Nature of interference potential of leaf debris of Ageratum conyzoides

The present study investigated the allelopathic interference of leaf debris of Ageratum conyzoides (billy goat weed; Asteraceae)—a weed of cultivated land—against rice (Oryza sativa). Seedling length and dry weight of rice were significantly reduced (16–20%) in soil from A. conyzoides infested fields compared to the soil from an area devoid of the weed. It indicated the presence of certain phytotoxins in the A. conyzoides infested soil. To explore the possible contribution of the weed in releasing these phytotoxins, growth studies involving leaf debris extracts and amended soils (prepared by incorporating leaf debris—5, 10, 20 g kg⁻¹ soil, w/w, or its extracts—0.5%, 1.0% and 2.0%, v/v) were conducted. The growth of rice was severely inhibited in A. conyzoides leaf debris- and debris extract-amended soils compared to unamended control soil. A significant amount of water-soluble phenolics, the potent phytotoxins, was found in the A. conyzoides infested soil, leaf debris, and debris-amended soils. These phenolics were identified as gallic acid, coumalic acid, protocatechuic acid, catechin and p-hydroxybenzoic acid. Among these, protocatechuic acid was in the maximum amount (35.72%) followed by coumalic acid (33.49%) and these two accounted for >69% of total phenolic compounds. Further, there was a significant increase in the available nutrient content in soil amended with A. conyzoides leaf debris thus ruling out the possibility of any resource depletion upon residue incorporation and their negative role in causing growth reduction. Based on the observations, the present study concludes that leaf debris of A. conyzoides deleteriously affects the early growth of rice by releasing water-soluble phenolic acids into the soil environment and not through soil nutrient depletion.     

Origins of root-mediated pH changes in the rhizosphere and their responses to environmental constraints: A review

The aim of the present review is to define the various origins of root-mediated changes of pH in the rhizosphere, i.e., the volume of soil around roots that is influenced by root activities. Root-mediated pH changes are of major relevance in an ecological perspective as soil pH is a critical parameter that influences the bioavailability of many nutrients and toxic elements and the physiology of the roots and rhizosphere microorganisms. A major process that contributes root-induced pH changes in the rhizosphere is the release of charges carried by H⁺ or OH^– to compensate for an unbalanced cation–anion uptake at the soil–root interface. In addition to the ions taken up by the plant, all the ions crossing the plasma membrane of root cells (e.g., organic anions exuded by plant roots) should be taken into account, since they all need to be balanced by an exchange of charges, i.e., by a release of either H⁺ or OH^–. Although poorly documented, root exudation and respiration can contribute some proportion of rhizosphere pH decrease as a result of a build-up of the CO₂ concentration. This will form carbonic acid in the rhizosphere that may dissociate in neutral to alkaline soils, and result in some pH decrease. Ultimately, plant roots and associated microorganisms can also alter rhizosphere pH via redox-coupled reactions. These various processes involved in root-mediated pH changes in the rhizosphere also depend on environmental constraints, especially nutritional constraints to which plants can respond. This is briefly addressed, with a special emphasis on the response of plant roots to deficiencies of P and Fe and to Al toxicity. Finally, soil pH itself and pH buffering capacity also have a dramatic influence on root-mediated pH changes.     

Allelochemicals and their transformations in the Ageratum conyzoides intercropped citrus orchard soils

Intercropping Ageratum conyzoides in citrus orchards may effectively suppress weeds and control other pests. Investigations showed that the inhibition of major weeds and soil pathogenic fungi in citrus orchards was significantly correlated with the allelochemicals released into the soil by intercropped A. conyzoides. Three flavones, ageratochromene, and its two dimers were isolated and identified from the A. conyzoides intercropped citrus orchard soil. These allelochemicals had different biological actions on major weeds and soil pathogenic fungi in the citrus orchard. Three flavones and ageratochromene could significantly inhibit the growth of weeds Bidens pilosa, Digitaria sanguinalis and Cyperus difformis, and spores germination of soil pathogenic fungi Phytophthora citrophthora, Pythium aphanidermatum and Fusarium solani. However, two dimers of ageratochromene had no inhibitory actions on them. The presence of these allelochemicals in soils suggests that they may be able to make a major contribution to control some weeds and diseases in citrus orchards. Further studies revealed that dynamic transformation between ageratochromene and its two dimers in the A. conyzoides intercropped citrus orchard soil was reversible, that is, ageratochromene released from ground A. conyzoides plants was transformed into its dimers, and the dimers can be remonomerized in the soils. However, this dynamic transformation did not occur in the soil with low organic matter and fertility. The dimerization was not correlated with microorganisms in the soil, but the biodegradation of both ageratochromene and its two dimers may have occurred, particularly in the soil with low organic matter and fertility. Our results strongly suggest that the reversible transformation between ageratochromene and its dimers in the A. conyzoides intercropped citrus orchard soil can be an important mechanism maintaining bioactive allelochemicals at an effective concentration, thus, sustaining the inhibition of weeds and pathogenic fungi in soil.     

