Allelopathy as a Tool in the Management of Biotic Resources in Agroecosystems

In modern agriculture, natural plant communities may be replaced by a single crop species. Weeds, some microorganisms, and viruses, as well as some herbivores are organisms that should be eliminated. Pesticides and fertilizers not only affect the pests and crops, but soil, non-pest species, water, food, and humans. In traditional agriculture weeds are components with an important ecological role in the maintenance of the system. Some weeds have been used as tools to control the growth of other weeds in traditional agroecosystems. Researchers on sustainable and organic agriculture get valuable information from traditional agriculture and currently are conducting research on plant breeding, soil fertility and tillage, crop protection, and cropping systems. Allelopathy and chemical ecology are directly involved in each of these fields and can play an important role in crop productivity, conservation of genetic diversity, and maintenance of ecosystems stability. Allelopathy has been shown to be related with problems of chemical interference between crops and weeds, crops and crops, toxicity of crops and weeds residues, and/or crops and weeds exudates. Problems of autotoxicity, orchard replanting, and forest regeneration are also referred as allelopathic. Allelopathy is strongly coupled with other stresses of the environment, including insects and disease, temperature extremes, nutrient and moisture variables, radiation, and herbicides. These stress conditions often enhance allelochemical production and increase the potential for allelopathic interference. Allelopathy offers potential for weed control through the production and release of allelochemicals from plants. Allelochemicals may impact the availability of nutrients through effects on the symbiotic microbes. Destruction and changes in the use of soils in the tropics have decreased biodiversity, bringing about the loss of valuable natural products. Many different types of useful products such as natural pesticides and drugs can arise from allelopathy studies. New methods must be generated for allelopathy as a part of the biotic resources management strategies.     

Screening methods for the evaluation of crop allelopathic potential

There is increasing interest in the development of allelopathic crop varieties for weed suppression. Allelopathic varieties are likely to be able to suppress weeds by natural exudation of bioactive allelochemicals, thereby reducing dependence upon synthetic herbicides. Screening bioassays are essential tools in identifying crop accessions with allelopathic potential. A number of crops have been screened for this allelopathic trait, and key issues in selecting and designing screening bioassays are reviewed. It is recommended that a combination of different bioassays be used in the evaluation of crop allelopathic potential. Laboratory bioassays, field testing, and chemical screening are important steps, and none of them can be precluded if conclusive evidence of crop allelopathy is to be established. More concerted efforts are needed in screening crop germplasm before the development of allelopathic varieties occurs.     

小麦化感作用研究进展

小麦是世界第一大粮食作物,在农业生产中占有重要地位．然而,由于人们为保证小麦产量往往施用大量的除草剂和杀菌剂,对环境造成了极大的危害．小麦化感作用是利用小麦活体或残体向环境中释放次生代谢物质对自身或其他生物产生作用,它克服了除草剂和杀菌剂等引起的环境污染问题,具有抑制杂草控制病害的潜力．本文对已有的小麦化感作用的研究进展情况进行了综合评述．其中小麦对杂草、虫害及病害产生防御功能的主要化感物质为异羟肟酸和酚酸类物质．小麦化感物质活性的发挥除了取决于化感物质的种类外,还由小麦自身的遗传因素、环境因素和生物因素的共同作用所决定．小麦化感物质在根际土壤中的滞留、迁移和转化过程、小麦化感作用与土壤生物的关系以及相关的作用机理是小麦化感作用研究的薄弱环节。其研究方法还需进一步探索改进．小麦化感作用在植物保护、环境保护以及作物育种等方面具有广泛的应用前景,促进了小麦抗逆性的增强以及产量和品质的提高．     

Unravelling the beneficial role of microbial contributors in reducing the allelopathic effects of weeds

The field of allelopathy is one of the most fascinating but controversial processes in plant ecology that offers an exciting, interdisciplinary, complex, and challenging study. In spite of the established role of soil microbes in plant health, their role has also been consolidated in studies of allelopathy. Moreover, allelopathy can be better understood by incorporating soil microbial ecology that determines the relevance of allelopathy phenomenon. Therefore, while discussing the role of allelochemicals in plant–plant interactions, the dynamic nature of soil microbes should not be overlooked. The occurrence and toxicity of allelochemicals in soil depend on various factors, but the type of microflora in the surroundings plays a crucial role because it can interfere with its allelopathic nature. Such microbes could be of prime importance for biological control management of weeds reducing the cost and ill effects of chemical herbicides. Among microbes, our main focus is on bacteria—as they are dominant among other microbes and are being used for enhancing crop production for decades—and fungi. Hence, to refer to both bacteria and fungi, we have used the term microbes. This review discusses the beneficial role of microbes in reducing the allelopathic effects of weeds. The review is mainly focused on various functions of bacteria in (1) reducing allelopathic inhibition caused by weeds to reduce crop yield loss, (2) building inherent defense capacity in plants against allelopathic weed, and (3) deciphering beneficial rhizospheric process such as chemotaxis/biofilm, degradation of toxic allelochemicals, and induced gene expression.     

