Allelopathy studies the interactions among plants, fungi, algae and bacteria with the organisms living in a certain ecosystem, interactions that are mediated by the secondary metabolites produced and exuded into the environment. Consequently, allelopathy is a multidisciplinary science where ecologists, chemists, soil scientists, agronomists, biologists, plant physiologists and molecular biologists offer their skills to give an overall view of the complex interactions occurring in a certain ecosystem. As a result of these studies, applications in weed and pest management are expected in such different fields as development of new agrochemicals, cultural methods, developing of allelopathic crops with increased weed resistance, etc. The present paper will focus on the chemical aspects of allelopathy, pointing out the most recent advances in the chemicals disclosed, their mode of action and their fate in the ecosystem. Also, attention will be paid to achievements in genomics and proteomics, two emerging fields in allelopathy. Rather than being exhaustive, this paper is intended to reflect a critical vision of the current state of allelopathy and to point to future lines of research where in the authors' opinion the main advances and applications could and should be expected.

In the search for a standard bioassay of phytotoxicity for allelochemicals, 22 commercial varieties of eight plant species [four dicotyledons: lettuce (Compositae), carrot (Umbelliferae), cress (Cruciferae), tomato (Solanaceae); and four monocotyledons: onion (Liliaceae), barley, wheat, and corn (Gramineae)] proposed as models for the most common weed families have been tested at different pH and solution volumes per set conditions. Nine commercial varieties selected as standard target species (STS) were tested with standard commercial herbicides to ensure their sensitivity to phytotoxic compounds. Results are discussed to establish the proper growth requirements, and sensitivity of commercial seeds of STS and to find the most suitable commercial herbicides that allow comparison with an internal standard to validate the response of potential allelochemicals.

Benzoxazinoids have been described as important allelochemicals from Gramineae as well as Acanthaceae, Rannunculaceae, and Scrophulariaceae plants. Several bioactivities have been described and evaluated for these compounds, including fungistatic, antifeedant, and phytotoxic. In ongoing studies about allelochemicals as natural herbicide models, the description of soil dynamics in phytotoxic agents has high importance, because the possible biotransformations developed by soil microorganisms could yield compounds with modified biological properties, affecting the overall allelopathic capability of the producer plant in a direct manner. Thus, a complete degradation study has been carried out for 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA) and 6-methoxybenzoxazolin-2(3H)-one (MBOA) in two soils cultivated with Triticum aestivum L. varieties (cv. Astron and cv. Ritmo). The main purpose was to identify degradation products and to elucidate biotransformation dynamics. Results show DIMBOA to degrade rapidly, yielding MBOA in both studied soils at different doses (t 1/2 ) 31 ( 1 h, n ) 12) and reaching high conversions (80 ( 4 h, n ) 42). MBOA, an intermediate in the degradation pathway from DIMBOA to 2-amino-7-methoxy-3H-phenoxazin-3-one (AMPO), was more resistant toward biodegradation (t 1/2 ) 5 ( 1 days, n ) 6). MBOA showed maximum conversions at a dose of 250 mg/kg of soil (36 ( 3 days, n ) 6). Soil belonging to T. aestivum cv. Ritmo crops showed higher degradation capacity than cv. Astron soil. AMPO was the final degradation product observed for DIMBOA in the soils and experimental conditions selected. Consequences for activity and stability of these compounds in relation to allelopathy are discussed.

Benzoxazinones 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA) and 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA) have been considered key compounds for understanding allelopathic phenomena in Gramineae crop plants such as corn (Zea mays L.), wheat (Triticum aestivum L.), and rye (Secale cereale L.). The degradation processes in the environment observed for these compounds, in which soil microbes are directly involved, could affect potential allelopathic activity of these plants. We present in this work a complete structure-activity relationships study based on the phytotoxic effects observed for DIMBOA, DIBOA, and their main degradation products, in addition to several synthetic analogues of them. Their effects were evaluated on standard target species (STS), which include Triticum aestivum L. . They showed high inhibitory activity over almost all species growth. The fact that APO is a degradation product from DIBOA with high phytotoxicity and stability makes it possible to assign an important ecological role regarding plant defense mechanisms. 2-Deoxy derivatives of natural benzoxazinones display a wide range of activities that allow proposing them as new leads for natural herbicide models with a 1,4-benzoxazine skeleton.