紫茎泽兰根区土壤酚酸类物质组成及其对土传病菌的影响

紫茎泽兰根系释放的化感物质是导致其土壤环境发生变化的重要因素,可为其成功入侵提供有利条件.以空白土样为对照,提取健康和感染棉蚜的紫茎泽兰根区土壤酚酸类化感物质,采用GC-MS分析其组分和相对含量.结果表明：不同处理根区土壤酚酸类化感物质的种类没有变化,但相对含量发生了改变.其中,虫害处理土样中苯甲酸和对羟基苯甲酸的相对含量均显著高于对照,但对羟基肉桂酸的相对含量显著低于对照,且苯甲酸、对羟基苯甲酸和对羟基肉桂酸的相对含量在3种处理土样中的比例存在差异.用上述3种物质单体及它们在3个处理土样的比例复配进行抑菌试验发现：在较低浓度（50～150 mg·L^-1）下,3种化感物质对5种土传性病菌有显著的抑制作用,但复配后对5种病菌的抑制作用各不相同,虫害处理的复配对其中3种病原菌的抑制作用均低于对照和健康植株.     

Seasonal dynamics of allelopathically significant phenolic compounds in globally successful invader Conyza canadensis L. plants and associated sandy soil

The seasonal dynamics of total phenolics and phenolic acids in the stems of the global invader Conyza canadensis, from March (young plants in the form of rosettes) to September (fruit abscission and the beginning of plant decline), and in sandy soil were monitored monthly in non-native areas. The highest amount of total free phenolics was found in its tissues (31,000 μg g⁻¹) during the flowering and fruiting time (August). Bound phenolics peaked (up to 8443 μg g⁻¹) during shoot elongation and intensive plant growth (May–June) and in September. In the stems, bound phenolic acids (p-coumaric, ferulic, p-hydroxybenzoic, vanillic and syringic) have a maximum twice, in May and in August, with ferulic acid predominating (up to 951.6 μg g⁻¹). Free phenolic acids in the plant's tissue peaked in May (plant elongation). In the soil under C. canadensis, the amount of bound phenolics decreased between March and June, before increasing up to the full bloom phase of the plants (August). The amount of bound phenolic acids was several times greater than that of free ones, with maximum values in August. C. canadensis is a highly important source of phenolics in the ruderal phytocoenosis in new areas. In order to better explain the mechanisms of the spread and domination of invasive plants in non-native areas, in which allelopathy plays a decisive role, it is necessary to measure the production of allelochemicals in tissue and their accumulation in soil at the shortest possible intervals and link this with the phases of plant development.     

三种根系分泌脂肪酸对花生生长和土壤酶活性的影响

为了探讨花生连作后土壤中脂肪酸类物质的累积与花生连作障碍间的关系,为花生连作障碍机理的研究提供新的理论依据,以田间土壤为介质,采用盆栽试验的方法研究了花生根系分泌物中3种长链脂肪酸,即:豆蔻酸、软脂酸和硬脂酸的混合物,对花生植株生长、产量和土壤酶活性的影响。结果表明,当土壤中脂肪酸的初始含量较低时(80 mg/kg土),对花生植株的生长和产量有微弱的促进作用(P>0.05),当土壤中脂肪酸的初始含量较高时(160 mg/kg土和240 mg/kg土),显著抑制了花生植株的生长和产量(P<0.05)。叶片叶绿素含量、根系活力、土壤酶(蔗糖酶、脲酶、磷酸酶)活性在低脂肪酸含量处理下升高,在高脂肪酸含量处理下显著降低(P<0.001)。光合产物、根际有效养分的减少和根系养分吸收能力的降低,可能是导致花生植株生长和产量降低的原因之一。花生连作土壤中豆蔻酸、软脂酸和硬脂酸的累积与花生的连作障碍有着密切关系。     

