Biochemical basis for wheat seedling allelopathy on the suppression of annual ryegrass (Lolium rigidum)

The chemical basis for wheat seedling allelopathy on the growth of annual ryegrass was investigated by the identification and quantification of multiple allelochemicals from wheat seedlings. Results indicated that 58 wheat accessions differed significantly in seedling allelopathy and inhibited the root growth of ryegrass from 10 to 91%, depending on accession. Analysis of allelochemicals by GC/MS/MS indicated that allelopathy was significantly correlated with the levels of measured allelochemicals in the shoots and roots of young wheat seedlings. Ryegrass root growth was also negatively correlated with the levels of p-hydroxybenzoic, vanillic, and trans-ferulic acids in root exudates. Wheat allelopathic potential was negatively correlated with the levels of the eight known allelochemicals quantified in the shoots, roots, and water-agar medium, with multiple regression coefficients (r) of -0.61, -0.71, and -0.71, respectively. In comparison with weakly allelopathic accessions, strongly allelopathic accessions produced significantly higher amounts of allelochemicals in the shoots and roots of the wheat seedlings and also exuded larger quantities of allelochemicals into the growth medium. Wheat accessions with strong seedling allelopathy might be useful for management of weeds during the establishment stage, thereby reducing the need for commercial herbicides in early-season application.     

Allelochemicals in wheat (Triticum aestivum l.): variation of phenolic acids in root tissues

Analysis by GC/MS/MS showed that a worldwide collection of 58 wheat accessions differed significantly in the production of seven phenolic acids in the roots of 17-day-old wheat seedlings. The allelochemical contents among wheat accessions ranged from 24.5 to 94.5, 19.9 to 91.7, 3.7 to 15.4, 2.2 to 38.6, 1.0 to 42.2, 19.3 to 183.6, and 11.7 to 187.6 mg/kg of root dry weight for p-hydroxybenzoic, vanillic, cis-p-coumaric, syringic, cis-ferulic, trans-p-coumaric, and trans-ferulic acids, respectively. trans-Ferulic acid was identified as the most predominant phenolic acid in the roots. Phenolic acids, with the exception of syringic acid, were more concentrated in roots than in shoots. Significant correlation was found between the roots and the shoots in the contents of vanillic, cis-p-coumaric, syringic, trans-p-coumaric, and trans-ferulic acids, and in the content of each structural group of phenolic acids. Wheat accessions with high levels of total identified phenolic acids in the roots were generally strongly allelopathic to the growth of annual ryegrass.     

Joint action of benzoxazinone derivatives and phenolic acids

The joint action of binary and ternary mixtures of benzoxazinone derivatives and phenolic acids was studied using the additive dose model (ADM) as reference model. The activity of fixed-ratio mixtures of phenolic acids [ferulic acid (FA), p-coumaric acid (CA), vanillic acid (VA), and p-hydroxybenzoic acid (HBA)] and benzoxazinone derivatives [2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), 6-methoxybenzoxazolin-2-one (MBOA), benzoxazolin-2-one (BOA), 2-aminophenol (AP), and N-(2-hydroxyphenyl)acetamide (HPAA)] on Lolium perenne and Myosotis arvensis root growth was assessed in Petri dishes. Root length was recorded 6 days after seeding, and EC(50) and EC(90) values were estimated using nonlinear regression analyses. The benzoxazinone derivatives were found to be more phytotoxic than the phenolic acids, particularly on M. arvensis. Binary mixtures of phenolic acids responded predominantly additively on both plant species. Deviations from additivity were species-specific with antagonistic responses on L. perenne and synergistic responses on M. arvensis. Similarly, binary mixtures of benzoxazinone derivatives also followed the ADM, although synergistic responses were observed for BOA + AP and BOA + HPAA. Binary and ternary mixtures of benzoxazinone derivatives and phenolic acids responded primarily antagonistically; however, a significant synergistic performance was observed with DIMBOA + FA and DIMBOA + VA on L. perenne. These results do not support the assumption that allelopathic effects of wheat can be attributed to synergistic effects of otherwise weakly active allelopathic compounds, and it is suggested that future research be directed toward identifying and studying the effects of other potential allelochemicals including the degradation products of the most abundant wheat allelochemicals.     

