NH⁺₄ or NO^?₃-N and unamended controls. The least take-all disease in 3 month-old barley plants was observed when N was supplied as foliar sprays of urea at 0.5 mg N kg^?1 soil (P < 0.01). There was no significant correlation between the degree of infection and the NH⁺₄-N to NO^?₃-N ratio in the rhizosphere soil     

Factors impacting the activity of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens against take-all of wheat

Take-all, caused by Gaeumannomyces graminis var. tritici, is an important soilborne disease of wheat worldwide. Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are biocontrol agents of take-all and provide natural suppression of the disease during wheat monoculture known as take-all decline. To identify factors that could contribute to the effectiveness of 2,4-DAPG producers in take-all suppression, P. fluorescens strains Q8r1-96 (genotype D) and Q2-87V1 (genotype B; reduced antibiotic production) were tested against three pathogen isolates differing in sensitivity to 2,4-DAPG (LD5, ARS-A1 and R3-111a-1) and two wheat cultivars (Tara and Buchanan). The antibiotic sensitivity of the pathogen and cultivar significantly affected the level of take-all suppression by Q8r1-96 and Q2-87V1; suppression was greatest with LD5 and Tara. Q8r1-96 suppressed ARS-A1 and R3-111a-1 on Tara but not Buchanan, and Q2-87V1 failed to suppress either pathogen isolate on either cultivar. Q8r1-96 colonized the rhizosphere of Tara and Buchanan grown in soil similarly, but 2,4-DAPG accumulation was higher on the roots of Buchanan than Tara. 2,4-DAPG at 7.5 μg mL⁻¹ reduced the growth of roots of both cultivars, and 10 μg mL⁻¹ caused brown necrosis and tissue collapse of seedling roots and reduced root hair development. The half-life of 2,4-DAPG in the rhizosphere was estimated to be 0.25 days. These results suggest that several interconnected factors including sensitivity of G. graminis var. tritici to 2,4-DAPG, wheat cultivar, fluctuations in populations of 2,4-DAPG producers, and antibiotics accumulation in the rhizosphere will impact the robustness of take-all suppression by P. fluorescens in the field.     

Antagonistic activity of the rhizosphere mycoflora against Gaeumannomyces graminis under conventional and zero-tillage

The effect of individual constituents of the rhizosphere mycoflora on Gaeumannomyces graminis was studied in conjunction with studies that were concerned with the general effects of conventional and zero-tillage on the mycoflora of the rhizosphere, rhizoplane and roots of winter wheat. Material for the isolation of micromycetes was sampled from plots that had been subjected to two extremes of tillage i.e., ploughing and zero-tillage. These plots were part of a long-term field experiment on the monoculture of cereals at Hrušovany u Brna, and they were situated in a corn-producing region.The occurrence of micromycetes was increased in the rhizosphere under direct drilling. The antagonistic activity of individual representatives of the mycoflora was evaluated in vitro, using the biotic-effect method. The total biotic effect of the rhizosphere was calculated from both the frequency and activity against Gaeumannomyces graminis of individual species (isolates) of micromycetes. The more pronounced antagonistic activity of the rhizosphere found under zero-tillage, fully agreed with the lower frequency of infested plants, ascertained by visually evaluating the health status of plants from the respective stands.     

Competition for amino acids between wheat roots and rhizosphere microorganisms and the role of amino acids in plant N acquisition

The direct uptake of organic nitrogen compounds from the soil solution by plant roots has been hypothesised to constitute a significant source of N to the plant particularly in N limiting ecosystems. The experiments undertaken here were designed to test whether wheat roots could out-compete the rhizosphere microflora for a pulse addition of organic N in the form of three contrasting amino acids, namely lysine, glycine and glutamate. Amino acids were added at a concentration reflecting reported soil solution concentrations (100 μM) and the uptake into either plant biomass or respiration or microbial biomass and respiration determined over a 24 h chase period. The results showed that the plant roots could only capture on average 6% of the added amino acid with the remainder captured by the microbial biomass. We therefore present direct in vivo evidence to support earlier work which has hypothesised that organic N may be of only limited consequence in high input agricultural systems. We suggest that this is a result of the higher concentrations of NO₃⁻ in agricultural soil solutions, the slow movement of amino acids in soil relative to NO₃⁻, the rapid turnover of amino acids by soil microorganisms, and the poor competitive ability of plant roots to capture amino acids from the soil solution.     