Allelochemicals released by rice roots and residues in soil

A few rice (Oryza sativa L.) varieties or rice straw produce and release allelochemicals into soil in which interfere with the growth of neighboring or successive plants. Allelopathic rice PI312777 and Huagan-1 at their early growth stages released momilactone B, 3-isopropyl-5-acetoxycyclohexene-2-one-1, and 5,7,4′-trihydroxy-3′,5′-dimethoxyflavone into soil at phytotoxic levels, but non-allelopathic rice Huajingxian did not. Both allelopathic and non-allelopathic rice residues released momilactone B and lignin-related phenolic acids (p-hydroxybenzoic, p-coumaric, ferulic, syringic and vanillic acids) into the soil during residue decomposition to inhibit successive plants. The results indicated that allelochemicals involved in rice allelopathy from living and dead plants are substantially different. Interestingly, the concentrations of the allelochemicals released from the allelopathic rice seedlings in soil increased dramatically when they were surrounded with Echinochloa crus-galli. The concentrations of the allelochemicals were over 3-fold higher in the presence of E. crus-galli than in the absence of E. crus-galli. However, the same case did not occur in non-allelopathic Huajingxian seedlings surrounded with E. crus-galli. In addition to allelochemical exudation being promoted by the presence of E. crus-galli, allelopathic rice seedlings also increased allelochemical exudation in response to exudates of germinated E. crus-galli seeds or lepidimoide, an uronic acid derivative exuded from E. crus-galli seeds. These results imply that allelopathic rice seedlings can sense certain allelochemicals released by E. crus-galli into the soil, and respond by increased production of allelochemicals inhibitory to E. crus-galli. This study suggests that rice residues of both allelopathic and non-allelopathic varieties release similar concentrations and types of allelochemicals to inhibit successive plants. In contrast, living rice plants of certain allelopathic varieties appear to be able to detect the presence of interspecific neighbors and respond by increased allelochemicals.     

Identification and Phytotoxicity Assessment of Phenolic Compounds in Chrysanthemoides monilifera subsp. monilifera (Boneseed)

Chrysanthemoides monilifera subsp. monilifera (boneseed), a weed of national significance in Australia, threatens indigenous species and crop production through allelopathy. We aimed to identify phenolic compounds produced by boneseed and to assess their phytotoxicity on native species. Phenolic compounds in water and methanol extracts, and in decomposed litter-mediated soil leachate were identified using HPLC, and phytotoxicity of identified phenolics was assessed (repeatedly) through a standard germination bioassay on native Isotoma axillaris. The impact of boneseed litter on native Xerochrysum bracteatum was evaluated using field soil in a greenhouse. Collectively, we found the highest quantity of phenolic compounds in boneseed litter followed by leaf, root and stem. Quantity varied with extraction media. The rank of phenolics concentration in boneseed was in the order of ferulic acid > phloridzin > catechin > p-coumaric acid and they inhibited germination of I. axillaris with the rank of ferulic acid > catechin > phloridzin > p-coumaric acid. Synergistic effects were more severe compared to individual phenolics. The litter-mediated soil leachate (collected after15 days) exhibited strong phytotoxicity to I. axillaris despite the level of phenolic compounds in the decomposed leachate being decreased significantly compared with their initial level. This suggests the presence of other unidentified allelochemicals that individually or synergistically contributed to the phytotoxicity. Further, the dose response phytotoxic impacts exhibited by the boneseed litter-mediated soil to native X. bracteatum in a more naturalistic greenhouse experiment might ensure the potential allelopathy of other chemical compounds in the boneseed invasion. The reduction of leaf relative water content and chlorophyll level in X. bracteatum suggest possible mechanisms underpinning plant growth inhibition caused by boneseed litter allelopathy. The presence of a substantial quantity of free proline in the target species also suggests that the plant was in a stressed condition due to litter allelopathy. These findings are important for better understanding the invasive potential of boneseed and in devising control strategies.     