田间旱育条件下不同化感潜力水稻根际土壤酚酸类和萜类物质分析

酚酸类和萜类是水稻化感作用研究中研究较多、争议较大的2大类化感物质,但目前有关水稻根系分泌物的研究多在实验室条件下进行。本文以国际公认的强化感水稻‘PI312777’和弱化感水稻‘Lemont’为材料,以未种植水稻的土壤为对照,研究了其在田间旱育条件下,不同土壤水分状态(旱地和湿地)时,根际土壤酚酸类和萜类物质的差异。结果表明,不同水分条件下不同化感潜力水稻品种和对照根际土壤中酚酸类物质和萜类物质的组成较为相似,但各物质含量存在一定差异。适度旱胁迫下,各处理根际土壤中所检测到的咖啡酸、对羟基苯甲酸、香草酸、阿魏酸和肉桂酸5种酚酸类物质及总量均有提高,其中强化感水稻‘PI312777’根际土壤中5种酚酸类物质提高最显著,比CK湿地处理提高2.84倍;在各处理根际土壤共检测到的27种萜类物质中,17种是含氧单萜;干旱胁迫导致各处理根际土壤单萜烯、含氧单萜、含氧倍半萜和总萜变化程度和变化趋势不同,单萜烯相对含量在强化感水稻‘PI312777’根际土壤中明显提高,而在弱化感水稻‘Lemont’和对照根际土壤中则降低。本文在此基础上讨论了由此导致2种水稻田间化感抑草效果差异的原因与机制。     

Bioavailability of allelochemicals as affected by companion compounds in soil matrices

Multicompound allelochemical interactions were studied using Centaurea maculosa as a model source to understand how the bioavailability of complex allelochemical mixtures is modified in soil-microbial systems. Litter decomposition of C. maculosa in sandy loam soil yielded five phenolic acids, namely, hydroxybenzoic, vanillic, protocatechuic, p-coumaric, and ferulic acids. The degradation studies were conducted by exogenous application of catechin, the primary allelochemical exuded by C. maculosa, and the phenolic acid cosolutes in a sandy loam and silt loam soil. Compared to a single-solute system, in a multisolute system the persistence of individual allelochemicals was significantly increased in both soils. Oxidation and sorption were primarily involved in the disappearance of allelochemicals. Mass spectrometric data showed that catechin rapidly underwent polymerization to form procyanidin dimer both in soil and in bioassay medium, resulting in reduced persistence and phytotoxicity. Hence, catechin phytotoxicity could occur only under conditions that would inhibit these condensation reactions. This study clearly demonstrates that various soil mechanisms including competitive sorption and preferential degradation would increase the persistence of allelochemical mixtures in a soil matrix.     

Free radical scavenging properties and phenolic content of Chinese black-grained wheat

Free radical scavenging properties and phenolic content of extracts from a novel Chinese black-grained wheat were evaluated for comparison with selected wheat controls. Extracts of bran and whole meal were compared for their scavenging activities against the 2,2-diphenyl-1-picryhydrazyl (DPPH) free radical. The total phenolic content and phenolic acid levels were determined using colorimetric and high-performance liquid chromatography (HPLC) methods, respectively. There were significant differences in radical scavenging activities and phenolic contents among bran or whole meal samples of Chinese black-grained wheat and selected wheat controls. Chinese black-grained wheat had the strongest scavenging activity and the highest total phenolic content among the wheat samples. The scavenging activity and total phenolic content of wheat bran was generally twice as high as that of whole meal. A positive correlation was found between DPPH radical scavenging activity and total phenolic content of bran (R = 0.86) and whole meal (R = 0.96). In addition, HPLC analysis detected the presence of gallic, p-hydroxybenzoic, caffeic, syringic, p-coumaric, vanillic, gentisic, o-coumaric acid, and ferulic acids in wheat bran. Ferulic acid content was highest among the phenolic acids. Chinese black-grained wheat may be considered as a potential source of natural antioxidants given its high free radical scavenging ability and phenolic content. Additional research is needed to further investigate other phenolic compounds and evaluate their contribution to the antioxidant activity in order to understand the nutraceutical value of the novel black-grained wheat genotype.     