Microbiology of roots infected with the take-all fungus (Gaeumannomyces graminis var. tritici) in phased sequences of winter wheat

Bacteria with possible relevance to the growth of the take-all fungus were counted from surfaces of lesioned and healthy roots of wheat growing in soil from a field monoculture system. Numbers showed short-term seasonal and long-term monocultural changes, which seemed to be genuinely associated with the monoculture. Bacteria were more numerous on lesioned than healthy roots. Only bacteria inhibitory to growth of Gaeumannomyces graminis on agar and Pseudomonas spp showed consistent changes irrespective of the source of the roots. Relationships were considered between the microflora on lesioned tissue and (a) severity of disease on roots supplying the lesions, and (b) infection produced on axenic seedlings inoculated with the lesioned tissue. Only total bacterial counts on the lesions from tillering and mature plants were positively correlated with disease on the donor roots. Only inhibitory bacteria on lesions from tillering and mature plants were positively correlated with disease on test seedlings. Pseudomonas spp showed no correlations. Interpretation of data differed with age of plant and the sequence in the monoculture from which plants or soil came.     

Age of wheat and barley roots and infection by gaeumannomyces graminis var tritici

Seminal roots of wheat and barley seedlings were inoculated with G. graminis var tritici on regions 0, 5- and 15-days old, and assessed for intensity and extent of infection after standard times. Wheat roots were most heavily infected on young regions, whereas barley roots were most heavily infected on old ones. The effect of root age in wheat was similar in both unsterile and aseptic conditions, so it could not be ascribed to saprophytic rhizosphere micro-organisms interacting with G. graminis.The contrasting results for wheat and barley are explained by a single hypothesis, based on decreasing host-resistance in the root cortex but increasing resistance at or near the endodermis as the roots age. It is suggested that, under some conditions, even small amounts of non-pathogenic root cortex death can enhance infection by G. graiminis. This interpretation may explain several aspects of take-all and its biological control by other dark mycelial parasites.     

抗病性不同大豆品种根面及根际微生物区系的变化 Ⅰ.非连作大豆(正茬)根面及根际微生物区系的变化

采用平板计数法测定了3个抗病性不同的大豆品种在生育期内根面和根际微生物区系的变化情况,并应用荧光计数法直接测定了根际细菌和真菌的生物量。结果表明,土体的微生物种类最丰富、根际的次之、根面的较单一。播种后从三叶期到鼓粒初期,根面和根际的可培养细菌总数随生育期逐渐增加,鼓粒初期达最大值,而成熟期则有明显的下降;大豆根际细菌生物量也存在相同的变化规律。抗病性不同的大豆品种其根面、根际可培养细菌总数存在差异;抗病品种大豆的根瘤重明显高于感病品种。种植一季后感病品种根际积累的病原生物(镰孢霉Fusarium.sp.和大豆胞囊线虫Heterodera.glycines的胞囊数)明显高于抗病品种。说明大豆根系分泌物对微生物具有选择性的促进或抑制作用,不同大豆品种以及同一大豆品种在不同生育时期根系分泌物的组成和数量不同,从而使大豆根面及根际形成了特定的微生物区系组成。     

Nutrient distribution across wheat rhizosphere with oxamide and ammonium sulfate as N source