水稻化感作用及其分子生态学研究进展

综述了近年来国际上研究水稻化感作用的新进展,比较分析了当前常用于室内评价水稻化感作用潜力的几种生物测试法的优缺点,指出了琼脂迟播共培法是较为理想的室内生物测试法并已广泛应用于化感作用研究中。在此基础上,分析了水稻化感作用的数量遗传特性及其QTL定位的研究现状;阐明了水稻化感作用的遗传多样性及其分子生态特性;并就当前普遍关注的焦点问题：逆境条件（如低氮或高伴生杂草密度胁迫）常引起水稻化感作用潜力增大的生理过程与分子机制作了阐述。结合近年来应用差异蛋白组学和生物信息学的研究实例,阐明了逆境引起水稻化感作用增强与其酚类合成代谢相关酶蛋白表达丰度增加,萜类合成代谢相关酶蛋白表达丰度下降有关。就究竟什么是水稻的化感物质及其作用方式等问题作了分析与讨论．指出水稻的化感抑草效应是其众多化感物质综合作用的结果,应重视区分化感物质对靶标杂草的原生作用和进入土壤生态系统中经生物转化后的次生作用。根据当前植物化感作用研究的发展趋势,阐明了进一步研究水稻化感作用的焦点问题,提出了水稻化感作用的根际生物学特性与分子生态学机制,是未来国际上竞相角逐的重点研究领域,并认为以现代系统生物学理论为指导,运用基因组学、蛋白质组学和代谢组学等技术方法,是揭示这一分子生态学过程与机制的重要技术选择和优先研究领域。     

Convergent or parallel molecular evolution of momilactone A and B: potent allelochemicals, momilactones have been found only in rice and the moss Hypnum plumaeforme

Plant second metabolites momilactone A and B, which act as potent phytoalexins and allelochemicals, have been found thus far only in rice and the moss Hypnum plumaeforme, although both plants are taxonomically quite distinct. The concentrations of momilactone A and B, respectively, in rice plants were 4.5-140 and 2.9-85 μg/g, and those in H. plumaeforme were 8.4-58.7 and 4.2-23.4 μg/g. Momilactone A and B concentrations in rice and H. plumaeforme plants were increased by UV irradiation, elicitor and jasmonic acid treatments. Rice and H. plumaeforme plants secrete momilactone A and B into the rhizosphere, and the secretion level was also increased by UV irradiation, elicitor and jasmonic acid treatments. In addition, although endogenous concentrations of momilactone A in rice and H. plumaeforme were greater than those of momilactone B, the secretion levels of momilactone B were greater than those of momilactone A in rice and H. plumaeforme, which suggests that momilactone B may be selectively secreted by both rice and H. plumaeforme. As momilactone A and B exert potent antifungal and growth inhibitory activities, momilactone A and B may play an important role in the defense responses in H. plumaeforme and rice against pathogen infections and in allelopathy. The secretion of momilactone A and B into the rhizosphere may also prevent bacterial and fungal infections and provide a competitive advantage for nutrients through the inhibition of invading root systems of neighboring plants as allelochemicals. Therefore, both plants, despite their evolutionary distance, may use same defense strategy with respect to the momilactone A and B production and secretion, which resulting from convergent or parallel evolutionary processes. In the case of parallel evolution, there may be plant species providing the missing link in molecular evolution of momilactones between H. plumaeforme and rice.     

Interference of allelopathic rice with paddy weeds at the root level

• Despite increasing knowledge of the involvement of allelopathy in negative interactions among plants, relatively little is known about its action at the root level. This study aims to enhance understanding of interactions of roots between a crop and associated weeds via allelopathy.
• Based on a series of experiments with window rhizoboxes and root segregation methods, we examined root placement patterns and root interactions between allelopathic rice and major paddy weeds Cyperus difformis, Echinochloa crus‐galli, Eclipta prostrata, Leptochloa chinensis and Oryza sativa (weedy rice).
• Allelopathic rice inhibited growth of paddy weed roots more than shoots regardless of species. Furthermore, allelopathic rice significantly reduced total root length, total root area, maximum root width and maximum root depth of paddy weeds, while the weeds adjusted horizontal and vertical placement of their roots in response to the presence of allelopathic rice. With the exception of O. sativa (weedy rice), root growth of weeds avoided expanding towards allelopathic rice. Compared with root contact, root segregation significantly increased inhibition of E. crus‐galli, E. prostrata and L. chinensis through an increase in rice allelochemicals. In particular, their root exudates induced production of rice allelochemicals. However, similar results were not observed in C. difformis and O. sativa (weedy rice) with either root segregation or root exudate application.
• The results demonstrate that allelopathic rice interferes with paddy weeds by altering root placement patterns and root interactions. This is the first case of a root behavioural strategy in crop–weed allelopathic interaction.