看麦娘叶片对化感小麦根水提液的生理响应

为阐明小麦化感抑草的生理机制,选择强化感小麦‘115/青海麦’、‘92L89’和弱化感小麦‘抗10103’,通过添加浓度为0.2%、0.5%和1.0%的小麦根水提液进行水培试验3周后,测定了各处理看麦娘的鲜重,分析叶片中叶绿素(SPAD值)、可溶蛋白、MDA、类黄酮、总酚的含量以及SOD、POD、CAT活性。结果表明,水提液处理显著抑制了看麦娘的生长,抑制率在不同处理浓度及小麦品种间均存在显著差异,强化感小麦的抑制率显著高于弱化感小麦。在处理浓度范围内,不同小麦根水提液的抑制率大小依次为‘115/青海麦’(24.7%~74.3%)‘92L89’(15.7%~71.6%)‘抗10103’(13.8%~61.4%);0.2%、1.0%和5.0%水提液处理的抑制率大小依次为13.8%~24.7%、41.7%~66.4%和61.4%~74.2%。看麦娘叶绿素含量(SPAD值)随处理浓度增大显著降低,可溶蛋白含量,SOD、POD、CAT活性,MDA、类黄酮含量随处理浓度增大显著升高,强化感小麦对看麦娘的生理刺激作用高于弱化感小麦。1.0%‘115/青海麦’及5.0%各小麦水提液处理的看麦娘总酚含量高于对照。可见,小麦化感胁迫提高了看麦娘的保护酶系统活性,增强了抗氧化物质代谢,但显著增强了细胞膜脂质过氧化和叶绿素降解,不利于靶标植物看麦娘的生长。     

连作植烟土壤具有显著积累特征的两种酚酸的化感效应评价

为科学评价连作植烟土壤中具有显著积累特征的两种酚酸的化感效应,丰富现有连作障碍机理,以烤烟品种K326为试验材料,基于前期田间实测结果设置不同浓度梯度(间苯三酚:0.20、1.00、5.00 mg·kg^-1,对羟基苯甲酸0.02、0.10、0.50 mg·kg^-1)进行盆栽试验。结果表明,两种酚酸物质均显著抑制了烤烟种子萌发。间苯三酚抑制了幼苗根系的生长,且浓度越大抑制作用越强;对羟基苯甲酸对幼苗根系生长的影响表现出“低促高抑”的浓度效应。随着间苯三酚浓度的增加,叶片超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性呈上升趋势,而过氧化物酶(POD)活性呈下降趋势;随着对羟基苯甲酸浓度的增加,叶片SOD、CAT和POD活性均呈先升后降趋势。此外,两种酚酸物质均降低了烤烟幼苗叶片叶绿素含量,增加了丙二醛(MDA)含量。随着处理时间的延长,胁迫作用趋势相同。由此可知,间苯三酚和对羟基苯甲酸不同程度地抑制了烤烟幼苗的生长,且存在浓度效应。间苯三酚表现为抑制作用,浓度越高抑制作用越强;对羟基苯甲酸浓度≤0.10 mg·kg^-1时表现为促进作用,>0.10 mg·kg^-1时表现为抑制作用。对羟基苯甲酸较间苯三酚具有更强的生物毒性。本研究可以丰富植烟土壤连作障碍机理,为今后连作障碍调控提供理论依据。     

