Effects of maize root exudates and organic acids on the desorption of phenanthrene from soils

The effects of maize root exudates and low-molecular-weight-organic anions （LMWOAs） on the desorption of phenanthrene from eight artificially contaminated soils were evaluated. A significant negative correlation was observed between the amounts of phenanthrene desorbed and the soil organic carbon （SOC） contents （P 〈 0.01）, and the influences of soil pH and clay content on phenanthrene desorption were insignificant （P 〉 0.1）. Neither maize root exudates nor oxalate and citrate anions influenced desorption of phenanthrene with the addition of NaN3. A faster phenanthrene desorption occurred without the addition of NaN3 in the presence of maize root exudates than oxalate or citrate due to the enhanced degradation by root exudates. Without the addition of NAN3, oxalate or citrate at different concentrations could inhibit phenanthrene desorption to different extents and the inhibiting effect by citrate was more significant than by oxalate. This study leads to the conclusion that maize root exudates can not enhance the desorption under abiotic condition with the addition of NaN3 and can promote the desorption of phenanthrene in soils without the addition of NaN3.

Artificial root exudates and soil organic carbon mineralization in a degraded sandy grassland in northern China

Plant root exudates contain various organic and inorganic components that include glucose, citric and oxalic acid. These components affect rhizosphere microbial and microfaunal activities, but the mechanisms are not fully known. Studies concerned from degraded grassland ecosystems with low soil carbon（C） contents are rare, in spite of the global distribution of grasslands in need of restoration. All these have a high potential for carbon sequestration, with a reduced carbon content due to overutilization. An exudate component that rapidly decomposes will increase soil respiration and CO2 emission, while a component that reduces decomposition of native soil carbon can reduce CO2 emission and actually help sequestering carbon in soil. Therefore, to investigate root exudate effects on rhizosphere activity, citric acid, glucose and oxalic acid（0.6 g C/kg dry soil） were added to soils from three biotopes（grassland, fixed dune and mobile dune） located in Naiman, Horqin Sandy Land, Inner Mongolia, China） and subjected to a 24-day incubation experiment together with a control. The soils were also analyzed for general soil properties. The results show that total respiration without exudate addition was highest in grassland soil, intermediate in fixed dune and lowest in mobile dune soil. However, the proportion of native soil carbon mineralized was highest in mobile dune soil, reflecting the low C/N ratio found there. The exudate effects on CO2-C emissions and other variables differed somewhat between biotopes, but total respiration（including that from the added substrates） was significantly increased in all combinations compared with the control, except for oxalic acid addition to mobile dune soil, which reduced CO2-C emissions from native soil carbon. A small but statistically significant increase in pH by the exudate additions in grassland and fixed dune soil was observed, but there was a major decrease from acid additions to mobile dune soil. In contrast, electrical conductivity decreased in grassland and fixed dune soil and increased in mobile dune. Thus, discrete components of root exudates affected soil environmental conditions differently, and responses to root exudates in soils with low carbon contents can differ from those in normal soils. The results indicate a potential for, e.g., acid root exudates to decrease decomposition rate of soil organic matter in low carbon soils, which is of interest for both soil restoration and carbon sequestration.

Accelerated degradation of a variety of aromatic compounds by Spirodela polyrrhiza-bacterial associations and contribution of root exudates released from S. polyrrhiza

Removal experiments of phenol, aniline, 2,4-dichlorophenol, nonylphenol and bisphenol A （BPA） using Spirodela polyrrhiza- bacterial associations revealed that all compounds but BPA underwent accelerated removal. The mechanisms differed depending on the substrates. It was found that S. polyrrhiza has a great ability to release phenolic compound-rich root exudates, and the exudates seem to stimulate bacterial degradation of a variety of aromatic compounds.