     

Effects of adjuvants and soil microbes on the phytotoxic activity of coumarin in combination with p-vanillin on goosegrass (Eleusine indica L.) seedling emergence and growth

Repeated use of herbicides has led to the occurrence of Eleusine indica biotypes evolving resistance to various groups of herbicides. The use of allelochemicals as an alternative method of weed control has been receiving a lot of attention recently. Coumarin and p-vanillin, two phenolic compounds belonging to this large class of allelochemicals, have proven to inhibit germination of E. indica greatly in a previous study. However, the efficacy of these two phenolic compounds is dependent on its absorption and is influenced by adjuvants. Thus, this study was conducted to determine the effectiveness of selected non-ionic surfactants (NIS) and an organosilicon (OS) in enhancing the phytotoxic activity of coumarin in combination with p-vanillin on emergence and growth E. indica seedlings under controlled laboratory conditions. Autoclaved and non-autoclaved soil samples were additionally used in the study to determine soil microbe effect on the activity of the phenolic mixture. Results have shown that the NIS, Tween-20 at 0.2% and Speed-Thru at 0.08% were effective in enhancing the phytotoxicity of the phenolic mixture. Two of the adjuvant concentrations used resulted in a reduced dosage of the phenolic mixture needed to reach 90% reduction in emergence (EM₉₀), fresh weight (FW₉₀), shoot length (SL₉₀) and root length (RL₉₀) by 2 to 3 fold. The NIS, Dupont at low dosage of 0.125% was effective in reducing the EM₉₀ and RL₉₀ values by 2.5 to 3.0 fold whereas the OS, Speedex at a recommended dosage of 0.1%, was found to be able to decrease the EM₉₀, SL₉₀ and RL₉₀ values by 2 to 3 fold. On the other hand, the phytotoxic effects of the phenolic mixture on E. indica grown in non-autoclaved soil were less than those observed in autoclaved soil, demonstrating the role of microbes in reducing the bioavailability of the phenolic mixture in the soil.     

Allelopathic potential of the phenolics from the roots of Pluchea lanceolata

Aqueous leachates of roots of the perennial weed Pluchea lanceolata (DC.) C. B. Clarke, its root-incorporated soil and rhizosphere soil, interfered with the seedling growth of certain plant species. The soils from the rhizosphere zone of this plant had significantly higher total phenolics and HPLC analysis revealed that phenolic fractions represented by retention times of 1.6, 1.9, 2.5 (simple phenol, chlorogenic acid and phloroglucinol respectively), 3.7 and 4.3 min were contributed by roots of the weed to the soil. The phenolic fraction represented by the retention time 3.3 (formononetin 7-O-glucoside) was detected in the weed's rhizosphere soils and not in the rootincorporated soils. UV spectral studies established the presence of phloroglucinol, simple phenol, chlorogenic acid, formononetin 7-O-glucoside, and methylated coumarins in the root leachate, which affect the seedling growth of mustard ( Brassica juncea ). Present research established the allelopathic potential of P. lanceolata roots, and the possible involvement of allelopathy in its interference success.     

An allelopathic investigation of the domination of the introduced invasive Conyza canadensis L.

The introduced, invasive species Conyza canadensis L. covers large areas of the sandy levees next to the River Tamiš (Serbia), forming dense microcomplexes and dominating the other herbaceous species in the ruderal phytocoenosis with its aboveground mass and abundance. In addition to this species, a further 28 plant species were found, but the abundance and cover of these was significantly lower. The allelopathic influence of the species C. canadensis was investigated through analyzing the total phenolics and phenolic acids, as the main allelochemicals, in dead and vegetative parts and the soil beneath them. Seed germination and seedling growth of the target plants (Dactylis glomerata L. and Trifolium repens L.), which grow in this community, served as a measure of the inhibitory capacity of this species. It was established that the content of total phenolics and phenolic acids (p-coumaric, ferulic, p-hydroxybenzoic, vanillic and syringic) varies, following the order: vegetative plant parts > dead plant parts > sandy soil under C. canadensis. Water leachate and soils inhibited seed germination and seedling growth of the test plants to varying degrees, following the order: vegetative parts > dead parts > sandy soil, which is directly related to the content of total phenolics and phenolic acids in them. It was concluded that the pioneer species C. canadensis plays a decisive role in the first phases of vegetation succession and the process of soil formation on the barren sandy levees, owing to the synthesis of secondary phenolic metabolites.     