Analysis of phenolic acids in barley by high-performance liquid chromatography

Phenolic acids from 30 barley varieties (combination of hulled/hulless/two-row/six-row/regular/waxy) were investigated by HPLC following four different sample treatments: (a) simple hot water extraction, (b) extraction after acid hydrolysis, (c) acid plus alpha-amylase hydrolysis, and (d) acid plus alpha-amylase plus cellulase hydrolysis treatments. The benzoic acid (p-hydroxybenzoic, vanillic, and protocatechuic acids) and cinnamic acid derivatives (coumaric, caffeic, ferulic, and chlorogenic acids) were identified, and some of the phenolic acids were quantified after each above-mentioned treatment. The data indicated that a combination of sequential acid, alpha-amylase, and cellulase hydrolysis treatments might be applicable for release of more phenolic acids from barley.     

外源酚酸在杨树人工林土壤中的吸附与滞留动态研究

采集不同连作代数杨树人工林土壤样品,采用外源添加酚酸的方法结合HPLC定量测定,研究了对羟基苯甲酸和苯甲酸在杨树人工林土壤中的吸附动态和滞留规律,探讨了两种酚酸在杨树人工林土壤中的累积机制。结果表明:(1)两种酚酸在杨树人工林土壤中的吸附可大致分为两个过程,即快速吸附阶段和慢吸附阶段,对羟基苯甲酸的快速吸附阶段大约在16h内完成,而苯甲酸约需24h。达到吸附平衡时,对羟基苯甲酸在土壤中的平均吸附率为58.78%,苯甲酸的平均吸附率为37.07%。(2)杨树人工林土壤对两种酚酸均具有较强的吸附能力,达到吸附平衡时,对羟基苯甲酸在土壤中的平均吸附量为2483.19μg/g,苯甲酸为1377.51μg/g,对羟基苯甲酸的吸附能力强于苯甲酸。(3)Langmuir动力学方程和一级动力学方程是描述两种酚酸在土壤中吸附过程的最优模型。(4)酚酸滞留量检测结果表明,酚酸添加15d后,对羟基苯甲酸在土壤中的残留率为4.3%～5.2%,而苯甲酸的残留率仅为1.3%～1.7%。两种酚酸在土壤中的滞留动态可以用准二级动力学方程进行描述,对羟基苯甲酸的消除半衰期平均为4.24d,苯甲酸消除半衰期平均为2.61d,对羟基苯甲酸在土壤中的滞留期长于苯甲酸。(5)两种酚酸的吸附与滞留行为与土壤有机质、金属离子、土壤质地等性质关系密切,微生物数量也是决定酚酸在土壤中滞留能力的重要因素。     

Microbial populations and phenolic acids in soil

Populations of bacteria, fungi and actinomycetes in Portsmouth A₁- and B,-soil material were affected in different ways by repeated enrichment with ferulic, p-coumaric, p-hydroxybenzoic or vanillic acids. Responses varied with type of soil material and phenolic acid, phenolic acid concentration, and inorganic nutrient status of the soil. Populations changed more frequently in B₁- than in A₁-soil material. Phenolic acids were readily metabolized by microorganisms, sometimes without detectable population changes, when adequate mineral nutrients were present. Induction of enzymes or selection of organisms capable of degrading individual phenolic acids were clearly evident. Results imply that microbial activity in bioassay systems should be defined for allelopathic studies, particularly when results from various bioassay systems are to be compared     