Exogenous Inoculation of Microorganisms Effect on Root Exudates and Rhizosphere Microorganism of Tobaccos

Studying the relationship between rhizosphere microorganisms and root exudates is of great significance for the interaction between rhizosphere microorganisms and plants, and the prevention and control of soil-borne diseases. This article analyzed the effects of different microorganisms on tobacco root exudates and rhizosphere microorganisms. It was found that the bacterial wilt pathogen can greatly increase acids and amines, while the probiotic B. amyloliquefaciens ZM9 can eliminate some acids and amines. The results of the study show that the root exudates of pathogenic bacteria may contain a variety of allelochemicals that cause soil-borne diseases.

Com parison of Amino Acids in Root Exudates of Flue-cured Tobacco Varieties with Different Resistances against Black Shank

In order to explore the differences of amino acids in root exudates and plant tissues, we selected four flue-cured tobacco varieties [ Yunyan 87 （ Ⅰ）, K326 （Ⅱ）, NC102（Ⅲ）, Hongda （Ⅳ）] as the test materials and studied the components and mass fractions of amino acids in rhizosphere soil, non rhizosphere soil, roots and leaves via high performance capillary zone electrophoresis. Ten amino acids were detected out from four varieties, including histidine, lysine, tryptophan, phenylalanine, valine, threonine, proline, serine, alanine and glycine. The mass fractions of amino acids in leaves and roots were higher than those in rhizosphere soil and non rhizosphere soil. In rhizosphere soil and non-rhizosphere soil, the mass fractions of lysine, phenylalanine, threonine, proline and glycine in susceptible varieties （ Ⅲ, Ⅳ） were higher than those in resistant varieties （ Ⅰ, Ⅱ）. In rhizosphere soil, the total amino acids successively were Ⅳ 〉 Ⅲ 〉 Ⅰ 〉 Ⅱ, and in non rhi- zosphere soil were Ⅲ 〉 Ⅳ 〉 Ⅰ 〉 Ⅱ. The total amino acids in roots of variety Ⅳ was the highest, up to 6 359. 195 μg/g. In leaves, the total amino acids in varieties I and If were higher than those in varieties IfI and IV. The results indicated that amino acids could secrete to rhizosphere via root secretion, and the compositions and mass fractions of amino acids varied with flue-cured tobacco varieties, thus the secretion amount of susceptible varieties was higher than that of resistant varieties.

Interspecific plant interaction via root exudates structures the disease suppressiveness of rhizosphere microbiomes

Terrestrial plants can affect the growth and health of adjacent plants via interspecific interaction.Here,we studied the mechanism by which plant root exudates affect the recruitment of the rhizosphere microbiome in adjacent plants—with implications for plant protection—using a tomato(Solanum lycopersicum)–potatoonion(Allium cepa var.agrogatum)intercropping system.First,we showed that the intercropping system results in a disease-suppressive rhizosphere microbiome that protects tomato plants against Verticillium wilt disease caused by the soilborne pathogen Verticillium dahliae.Second,16S rRNA gene sequencing revealed that intercropping with potatoonion altered the composition of the tomato rhizosphere microbiome by promoting the colonization of specific Bacillus sp.This taxon was isolated and shown to inhibit V.dahliae growth and induce systemic resistance in tomato plants.Third,a belowground segregation experiment found that root exudates mediated the interspecific interaction between potatoonion and tomato.Moreover,experiments using split-root tomato plants found that root exudates from potatoonion,especially taxifolin—a flavonoid compound—stimulate tomato plants to recruit plant-beneficial bacteria,such as Bacillus sp.Lastly,ultra-high-pressure liquid chromatography–mass spectrometry analysis found that taxifolin alters tomato root exudate chemistry;thus,this compound acts indirectly in modulating root colonization by Bacillus sp.Our results revealed that this intercropping system can improve tomato plant fitness by changing rhizosphere microbiome recruitment via the use of signaling chemicals released by root exudates of potatoonion.This study revealed a novel mechanism by which interspecific plant interaction modulates the establishment of a disease-suppressive microbiome,thus opening up new avenues of research for precision plant microbiome manipulations.