Analysis of the behavior of slow releasing nitrogen fertilizers in the rhizosphere soil of agricultural crops is essential for their effective use. This work presents a comparative study of the effects of oxamide and ammonium sulfate as different N sources on the relative mobility of some nutritionally important anions and cations and their relevance to plant growth, root development, and pH distribution pattern in the rhizosphere of wheat (Triticum aestivum cv. Nourin No. 61). Experiment was conducted on a Japanese upland alluvial soil through a rhizobox approach. In the central compartment (CC) of the oxamide and ammonium sulfate treated rhizoboxes, four uncontaminated and homogeneous wheat seedlings germinated on a sterile petridish were grown for nearly two months. During the growing period plant height was recorded weekly. After the growing period, rhizoboxes were dismantled for soil sampling and photography of roots. Soil samples from each compartment of the two rhizoboxes were used for the analysis of CI^-, N0₃ ^-, SO₄ ^2-, Ca²⁺, Mg²⁺, K⁺, NH₄ ⁺-N, N0₃ ^--N and measurement of pH. Compared with ammonium sulfate, the distribution of anions and cations from the distant compartments to the central ones over a distance of 3–5 mm along with a better plant growth, high rooting density, and high pH confirmed that oxamide was an efficient nitrogen fertilizer.     

Influence of long-term conservation tillage on soil and rhizosphere microorganisms

 In long-term field experiments on sandy loam and loamy sand soils, the influence of conservation and conventional tillage on soil and rhizosphere microorganisms was studied. Conservation tillage stimulated rhizosphere bacteria on winter wheat, winter barley, winter rye and maize in different soil layers. Particularly the populations of Agrobacterium spp. and Pseudomonas spp. were increased. On the sandy loam, N₂ fixation and nodulation of pea plants were significantly increased. No influence of different soil tillage was determined on the colonization of the rhizosphere by mycorrhiza and saprophytic fungi. Stubble residues infected with Gaeumanomyces graminis were infectious for a longer time on the soil surface than after incorporation into the soil. Received: 10 March 1998     

Carbon loss from roots of wheat cultivars

The amounts of organic materials released into soil from roots during the first 4 weeks of growth were determined for 11 cultivars of wheat (Triticum aestivum L.). Carbon loss from roots was measured by supplying ¹⁴CO₂ continuously to the shoots and measuring the ¹⁴C content of the roots, root-free soil, water-soluble material and CO₂ flushed from the root chamber. Six cultivars were compared in each of two experiments, with the cultivar Condor common to both experiments. There were no significant differences between cultivars, relative to Condor, for ¹⁴C activity present in soil, roots, water-soluble material or rhizosphere CO₂. There was a significant difference between cultivars in experiment 1, but not in experiment 2, for the variate log₁₀ (¹⁴C lost from roots: ¹⁴C translocated to roots).There was evidence that a reduction in growth temperature, within the range 10–15°C, increased carbon loss from wheat roots into the rhizosphere.     

Nitrogenase (C2H2) Activity in Roots of Non-Cultivated and Cereal Plants: Influence of Nitrogen Fertilizer on Populations and Activity of Nitrogen-Fixing Bacteria

Root samples of 11 non-cultivated monocotyledonous and 7 dicotyledonous species taken during a wet summer had low mean nitrogenase activities of 10.2 and 7.1 nmol C₂H₄·g^?1 DW·h^?1 after preincubation at pO₂ 0.02, respectively. Maxima of 139–169 nmol·g^?1·h^?1 were observed with Agrostis vulgaris and Agropyron repens on a sandy soil poor in C_org. Three of 6 early, but none of 4 late fodder maize cultivars had a very low activity up to 0.5 nmol·g^?1h^?1. Oat, rye and wheat roots from plots with organic or mineral N fertilizers had activities between 1.3 and 7.3 nmol·g^?1h^?1 at flowering, which were not correlated with their Azospirillum populations (10²-10⁷·g^?1 after preincubation). Winter wheat and barley roots given 0, 40, 80 and 120 kg. ha^?1 NH₄NO₃-N in 0–3 applications had mean activities of 0.08, 4.06, 0.09 and 0.08 nmol or 1.77, 2.67, 0.36 and 0.23 nmol C₂H₄g^?1·h^?1 after flowering, respectively. An appreciable part of this activity could be removed by root washing. In preincubated rhizosphere soil of wheat and barley populations of N₂-fixing, facultative anaerobic Klebsiella and Enterobacter spp. were 10–100 times higher than those of Azospirillum sp., both being higher in O N than in 80 kg N·ha^?1 trials.     