Release and activity of allelochemicals from allelopathic rice seedlings

3-Isopropyl-5-acetoxycyclohexene-2-one-1 (1), momilactone B (2), and 5,7,4'-trihydroxy-3',5'-dimethoxyflavone (3) were isolated and identified from an allelopathic rice accession PI312777. These three compounds at low concentrations could inhibit the growth of weeds Echinochloa crusgalli and Cyperus difformis associated with rice, especially mixtures of the compounds had stronger inhibitory activity than did individual compounds. Studies with hydroponic culture, continuous root exudates trapping system (CRETS), and direct resin adsorption methods showed that a total of 7.6 n moles 1, 2, and 3 were exuded from living roots of each seedling into the environment at 10 days after seedlings were transplanted. Furthermore, 1, 2, and 3 were found in the soil growing PI312777 seedlings at day 15 after seedlings emergence and reached a total of 39.5 microg/g soil at day 30. The results indicated that PI 312777 seedlings could release sufficient quantities of 1, 2, and 3 into the environment to act as allelochemicals inhibiting the growth of associated weeds. Investigations on the distribution of 1, 2, and 3 in PI 312777 plant, and its root exudates showed that the levels of 1, 2, and 3 were significantly higher in the shoots and root exudates than in the roots, and only trace 1 was observed in the roots. The results suggest that the roots of rice seedlings are not major site of synthesis or accumulation 1, 2, and 3, but a pathway for their release into the environment. The levels of 1, 2, and 3 in the root exudates were over 2-folds higher under direct resin adsorption than under hydroponic culture and CRETS, and hence, it is the preferred method to collect and identify active allelochemicals in rice exudates in future studies on rice allelopathy.     

Root-mediated allelopathic interference of bhringraj (Eclipta alba L.) Hassk. on peanut (Arachis hypogaea) and mung bean (Vigna radiata)

The root mediated allelopathic interference of Eclipta alba infested soil on growth, physiological parameters and antioxidant enzyme activity was conducted on Arachis hypogaea L. and vigna radiata L. It was found that rhizosphere soil significantly reduced the germination percentage, seedling growth and dry biomass depending upon the species sensitivity. The germination inhibition was correlated with membrane deterioration as proved by a strong electrolyte leakage, increase in malondialdehyde (MDA) and H₂O₂ content. The physiological parameters like chlorophyll content, photosynthetic rate (P_n), intercellular CO₂ concentration (C_i), stomatal conductance (G_s), and transpiration (E) also showed significant reduction in E. alba infested soil and non-significant increase in leaf temperature (L_t) of two test species. The test seedlings have circumvented the allelochemicals stress, by both significant decrease and non-significant increase in the antioxidant activities in E. alba infested soil in contrast to control soil. Rhizosphere soil contained significantly higher amount of water-soluble phenolics as the putative allelochemicals, which were vanillic acid, benzoic acid, ferulic acid, and p-coumaric acid. The study concluded that rhizosphere soil exerts an allelopathic influence on peanut and mung bean by releasing water soluble phenolic acids as putative allelochemicals in soil.     

Exudation of allelopathic substances in buckwheat (Fagopyrum esculentum Moench)

Root exudates of the common buckwheat, especially phenolic compounds, were studied. Their contents, both in the soil during the growing season and in agar medium during germination, were determined by HPLC and GC-MS. The allelopathic activity of the soil from a buckwheat stand was evaluated, as well. Palmitic acid, squalene, epicatechin, vitexin, a gallic acid derivative, and a quercetin derivative were the main compounds of the agar medium. In the soil, palmitic acid methyl ester, vanillic acid, rutin, a gallic acid derivative, and a 4-hydroxyacetophenone derivative were identified. The effects of vitexin, squalene, epicatechin, 4-hydroxyacetophenone, and vanillic and gallic acids were tested on eight plant species. Inhibitive effects were observed in the cases of 4-hydroxyacetophenone and vanillic and gallic acids. Comparisons of the identified compounds and inhibitive effects of soil extracts indicated that palmitic acid and the gallic acid derivative probably have an important function in the allelopathic root response of buckwheat.     