Allelopathy and its coevolutionary implications between native and non‐native neighbors of invasive Cynara cardunculus L.

Invasive plants apply new selection pressures on neighbor plant species by different means including allelopathy. Recent evidence shows allelopathy functions as remarkably influential mediator for invaders to be successful in their invaded range. However, few studies have determined whether native and non‐native species co‐occurring with invaders have evolved tolerance to allelopathy. In this study, we conducted germination and growth experiments to evaluate whether co‐occurring native Juncus pallidus and non‐native Lolium rigidum species may evolve tolerance to the allelochemicals induced by Cyanara cardunculus in Australian agricultural fields. The test species were germinated and grown in pots filled with collected invaded and uninvaded rhizosphere soil of C. cardunculus with and without activated carbon (AC). Additionally, a separate experiment was done to differentiate the direct effects of AC on the test species. The soil properties showed invaded rhizosphere soils had higher total phenolic and lower pH compared with uninvaded soils. We found significant reduction of germination percentage and seedling growth in terms of above‐ and belowground biomass, and maximum plant height and root length of native in the invaded rhizosphere soil of C. cardunculus, but little effect on non‐native grass species. Even soil manipulated with AC showed no significant differences in the measured parameters of non‐native except aboveground biomass. Taken together, the results indicate allelochemicals induced by C. cardunculus exert more suppressive effects on native than non‐native linking the coevolved tolerance of those.     

Allelopathic effects of root exudates from watermelon and rice plants on Fusarium oxysporum f.sp. niveum

Root exudates have a key role in communication between plants and microbes in the rhizosphere. Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (Fusarium oxysporum), drastically reduces watermelon yields in continuous cultivation systems, but it can be significantly alleviated using watermelon/aerobic rice intercropping system as shown by the research carried out in this laboratory. It is important to evaluate the interaction between root exudates from the two crops and the pathogen and thus to clarify the mechanism of disease suppressiveness in the intercropping system. The effects of phenolic acids, sugars and free amino acids in root exudates from watermelon (REW) and rice (RER) on the growth of Fusarium oxysporum were studied. The results obtained are listed as follows: (1) REW significantly increased spore germination and sporulation, whereas RER had inhibitory effects on those two parameters. (2) HPLC analysis showed that salicylic acid, p-hydroxybenzoic acid and phthalic acid were identified in exudates from both plants, but p-coumaric acid was only detected in rice and ferulic acid only in watermelon. Moreover, of the total rice exudates a high proportion (37.9 %) of p-coumaric acid was detected and the total amount of phenolic acids was 1.4-fold as high as that in watermelon. (3) Considerable differences in the components and contents of both sugars and amino acids were found between REW and RER exudates. (4) Exogenously applied alanine (Ala) increased spore germination and sporulation. In contrast, addition of exogenous p-coumaric acid reduced spore germination and sporulation, relative to controls. It was concluded that the rice root exudates had anti-fungal properties while that from watermelon promoted pathogen growth. This discovery provided a scientific basis for practicing watermelon/aerobic rice intercropping to control Fusarium wilt in watermelon.     

Root-Derived Short-Chain Suberin Diacids from Rice and Rape Seed in a Paddy Soil under Rice Cultivar Treatments