Root exudates increased arsenic mobility and altered microbial community in paddy soils

Root exudates are crucial for plants returning organic matter to soils,which is assumed to be a major source of carbon for the soil microbial community.This study investigated the influence of root exudates on the fate of arsenic(As)with a lab simulation experiment.Our findings suggested that root exudates had a dose effect on the soil physicochemical properties,As speciation transformation and the microbial community structure at different concentrations.The addition of root exudates increased the soil pH while decreased the soil redox potential(Eh).These changes in the soil pH and Eh increased As and ferrous(Fe(Ⅱ))concentrations in soil porewater.Results showed that 40 mg/L exudates addition significantly increased arsenite(As(Ⅲ))and arsenate(As(Ⅴ))by 541 and 10 times respectively within 30 days in soil porewater.The relative abundance of Fe(Ⅲ)-reducing bacteria Geobacter and Anaeromyxobacter increased with the addition of root exudates,which enhanced microbial Fe reduction.Together these results suggest that investigating how root exudates affect the mobility and transformation of As in paddy soils is helpful to systematically understand the biogeochemical cycle of As in soil-rice system,which is of great significance for reducing the health risk of soil As contamination.

Bioassay and Identification of Root Exudates of Three Fruit Tree Species

A laboratory bioassay was designed to determine the aileiopathic potential of root exudates of three fruit tree species on apple germination. The results showed that root exudates of apple （Malus pumila L.） and peach （Prunus persica L.）, each at concentrations of 0.02 and 0.2 mg/L, inhibited germination and radicle growth of apple seeds by 56.7%, 60.7%, 51.5%, and 59.3%, respectively. The corresponding shoot growth inhibition rate was 49.5%, 46.7%, 36. 4%, and 44%, respectively. Root exudates of jujube （Ziziphus jujuba Mill.） had no significant effect on apple seeds. Qualitative determination of root exudates of apple, peach, and jujube tree was developed with gas chromatography-mass spectrometry. The root exudates of apple seedlings mainly contain organic acids, glycol, esters, and benzenphenol derivatives. Peach root exudates contained phenolic acids and benzenphenoi derivatives in addition to two unidentified compounds. The root exudates of jujube did not contain any phenolic acids.

Molecular characterization of root exudates using Fourier transform ion cyclotron resonance mass spectrometry

The release of root exudates(REs) provides an important source of soil organic carbon. This work revealed the molecular composition of REs of different plant species including alfalfa( Medicago sativa L.), bean( Phaseolus vulgaris L.), barley( Hordeum vulgare L.), maize( Zea mays), wheat( Triticum aestivum L.), ryegrass( Lolium perenne L.) and pumpkin( Cucurbita maxima) using electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry(ESI FT-ICR MS). The combination of positive ion mode( + ESI) and negative ion mode(-ESI) increased the number of the molecules detected by ESI FT-ICR MS, and a total of 8758 molecules were identified across all the samples. In detail, lipids and proteins and unsaturated hydrocarbons were more easily detected in + ESI mode, while aromatic compounds with high O/C were readily ionized in-ESI mode, and only 38% of the total assigned formulas were shared by -ESI and + ESI modes. Multivariate statistical analysis of the formulas indicated that the close related plants species secreted REs with similar molecular components. Moreover, the unsaturation degree and nitrogen content were the two key parameters able to distinguish the similarities and differences of molecular components of REs between plant species. The results provided a feasible analysis method for characterization of the molecular components of REs and for the first time characterized the molecular components of REs of a variety of plant species using ESI FT-ICR MS.