The phenolic acid content of cashew leaves (Anacardium occidentale L.) and of the associated humus layer,Senegal

Phenolic acid content was studied in NaOH- and water-extracts from cashew leaves and from the associated humus profile. Free and bound forms of water-soluble phenolic acids were evaluated separately. Phenolic acid composition in the humus layers is to a high degree determined by the phenolic substances inherited from the leaves. Alkali- and water-extracts were dominated by gallic acid, because of the high contents of hydrolisable gallotannin in cashew leaves. Gallic, protocatechuic, p-hydroxybenzoic, cinnamic, p-coumaric and ferulic acids were derived from the leaves, whereas vanillic acid only occurred in the soil. Considerable proportions of the total phenolic acids are water-soluble and lost by leaching from the humus layers.     

Antioxidant properties of bran extracts from Trego wheat grown at different locations

The effects of growing conditions during the grain-filling period, including high temperature stress, total solar radiation, and average daily solar radiation, on the antioxidant properties of Trego wheat were evaluated. Bran extracts were prepared from Trego wheat, grown at four nonirrigated and one irrigated location in Colorado, and compared for their radical scavenging activities against ABTS*+ and DPPH*, Fe(2+) chelating capacities, and total phenolic contents. Significant differences in radical scavenging activities, chelating capacities, and total phenolic contents were detected among Trego bran samples grown at different locations, suggesting that growing conditions may influence the antioxidant properties of wheat. The bran sample obtained from Fort Collins had the strongest scavenging activity against either ABTS*+ or DPPH* radicals and the greatest chelating activity, whereas the highest total phenolic content was detected in bran samples from Walsh, indicating that each antioxidant activity may respond to the environmental changes differently. Positive correlations were detected between the DPPH* scavenging activity and either total solar radiation (r = 0.97, p = 0.03) or average daily solar radiation (r = 0.97, p = 0.03). In addition, HPLC analysis detected the presence of ferulic, syringic, vanillic, p-hydroxybenzoic, and coumaric acids in wheat bran. Additional research is needed to further investigate the effects of environmental conditions and the interactions between genotype and environmental factors on the antioxidant properties of wheat to promote the production of wheat with improved antioxidant properties by optimizing the growing conditions for a selected genotype.     

Effects of phenolic acids on human phenolsulfotransferases in relation to their antioxidant activity

Sulfate conjugation by phenolsulfotransferase (PST) enzyme is an important process in the detoxification of xenobiotics and endogenous compounds. There are two forms of PST that are specific for the sulfation of small phenols (PST-P) and monoamines (PST-M). Phenoilc acids have been reported to have important biological and pharmacological properties and may have benefits to human health. In the present study, human platelets were used as a model to investigate the influence of 13 phenolic acids on human PST activity and to evaluate the relationship to their antioxidant activity. The results showed that chlorogenic acid, syringic acid, protocatechuic acid, vanillic acid, sinapic acid, and caffeic acid significantly (p < 0.05) inhibited the activities of both forms of PST by 21-30% at a concentration of 6.7 microM. The activity of PST-P was enhanced (p < 0.05) by p-hydroxybenzoic acid, gallic acid, gentisic acid, o-coumaric acid, p-coumaric acid, and m-coumaric acid at a concentration of 6.7 microM, whereas the activity of PST-M was enhanced by gentisic acid, gallic acid, p-hydroxybenzoic acid, and ferulic acid. The phenolic acids exhibited antioxidant activity as determined by the oxygen radical absorbance capacity (ORAC) assay and Trolox equivalent antioxidant capacity (TEAC) assay, especially gallic acid, p-hydroxybenzoic acid, gentisic acid, and coumaric acid, which had strong activity. The overall effect of phenolic acids tested on the activity of PST-P and PST-M was well correlated to their antioxidant activity of ORAC value (r = 0.71, p < 0.01; and r = 0.66, p < 0.01). These observations suggest that antioxidant phenolic acids might alter sulfate conjugation.     