Suberin-derived substituted fatty acids have been shown to be potential biomarkers for plant-derived carbon (C) in soils across ecosystems. Analyzing root derived suberin compounds bound in soil could help to understand the root input into a soil organic carbon pool. In this study, bound lipids were extracted and identified in root and topsoil samples. Short-chain suberin diacids were quantified under rice (Oryza sativa L.) and rape (Brassica campestris) rotations with different cultivar combinations in a Chinese rice paddy. After removal of free lipids with sequential extraction, the residual bound lipids were obtained with saponification and derivatization before analysis using gas chromatography–mass spectrometry (GC-MS). Diacids C16 and C18 in bound lipids were detected both in rice and rape root samples, while diacids C20 and C22 were detected only in rape root samples. Accordingly, diacids were quantified in both rhizosphere and bulk soil (0–15 cm). The amount of total root-derived diacids in bulk soil varied in a range of 5.6–9.6 mg/kg across growth stages and crop seasons. After one year-round rice-rape rotation, root-derived suberin diacids were maintained at a level of 7–9 mg/kg in bulk soil; this was higher under a super rice cultivar LY than under a hybrid cultivar IIY. While concentrations of the analyzed diacids were generally higher in rhizosphere than in bulk soil, the total diacid (DA) concentration was higher at the time of rape harvest than at rice harvest, suggesting that rape roots made a major contribution to the preservation of diacids in the paddy. Moreover, the net change in the concentration and the ratios of C_16:0 DA to C_18:1 DA, and of C_16:0 DA to C_18:0 DA, over a whole growing season, were greater under LY than under IIY, though there was no difference between cultivars within a single growth stage. Overall, total concentration of root-derived suberin diacids was found to be positively correlated to soil organic carbon concentration both for bulk soil and rhizosphere. However, the turnover and preservation of the root suberin biomolecules with soil property and field conditions deserve further field studies.     

Soil microorganisms interacting with residue-derived allelochemicals effects on seed germination

Despite the knowledge regarding allelopathy, known as a major ecological mechanism for biological weed control, had increased greatly, the role of soil microorganisms in that field remained controversial. The study sought to evaluate the interference potential of soil microorganisms, residues-derived allelochemicals and their interaction on seed germination and understand the variation of microbial community in allelopathic activities. Three different rice residues-derived fractions from variety PI312777 (extracts, straw fraction and fresh residue) were applied to sterile and live soils to disentangle the interference potential of soil microorganisms, residues-derived allelochemicals and their interaction concerned allelopathic activities. The results demonstrated that microbe-only and residues-only exerted onefold promotion and inhibition effects on lettuce (Lactuca sativa Linn.) seed germination, respectively, whereas, microbe-by-residues interaction showed an inhibition at the beginning, and a feeble promotion later. The 20 most dominant genera of microbes were classified into three clusters, with 13 genera in one cluster, only 1 in the second cluster and 6 in the third one. The genera in the first cluster commonly exerted negative effects on phenol content, while showed positive correlation with seed germination. Interestingly, Bacillus, clustered in the second cluster, had an opposite effect alone. The third cluster genera somehow had a weak correlation with both germination as well as the release of the allelochemicals. Overall, we incorporated molecular methodology for tracking bacterial impacts during incubation with allelochemicals, and demonstrated the mutable role of soil microbes in allelopathy. It may be potentially important for stimulating the beneficial roles of microbes for environmentally friendly weed management.     

Different Bacterial Populations Associated with the Roots and Rhizosphere of Rice Incorporate Plant-Derived Carbon

Microorganisms associated with the roots of plants have an important function in plant growth and in soil carbon sequestration. Rice cultivation is the second largest anthropogenic source of atmospheric CH₄, which is a significant greenhouse gas. Up to 60% of fixed carbon formed by photosynthesis in plants is transported below ground, much of it as root exudates that are consumed by microorganisms. A stable isotope probing (SIP) approach was used to identify microorganisms using plant carbon in association with the roots and rhizosphere of rice plants. Rice plants grown in Italian paddy soil were labeled with ¹³CO₂ for 10 days. RNA was extracted from root material and rhizosphere soil and subjected to cesium gradient centrifugation followed by 16S rRNA amplicon pyrosequencing to identify microorganisms enriched with ¹³C. Thirty operational taxonomic units (OTUs) were labeled and mostly corresponded to Proteobacteria (13 OTUs) and Verrucomicrobia (8 OTUs). These OTUs were affiliated with the Alphaproteobacteria, Betaproteobacteria, and Deltaproteobacteria classes of Proteobacteria and the “Spartobacteria” and Opitutae classes of Verrucomicrobia. In general, different bacterial groups were labeled in the root and rhizosphere, reflecting different physicochemical characteristics of these locations. The labeled OTUs in the root compartment corresponded to a greater proportion of the 16S rRNA sequences (∼20%) than did those in the rhizosphere (∼4%), indicating that a proportion of the active microbial community on the roots greater than that in the rhizosphere incorporated plant-derived carbon within the time frame of the experiment.