Root exudates mediate plant defense against foliar pathogens by recruiting beneficial microbes

Plants are capable of releasing specific root exudates to recruit beneficial rhizosphere microbes upon foliar pathogen invasion attack,including long-chain fatty acids,amino acids,short-chain organic acids and sugars.Although long-chain fatty acids and amino acids application have been linked to soil legacy effects that improve future plant performance in the presence of the pathogen,the precise mechanisms involved are to a large extent still unknown.Here,we conditioned soils with long-chain fatty acids and amino acids application(L+A)or short-chain organic acids and sugars(S+S)to examine the direct role of such exudates on soil microbiome structure and function.The L+A treatment recruited higher abundances of Proteobacteria which were further identified as members of the genera Sphingomonas,Pseudomonas,Roseiflexus,and Flavitalea.We then isolated the enriched bacterial strains from these groups,identifying ten Pseudomonas strains that were able to help host plant to resist foliar pathogen infection.Further investigation showed that the L+A treatment resulted in growth promotion of these Pseudomonas strains.Collectively,our data suggest that long-chain fatty acids and amino acids stimulated by foliar pathogen infection can recruit specific Pseudomonas populations that can help protect the host plant or future plant generations.

Influence of Indian mustard (Brassica juncea) on rhizosphere soil solution chemistry in long-term contaminated soils:A rhizobox study

This study investigated the influence of Indian mustard （Brassicajuncea） root exudation on soil solution properties （pH, dissolved organic carbon （DOC）, metal solubility） in the rhizosphere using a rhizobox. Measurement was conducted following the cultivation of Indian mustard in the rhizobox filled four different types of heavy metal contaminated soils （two alkaline soils and two acidic soils）. The growth of Indian mustard resulted in a significant increase （by 0.6 pH units） in rhizosphere soil solution pH of acidic soils and only a slight increase （〈 0.1 pH units） in alkaline soils. Furthermore, the DOC concentration increased by 17-156 mg/L in the rhizosphere regardless of soil type and the extent of contamination, demonstrating the exudation of DOC from root. Ion chromatographic determination showed a marked increase in the total dissolved organic acids （OAs） in rhizosphere. While root exudates were observed in all soils, the amount of DOC and OAs in soil solution varied considerably amongst different soils, resulting in significant changes to soil solution metals in the rhizosphere. For example, the soil solution Cd, Cu, Pb, and Zn concentrations increased in the rhizosphere of alkaline soils compared to bulk soil following plant cultivation. In contrast, the soluble concentrations of Cd, Pb, and Zn in acidic soils decreased in rhizosphere soil when compared to bulk soils. Besides the influence of pH and DOC on metal solubility, the increase of heavy metal concentration having high stability constant such as Cu and Pb resulted in a release of Cd and Zn from solid phase to liquid phase.

Microbial activity related to N cycling in the rhizosphere of maize stressed by heavy metals

A greenhouse experiment was carried out to compare differences in potential activities of ammonification, nitrification and denitrification in rhizosphere and bulk soil in a heavy-metal-stressed system. Exchangeable fractions of Cd, Cu and Cr were all higher in the rhizosphere of maize than in bulk soil. Results showed that the mineralization of N in soil was stimulated by low concentration of Cd. Addition of Cd at low levels stimulated the ammonifying and nitrifying activity in soil, while inhibitory influences were shown at high levels. Nitrifying bacteria was proved to be the most sensitive one, whilst the effect on denitrifying bacteria was very limited. Comparing Cd, Cu and Cr(VI) at 20 mg/kg soil, Cd was the most effective inhibitor of ammonification and denitrification, while Cr(VI) had the strongest inhibitory influence on nitrifying activity. Root exudates played important roles on the different exchangeable metal fractions and bacterial activities between rhizosphere and non-rhizosphere. Nitrate was the main form of mineral N in soil, as well as the main form of N absorbed by plants, but the formation and relative absorption of ammonium were promoted in response to high Cd exposure.