Effects of different N sources on N nutrient and microbial distribution across soybean rhizosphere

Abstract

Nitrogen is the most important nutrient for plant growth. In the present study, investigations were carried out on the effects of sodium nitrate, ammonium sulfate, urea, and two types of controlled-release coated urea (LP-40 and LP-70) fertilizers on the NO₃ ^?-N, NH₄ ⁺-N concentrations, and microbial numbers as well as pH distribution across the rhizosphere of soybean (Glycine max L. Merrill, var. Heinong 35). The study was conducted on a typical black soil using a rhizobox system. The results showed that NO₃ ^?-N was the main source of nitrogen, which was deficient in the rhizosphere in the treatments of ammonium sulfate, urea, LP-40, and LP-70, but accumulated considerably in the sodium nitrate treatment. The NH₄ ⁺-N concentration slightly increased in the rhizosphere in the ammonium sulfate treatment, and decreased in the rhizosphere when the other four kinds of N fertilizers were supplied. In an the treatments, bacterial and fungal numbers were highest in the central compartment (C.C.) of the rhizoboxes where the soybean root system was confined, but the rhizosphere width estimated from the increase in the microbial abundance differed among different N fertilizers. The experimental results also indicated that the fungal composition in the C.C. was less diverse than in other parts of the rhizobox compartments, and that the majority of fungal groups was represented by Penicillium spp., suggesting that the microbial distribution across the soybean rhizosphere differed both quantitatively and qualitatively.     

Cross-protection against the wheat and oat take-all fungi by Gaeumannomyces graminis var. graminis

Glasshouse experiments have shown that the prior colonisation of wheat roots by Gaeumannomyces graminis var. graminis, a fungus closely related to the wheat and oat take-all fungi but non-pathogenic to temperate cereals, reduced take-all infection along the roots. Cross-protected wheat plants produced grain yields significantly greater than those of unprotected plants but not significantly different to those of healthy wheat plants. A Phialophora-like fungus from grass roots did not confer the same degree of protection. There is some evidence that the cross-protection mechanism may be a specific host response nduced by var. graminis. The possible use of var. graminis in the biological control of take-all is discussed.     

The dynamics of oxygen concentration, pH value, and organic acids in the rhizosphere of Juncus spp.

A novel type of planar optodes for simultaneous optical analysis of pH and oxygen dynamics in the rhizosphere is introduced. The combination of the optical, non-invasive measurement of these parameters with sterile sampling of rhizosphere solution across and along growing roots by use of a novel type of rhizobox provides a methodical step forward in the investigation of the physicochemical dynamics of the rhizosphere and its underlying matter fluxes between roots and soil. In this study, this rhizobox was used to investigate the effect of oxygen releasing roots of three Juncus species on the amount and distribution of organic acids in reductive, oxygen-deficient soils of different pH (pH 3.9-pH 5.9). Pronounced diurnal variations of oxygen concentration and pH along the roots, particularly along the elongation zone were observed. Long-term records over more than eight weeks revealed considerable spatial and temporal patterns of oxygen over a range of almost 200 μmol O₂ L⁻¹ and pH dynamics of ±1.4 pH units in the rhizosphere. A strong effect of oxidative acidification due to oxygen release by the plant roots was clearly visible for Juncus effusus, whereas the roots of Juncus articulatus alkalinized the rhizosphere. In contrast, roots of Juncus inflexus induced no effects on rhizospheric pH. Only four different organic acids (oxalate, acetate, formate and lactate) were detectable in all soil solutions. Maximal concentration of all organic acids occurred at pH 3.9, whereas the lowest concentration of each organic acid was found at pH 5.9. Hence, considering the pH-dependence of the redox potential, the acid soil provided increased reductive conditions leading to slower anaerobic degradation of organic acids to CO₂ or methane (CH₄). The concentration of organic acids decreased by up to 58% within a distance of only 4 mm from the bulk soil to the root surface, i.e. reciprocal to the pronounced O₂-gradient. The decreasing presence of organic acids toward the oxygen releasing roots is possibly due to a change in the composition of the microbial community from anaerobic to aerobic conditions. The present study highlights the dynamic interplay between O₂ concentration, pH and organic acids as key parameters of the physicochemical environment of the rhizosphere, particularly for wetland plants growing in oxygen-deficient waterlogged soils.