Allelopathy in wheat (Triticum aestivum)

Wheat (Triticum aestivum) allelopathy has potential for the management of weeds, pests and diseases. Both wheat residue allelopathy and wheat seedling allelopathy can be exploited for managing weeds, including resistant biotypes. Wheat varieties differ in allelopathic potential against weeds, indicating that selection of allelopathic varieties might be a useful strategy in integrated weed management. Several categories of allelochemicals for wheat allelopathy have been identified, namely, phenolic acids, hydroxamic acids and short‐chain fatty acids. Wheat allelopathic activity is genetically controlled and a multigenic model has been proposed. Research is underway to identify genetic markers associated with wheat allelopathy. Once allelopathic genes have been located, a breeding programme could be initiated to transfer the genes into modern varieties for weed suppression. The negative impacts of wheat autotoxicity on agricultural production systems have also been identified when wheat straws are retained on the soil surface for conservation farming purposes. A management package to avoid such deleterious effects is discussed. Wheat allelopathy requires further study in order to maximise its allelopathic potential for the control of weeds, pests and diseases, and to minimise its detrimental effects on the growth of wheat and other crops.     

小麦化感作用研究进展

小麦是世界第一大粮食作物,在农业生产中占有重要地位.然而,由于人们为保证小麦产量往往施用大量的除草剂和杀菌剂,对环境造成了极大的危害.小麦化感作用是利用小麦活体或残体向环境中释放次生代谢物质对自身或其他生物产生作用,它克服了除草剂和杀菌剂等引起的环境污染问题,具有抑制杂草控制病害的潜力.本文对已有的小麦化感作用的研究进展情况进行了综合评述.其中小麦对杂草、虫害及病害产生防御功能的主要化感物质为异羟肟酸和酚酸类物质.小麦化感物质活性的发挥除了取决于化感物质的种类外,还由小麦自身的遗传因素、环境因素和生物因素的共同作用所决定.小麦化感物质在根际土壤中的滞留、迁移和转化过程、小麦化感作用与土壤生物的关系以及相关的作用机理是小麦化感作用研究的薄弱环节,其研究方法还需进一步探索改进.小麦化感作用在植物保护、环境保护以及作物育种等方面具有广泛的应用前景,促进了小麦抗逆性的增强以及产量和品质的提高.     

模拟化感作物种植密度对杂草种群数量的作用

具化感作用的作物其种植密度会影响杂草的种群动态。本研究应用数学模型作为分析问题的工具,对作物化感作用下杂草种群动态进行了模型分析和计算机模拟,结果表明,当作物的种植密度较大时,杂草的种群动态数量可以得到有效的控制;而当作物种植密度低于一定数量时,其化感作用对杂草的种群动态数量影响不大。     

Weed Management in Rice: Wheat Rotation by Allelopathy

Rice is major crop in India and its cultivation in northwest India started 25 to 30 years ago in assured irrigation areas during the summer rainy season. In this region, rice-wheat rotation became most popular owing to its high yields; however, these crops are highly infested by the weeds, thus farmers use herbicides for their control. Hence, this rotation consumes a maximum quantity of herbicides in this region, which has resulted in several problems (environmental pollution, human health hazards, development of herbicide resistance in weeds). Thus, serious ecological questions about the reliance on herbicides for weed control in this rotation have been raised. One of the alternatives to overcome these problems is with the use of allelopathic strategies, including the use of weed-smothering crops for weed management and for the sustainability of agriculture. The field, pot culture, and laboratory studies have shown that inclusion of weed-smothering crops in rotation considerably reduced the weed population in the current and succeeding crops. Early summer (April-June) fodder crops of sorghum, pearlmillet and maize drastically smothered the weed population and biomass. The residual suppression effect of peralmillet also persisted in the next crop up to 45 days. Thus, it is conceptualized that the inclusion of such summer fodder crops before the rice crop in the rice-wheat rotation may provide satisfactory weed control in the succeeding rice crop and may minimize the use of herbicides. Likewise, the replacement of wheat by winter fodder crops of oat and berseem (Trifolium alexandrinum) may also help in the control of winter weeds. Hence, further studies in this direction may provide satisfactory weed management in rice-wheat rotation and may minimize the use of herbicides and thereby help indeveloping sustainable agricultural practices.     