Subcellular distribution and chemical form of Pb in hyperaccumulator Arenaria orbiculata and response of root exudates to Pb addition

Solution culture was conducted in order to understand accumulation characteristics and chemical forms of Pb in Arenaria orbiculata （A. orbiculata） and the response of root exudates to Pb addition. The results showed that： 1） Pb contents in the shoot and root of A. orbiculata increased with increasing in Pb concentrations in solution. 2） The contents of Pb chemical forms under Pb addition followed as： HAc extractable fraction＇（FriAC）〉 HC1 extractable fraction （FHcl）〉 NaCl extractable fraction （FNacl） 〉 ethanol-extractable fraction （FE） 〉 water extractable fraction （Fw）. 3） Increased Pb level in the medium caused increases in Pb contents in the four subcellular fractions of shoots and roots, with most accumulation in FIV （Fraction ＇IV, sbluble fraction） in shoots and FI （Fraction I, cell wall fraction） in roots. 4） Contents of soluble sugar and free amino acid of root exudates increased with increasing Pb concentration in solution. Significantly positive correlations between Pb and contents of soluble sugar and free amino acid were observed. 5） With Pb concentrations in solution, low molecular weight organic acids （LMWOAs） contents followed the tendency： tartaric acid 〉 acetic acid 〉 malic acid 〉 citric acid. Significantly positive correlation was observed between Pb and citric acid contents. The results indicate that soluble sugars, free amino acid and citric acid in root exudates of A.orbiculata facilitate the absorption and accumulation of Pb, which exist in NaCl-, HCI- and HAc- extractable Pb forms, FI and FIV fractions, resulting in tolerance of A.orbiculata to Pb.

Physiological response and phenolic metabolism in tomato (Solanum lycopersicum) mediated by silicon under Ralstonia solanacearum infection

Bacterial wilt, caused by Ralstonia solanacearum（Rs） is a serious soil-borne disease and silicon can enhance tomato resistance against this disease. However, few studies have focused on the mechanisms of Si-mediated pathogen resistance from the rhizosphere perspective. In this study, two tomato genotypes, HYT（susceptible） and H7996（resistant）, were used to investigate the effects of silicon application on disease inhibition, root growth, and organic acid content in both roots and root exudates under R. solanacearum infection. The results showed that Si application significantly suppressed bacterial wilt in HYT, but had no effect in H7996. Silicon concentrations in roots, stems and leaves of tomato were significantly increased by Si treatment under R. solanacearum inoculation. In HYT, Si application increased root dry weight by 22.8-51.6% and leaf photosynthesis by 30.6-208.0%, and reduced the concentrations of citric acid in root exudates by 71.4% and in roots by 83.5%. However, organic acids did not influence R. solanacearum growth. Results also demonstrated that salicylic acid（SA） content in roots was significantly increased by silicon addition for H7996 and exogenous SA application could reduce bacterial wilt disease index. Collectively, these results suggest that Si-modulated phenolic compound metabolism in roots or root exudates, especially citric acid and SA, may be a potential mechanism in the amelioration of bacterial wilt disease by Si.

Effect of Zn deficiency and excessive bicarbonate on the allocation and exudation of organic acids in two Moraceae plants

The effect of zinc(Zn) deficiency and excessive bicarbonate on the allocation and exudation of organic acids in plant organs(root, stem, and leaf) and root exudates of two Moraceae plants(Broussonetia papyrifera and Morus alba) were investigated. Two Moraceae plants were hydroponically grown and cultured in nutrient solution in four different treatments with 0.02 mM Zn or no Zn,combined with no or 10 mM bicarbonate. The variations of organic acids in different plant organs were similar to those of root exudates in the four treatments except B. papyrifera, which was in a treatment that was a combination of 0.02 mM Zn and no bicarbonate. The response characteristics in the production, translocation, and allocation of organic acids in the plant organs and root exudates varied with species and treatments. Organic acids in plant organs and root exudates increased under Zn-deficient conditions,excessive bicarbonate, or both. An increase of organic acids in the leaves resulted in an increase of root-exuded organic acids. B. papyrifera translocated more oxalate and citrate from the roots to the rhizosphere than M. alba under the dual influence of 10 mM bicarbonate and Zn deficiency. Organic acids of leaves may be derived from dark respiration and photorespiration. By comparison, organic acids in stems, roots, and root exudates may be derived from dark respiration and organic acid translocation from the leaves. These results provide evidence for the selective adaptation of plants to environments with low Zn levels or high bicarbonate levels such as a karst ecosystem.