Alteration of gymnosperm and angiosperm lignin during decomposition in forest humus layers

In the present investigation an attempt was made to characterize lignin decomposition in three forest humus profiles by the examination of their contents of simple lignin-derived phenolic compounds obtained from alkaline CuO oxidation. The total amounts of p-hydroxybenzoic, syringic, vanillic, p-coumaric and ferulic acids, p-hydroxybenzaldehyde, syringaldehyde and vanillin were determined. The acid to aldehyde weight ratios of the syringyl and vanillyl units, the weight ratio p-coumaric/ferulic acids and the weight ratios of syringyl and cinnamyl phenols, respectively, to vanillyl phenols were used for the characterization of the gymnosperm and angiosperm lignin in different stages of the decomposition process. The patterns of these parameters with depht point to a considerable decomposition and chemical alteration of the lignin molecule during biodegradation.     

低氮胁迫下小麦抑草作用的化感效应与资源竞争分析

化感效应和资源竞争在作物抑草中起重要作用。为区分低氮胁迫下小麦抑草作用的化感效应与资源竞争,以强化感小麦品种‘115/青海麦’、‘92L89’和弱化感小麦‘抗10103’为材料,设置氮水平为25 mg·L-1、50mg·L-1、100 mg·L-1和200 mg·L-1的水培试验,通过小麦与看麦娘共培、应用含小麦根系分泌物的小麦-看麦娘共培液处理看麦娘,分别测定了小麦对看麦娘的生物干扰作用、化感效应及资源竞争效应。结果表明,不同化感潜力的小麦对看麦娘的抑制能力存在显著差异,抑制率大小为‘115/青海麦’(55.1%~73.9%)‘92L89’(48.9%~65.9%)‘抗10103’(15.4%~45.5%),且抑制率随氮水平的降低而增大。在小麦的抑草作用中,化感效应与资源竞争并存。低氮胁迫条件下,强化感小麦抑草能力增强主要通过提高化感效应实现;弱化感小麦抑草能力增强主要通过提高资源竞争实现。不同化感潜力小麦的抑草作用在低氮胁迫下表现出不同的生态策略。小麦抑草作用的化感效应均随氮水平降低而增大,其抑制率随氮水平的变化可用一元二次方程加以拟合。     

Activity and allelopathy of soil of flavone o-glycosides from rice

Two flavone O-glycosides were isolated from allelopathic rice seedlings and have been identified as 5,4'-dihydroxy-3',5'-dimethoxy-7-O-beta-glucopyranosylflavone and 7,4'-dihydroxy-3',5'-dimethoxy-5-O-beta-glucopyranosylflavone. Considerable levels of these glycosides could be found in allelopathic rice tissues. They could not be detected in the soils growing these allelopathic rice seedlings. Only their aglycone, 5,7,4'-trihydroxy-3',5'-dimethoxyflavone, could be found in the soil. Further experiments showed that two flavone O-glycosides were exuded from allelopathic rice roots to the rihzosphere and then transformed into their aglycone form, that is, 5,7,4'-trihydroxy-3',5'-dimethoxyflavone, with a great diversity of biological activities on associated weeds and microbes by soil interactions once released. The glycosides degraded rapidly (t1/2 < 2 h), whereas their aglycone was more resistant toward degradation in paddy soils, in which the half-life (t1/2) at low (25 mug/g) and high (200 mug/g) doses reached 19.86 +/- 3.64 h (r 2 = 0.97) and 28.78 +/- 3.72 h (r 2 = 0.98), respectively. Furthermore, the mobility of both glycosides and their aglycone in paddy soil was evaluated by soil TLC with bioassay. The mobility of the glycosides (Rf = 0.418 +/- 0.069, n = 18) is higher than that of the aglycone (Rf = 0.361 +/- 0.048, n = 18). The results suggested that two flavone O-glycosides are formed in rice biosynthesis and that storage of the allelochemicals and their aglycone 5,7,4'-trihydroxy-3',5'-dimethoxyflavone is the agent of alleloapthic rice which interferes with weeds or microbes in paddy soil.