杂草间的他感作用及其在杂草生防中的应用

动、植物所产生的有机化合物,除了营养自身之外,还在很多方面影响着其它植物和动物的生长、健康、行为及种群生物学特征。很多人都熟悉由微生物如放线菌、细菌、真菌所产生的抗生素,它们影响着其他微生物的生长     

化感作用的研究意义及发展前景

对化感作用在生态学、林业、农业等领域的理论及实践意义进行综述与探讨,并分析了其今后的发展方向.化感作用有助于更加合理地解释生态系统中植物组成与分布、群落演替、协同进化和生物入侵等现象;在林业生产的森林更新、混交林培育中有时会起到决定性作用;在农业上,单作、轮作、覆盖等各种种植方式均受化感影响,此外,一些化感物质可用作杀虫剂和除草剂,从而减轻对环境的污染水体中同样存在化感作用.培育新的抗虫害、抑草品种,分离、鉴定新的化感物质,深入揭示化感作用的机制,是今后化感作用的研究重点.     

Allelopathy in the natural and agricultural ecosystems and isolation of potent allelochemicals from Velvet bean (Mucuna pruriens) and Hairy vetch (Vicia villosa).

We have studied on allelopathy of plants and developed methods to identify the effective substances in root exudates, leaf leacheate, and volatile chemicals emitted from plants. We found traditional cover plants that show allelopathic activity are useful for weed control. It could eliminate the use of synthetic chemicals for this purpose. Allelopathy is a natural power of plants to protect themselves by producing natural organic chemicals. Some endemic plants in Asia, already known by farmers in the region, as either cover crops used in intercropping, hedgerow, or agroforestry, were found to possess strong allelopathic abilities. Our group identified several allelochemicals from these plants. These allelopathic cover crops, mostly leguminous plants, provide protein rich food, and grow easily without artificial fertilizers, herbicides, insecticides and fungicides. In this regards, these allelopathic cover crops could save food shortage in rural area, and are useful for environmental conservation. Screenings of allelopathic plants by specific bioassays and field tests have been conducted. Hairy vetch (Vicia villosa) and Velvet bean (Mucuna pruriens) are two promising species for the practical application of allelopathy. An amino acid, L-DOPA, unusual in plants, plays an important role as allelochemical in Velvet bean (Mucuna pruriens). Hairy vetch is the most promising cover plant for the weed control in orchard, vegetable and rice production and even for landscape amendment in abandoned field in Japan. We have isolated "cyanamide", a well known nitrogen fertilizer, from Hairy vetch. This is the first finding of naturally produced cyanamide in the world.     

Weed and Crop Allelopathy

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization, and root exudation. Allelochemical structures and modes of action are diverse and may offer potential for the development of future herbicides. We have focused our review on a variety of weed and crop species that establish some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are more highly weed suppressive in managed settings. Both environmental and genotypic effects impact allelochemical production and release over time. A new challenge that exists for future plant scientists is to generate additional information on allelochemical mechanisms of release, selectivity and persistence, mode of action, and genetic regulation. In this manner, we can further protect plant biodiversity and enhance weed management strategies in a variety of ecosystems. Referee: Dr. Steve Weller, Purdue University, Dept. of Horticulture, West Lafayette, IN 47907     

Application of allelochemicals to agriculture.

The study of allelopathy has a long history, and its application to agricultural production has long been anticipated. Recently, researchers have found allelopathic plants that are now used as cover crops, and allelochemicals which may lead to new herbicides. This paper reviews three studies introduced in this symposium, and discusses the possible application of allelopathy to agriculture.     

水稻化感作用研究综述

主要对水稻化感现象发现,化感生物检测,化感物质分离和鉴定,化感种质资源等方面的研究进展了系统的综合论述,在此基础上,进一步对化感作用研究中的主要问题,例如;生物检测手段,化感作用利用途径等进行了分析。并对我国的化感作用研究的存在问题及发展方向作出了论述。     

Molecular biology of weed control

The vast commercial effort to utilize chemical and molecular tools to solve weed control problems has had a major impact on the basic biological sciences as well as benefits to agriculture, and the first generation of transgenic products has been successful, while somewhat crude. More sophisticated products are envisaged and expected. Biotechnologically-derived herbicide-resistant crops have been a considerable benefit, yet in some cases there is a risk that the same useful transgenes may introgress into related weeds, specifically the weeds that are hardest to control without such transgenic crops. Biotechnology can also be used to mitigate the risks. Molecular tools should be considered for weed control without the use of, or with less chemicals, whether by enhancing crop competitiveness with weeds for light, nutrients and water, or via allelochemicals. Biocontrol agents may become more effective as well as more safe when rendered hypervirulent yet non-spreading by biotechnology. There might be ways to disperse deleterious transposons throughout weed populations, obviating the need to modify the crops. This revised version was published online in August 2006 with corrections to the Cover Date.     