Beneficial rhizobacterium provides positive plant–soil feedback effects to Ageratina adenophora

Rhizosphere microbial communities play important roles in facilitating or inhibiting the establishment of exotic species.Since some invasive plants interact with soil microbial communities such as rhizosphere bacteria,changes triggered by rhizosphere bacteria may alter competitive interactions between exotic and native plants.This study compared the Bacillus cereus content in soils with different degrees of Ageratina adenophora invasion,and investigated the effects of A.adenophora allelochemicals on B.cereus growth and soil characteristics and the feedback effects of B.cereus on A.adenophora growth.Bacillus cereus content in the rhizosphere of A.adenophora increased with intensification of the invasion process,and newly invaded soil contained almost twice as much bacteria as noninvaded soil.When rhizosphere soil was added to the root exudates of A.adenophora,the contents of B.cereus were twice as much as the control,except on the first day.Certain soil parameters increased significantly,such as ammonium nitrogen(NH_(4)^(+)-N)and available phosphorus(AP),which were increased by 41 and 27%,respectively.Soil treatment with B.cereus promoted the degradation of two allelochemicals from the rhizosphere of A.adenophora,amorpha-4,7(11)-dien-8-one and 6-hydroxy-5-isopropy1-3,8-dimethyl-4 a,5,6,7,8,8 ahexahydraphthalen-2(1 H)-one,to varying degrees;and increased the germination rate by 50%,root length by 117%,shoot length by 48%and fresh weight by 81%for A.adenophora compared to those of untreated soil.Our results confirmed that the invasion of A.adenophora will promote an increase of B.cereus,a beneficial rhizosphere bacterium,which in turn induces a positive feedback effect on A.adenophora.

Transcriptional Analysis of Pseudomonas stutzeri A1501 Associated with Host Rice

Pseudomonas stutzeriA1501, associative and endophytic nitrogen-fixing bacterium showed the capacity of colonization in the rice roots and considered as the good colonizer in the rice plant. The experiment was conducted to study the expression of genes potentiality relevant to the association of nitrogen fixing Pseudomonas stutzeri with host rice and reveal the molecular mechanism by which underlying interaction between bacteria and host rice. The bacteria were shown to be uniformly distributed on the rhizoplane of the root and the density of bacteria was found at the intercellular junction and micro colony developed on the surface of the epidermal cells and on the cellular junctions. Root exudates of rice were the major components of carbon and energy sources for bacteria. RT-PCR analyses of pilK, metE, rpoN and fdhE genes expression of P. stutzeri A1501 were performed at positive and negative (control) conditions. After 1 h, it was found that pilK, metE and rpoN transcription were increased 5.7, 6.4 and 3.4-fold, respectively, whereas in the fdhE gene has no expression. Consequently, after 4 h pilk, fdhE, metE and rpoN were decreased -1.9, -4.4, -0.2 and -0.8-fold, respectively. The gene pilK, expression was up-regulation after 1 h and down-regulation after 4 h that has twitching motility to convey the bacterial cell to point of attachment in to host plant. The gene expressions of the bacteria, pilK, metE, rpoN and fdhE were up- and down-regulated during the influence of root exudates which regulated the colonization of bacteria during plant-microbe interaction.