Allelopathic potential of an annual weed,Polypogon monspeliensis,in crops in India

The question of whether annual weeds are allelopathic under natural conditions still remains to be critically answered. Investigations were carried out to understand the involvement and mode of operation of allelopathy in an annual weed, Polypogon monspeliensis. Comparative studies of soils associated with and without the weed under field conditions revealed that there was no significant difference in toxicity of the two soils, and thus the possibility of its allelopathic effect on crops grown in the same season could be ruled out. However, soil amended with weed straw had significantly higher total phenolics including higher relative concentrations of phenolic fractions that were not detected in unamended soil. Phenolic fractions significantly affected the seedling growth of radish and cluster bean. It is likely that P. monspeliensis did not interfere chemically with the crops cultivated during the same season, but interfered with the following season crop through incorporated straw. These results indicate how a monocarpic annual such as P. monspeliensis can be allelopathic under field conditions and allelopathic potential can be managed. We suggest that before detailed investigations on allelopathy are performed as per earlier recommended protocols, data on weed life cycle pattern and agricultural practices should be collected.     

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.

     

Roles of Allelopathy in Plant Biodiversity and Sustainable Agriculture

Allelopathic compounds are metabolites released from plants that might be beneficial or detrimental to the growth of receptor plants. These compounds are involved in the environmental complex of managed or natural ecosystems. Allelopathic compounds have been shown to play important roles in the determination of plant diversity, dominance, succession, and climax of natural vegetation and in the plant productivity of agroecosystems. The overuse of synthetic agrochemicals often causes environmental hazards, an imbalance of soil microorganisms, nutrient deficiency, and change of soil physicochemical properties, resulting in a decrease of crop productivity. The incorporation of allelopathic substances into agricultural management may reduce the use of synthetic herbicides, fungicides, and insecticides and lessen environmental deterioration. Scientists in many different habitats around the world have demonstrated the above examples previously. It is known that most volatile compounds, such as terpenoids, are released from plants in drought areas. In contrast, water-borne phytotoxins, such as phenolics, flavonoids, or alkaloids, are released from plants in humid zone areas. Both allelopathy and autointoxication play an important mechanism in regulating plant biodiversity and plant productivity. A unique case study of a pasture-forest intercropping system, which is particularly emphasized here, could be used as a model for forest management. After the deforestation of coniferous or hardwood forests, a pasture grass, kikuyu grass (Pennisetum clandestinum), was transplanted onto the land. The grass was quickly established within 6 months. Significant suppression of weed growth by the kikuyu grass was found; however, the growth of coniferous or hardwood plants was not suppressed but stimulated. This example as well as others described in this text clearly indicate that allelopathy plays a significant role in sustainable agriculture. Nevertheless, room for allelopathic research in the next century is available for biologists, biochemists, biotechnologists, and chemists. Future allelopathic research should focus on the following tasks: (1) a continuous survey of potential allelochemicals from natural vegetation or microorganisms, (2) the establishment of practical ways of using allelochemicals in the field, (3) to understand the mode of action of allelopathic chemicals in receptor organisms, (4) to understand the role of allelopathic chemicals in biodiversity and ecosystem function, (5) to explore advanced biotechnology for allocating allelopathic chemical genes in plants or microorganisms for biological control, and (6) to challenge the natural product chemists to develop a better methodology for isolating allelopathic compounds or their degraded compounds from the environment, particularly the soil environment.     

菊科植物化感作用研究进展

对菊科植物化感作用的研究进展进行了综述。菊科植物中至少有 39个属存在化感作用 ,特别是一枝黄花属、向日葵属、胜红蓟属、银胶菊属、蒿属植物等有较多的研究报道。鉴定出的化感物质多为萜类、聚乙炔类、酚类、有机酸类等 ,这些化感物质对多种受体植物表现出程度不同的抑制或促进的效应。其化感作用机理表现在破坏受体膜系统的稳定性及水分平衡关系、抑制氧化磷酸化、促进或阻滞叶绿素的合成、影响矿质元素的吸收利用等。并对菊科植物化感物质在植物生长调节剂、天然除草剂和生物杀虫剂 ,或人工合成除草剂和杀虫剂上应用的前景进行了探讨。本文显示菊科植物的化感作用将在控制外来恶性杂草及维护生态平衡上扮演重要的角色。在当前菊科植物化感作用研究的基础上 ,提出了进一步研究的 6个方向 :(1)化感物质的生物合成途径与关键酶的特性研究 ;(2 )具化感潜势物种资源的调查评价及利用研究 ;(3)化感作用在自然生态系统中的演变规律 ;(4 )菊科重要作物自毒的生化机制及克服途径 ;(5 )具应用前景的菊科植物化感关键酶的基因克隆和转基因 ,并对受体植物基因的表达与调控进行研究 ;(6 )化感作用在可持续发展农业应用上的研究与开发 ,特别是作为天然除草剂及杀虫剂