RIN enhances plant disease resistance via root exudate-mediated assembly of diseasesuppressive rhizosphere microbiota

The RIPENING-INHIBITOR(RIN)transcriptional factor is a key regulator governing fruit ripening.While RIN also affects other physiological processes,its potential roles in triggering interactions with the rhizosphere microbiome and plant health are unknown.Here we show that RIN affects microbiome-mediated disease resistance via root exudation,leading to recruitment of microbiota that suppress the soil-borne,phytopathogenic Ralstonia solanacearum bacterium.Compared with the wild-type(WT)plant,RIN mutants had different root exudate profiles,which were associated with distinct changes in microbiome composition and diversity.Specifically,the relative abundances of antibiosis-associated genes and pathogensuppressing Actinobacteria(Streptomyces)were clearly lower in the rhizosphere of rin mutants.The composition,diversity,and suppressiveness of rin plant microbiomes could be restored by the application of 3-hydroxyflavone and riboflavin,which were exuded in much lower concentrations by the rin mutant.Interestingly,RIN-mediated effects on root exudates,Actinobacteria,and disease suppression were evident from the seedling stage,indicating that RIN plays a dual role in the early assembly of diseasesuppressive microbiota and late fruit development.Collectively,our work suggests that,while plant disease resistance is a complex trait driven by interactions between the plant,rhizosphere microbiome,and the pathogen,it can be indirectly manipulated using"prebiotic"compounds that promote the recruitment of disease-suppressive microbiota.

Sizes of Anode and Cathode Affect Electricity Generation in Rice Paddy-Field Microbial Fuel Cells

Rice paddy-field microbial fuel cells (RPF-MFCs) are devices that exploit rhizosphere bacteria to generate electricity from soil organic matter, including those excreted from roots. Previous studies have examined factors affecting electric outputs from RPF-MFCs and demonstrated that RPFMFC was able to generate electricity up to 80 mW·m-2 (based on the projected area of anode). The present study operated RPF-MFCs with different sizes of anodes and cathodes and examined how electrode sizes affected electricity generation. We show that anodes are the limiting factor for electricity generation immediately after commencing the operation, while cathodes become the limiting factor after anode performances are sufficiently increased. RPF-MFC achieved the maximum power density of 140 mW·m-2 (based on the projected area of anode), when the cathode is sufficiently larger than the anode. Results suggest that the cathode needs to be improved for eliciting the maximum capacity of rhizosphere bacteria for electricity generation in RPF-MFC.

Weed-Suppressing Effect and Mechanism of Allelopathic Rice Accessions

Two allelopathic rice accessions, PI312777 and Allelopathy1, significantly suppressedthe growth of associated weeds in the field. Moreover, their weed-suppressing effectswere correlated with the cultivation patterns. The weed-suppressing effects of throwingand transplanting were more effective than that of direct seeding. Furthermore, theamounts of allelochemicals (resorcinols, flavones and hydroxamic acids) produced andreleased from two allelopathic rice accessions were much higher than that from a non-allelopathic rice variety Hua-Jing-Xian1, and reached the maximum concentration at the6th leaf stage. Differences in the weed-suppressing effects of rice accessions appear toresult from the accessions producing and releasing different amounts of allelochemicalsin the field. Further research confirmed that in PI312777 plants, allelochemicals weresynthesized by the above-ground parts, and then secreted through the root tissues. Roottissues of PI312777 plants never produced the allelochemicals. Root exudates fromPI312777 could significantly inhibit the growth of E. crus-galli surrounding rice plantsin water culture. However, when activated carbon was added to the culture solution, whichcould absorb allelochemicals from root exudates, the growth of E. crus-galli was nolonger significantly inhibited. Weed-suppressing effects of rice accessions depended onallelopathy, cultivation patterns and other factors in rice fields, while allelopathywas one of important factors. Interestingly, the amounts of allelochemicals produced andreleased from allelopathic rice plants may be induced by the presence of E. crus-galli.This suggests that there is a possible chemical recognition between rice and E. crus-galli.