Wheat (Triticum aestivum L.) has been found to possess allelopathic potential and studies have been conduced to apply wheat allelopathy for biological weed control. 2,4-Dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA) is a common product found in wheat, corn, and rye exudates and it can be released to the environment by that way. In this report, the stability of DIBOA is studied in two soils from crop lands of wheat cv. Astron and cv. Ritmo. These varieties were selected by their concentrations of DIBOA and 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA) from aerial parts and by the bioactivities of their aqueous extracts in the growth of wheat coleoptile sections. The degradation rate of DIBOA in these soils was measured in laboratory tests during 90 h by high-pressure liquid chromatography methods. These analyses demonstrate that DIBOA was transformed primarily into 2-benzoxazolinone (BOA). This transformation was similar in both soil types with an average half-life of 43 h. The degradation studies for BOA show its biotransformation to 2-aminophenoxazin-3-one (APO) with a half-life of 2.5 days. Therefore, BOA is an intermediate product in the biotransformation from DIBOA to APO in these wheat crop soils and is consistent with previous findings. APO was not degraded after three months in soil, suggesting that its degradation rate in soil is very slow.KEYWORDS: Benzoxazinoids; DIBOA-Glc; DIBOA, BOA; biodegradations; soil; Triticum aestivum; bioactivity INTRODUCTIONSome cereal plants produce a series of benzoxazinoid compounds (cyclic hydroxamic acids). The number of this group of natural products is small (1), but they possess diverse biological activities. These compounds are involved in the defense of plants against fungi (2) and insects (3) as well as in allelopathic interactions (4,5). The most important benzoxazinoids reported (Figure 1) are 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA) and 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA), which are present in wheat, maize, and rye and have been found in members of families Acanthaceae, Rannunculaceae, and Scrophulariaceae (6). These compounds are present as glycosides (Figure 1) in plants, being released as aglycones by the activity of the enzyme -glucosidase (7,8).Moreover, these aglycones are unstable in solution and soil, being transformed to 2-benzoxazolinone (BOA), 7-methoxy-2-benzoxazolinone (MBOA), and other degradation products (1) (Figure 1). These transformations depend on the chemical and biological conditions. Some of these transformation products are more biologically active than the original ones (9).Transport of allelochemicals to the soil can occur mainly by leaching of the foliar parts (10, 11), exudation from root (12), decomposition of plant residues by microbial action (13), or by direct transformation by microbes associated to the roots (14).Previous publications have dealt with the isolation, characterization, and biological activity of the degradation products (15-17). However, the dynamic aspects of...

Allelopathy is the biological phenomenon of chemical interactions between living organisms in the ecosystem, and must be taken into account in addressing pest and weed problems in future sustainable agriculture. Allelopathy is a multidisciplinary science, but in some cases, aspects of its chemistry are overlooked, despite the need for a deep knowledge of the chemical structural characteristics of allelochemicals to facilitate the design of new herbicides. This review is focused on the most important advances in allelopathy, paying particular attention to the design and development of phenolic compounds, terpenoids and alkaloids as herbicides. The isolation of allelochemicals is mainly addressed, but other aspects such as the analysis and activities of derivatives or analogs are also covered. Furthermore, the use of allelopathy in the fight against parasitic plants is included. The past 12 years have been a prolific period for publications on allelopathy. This critical review discusses future research areas in this field and the state of the art is analyzed from the chemist's perspective. © 2019 Society of Chemical Industry

Avena fatua L. (wild oat) and Lolium rigidum Gaud. (rigid ryegrass) are highly problematic weeds affecting a wide variety of cereal crops worldwide. The fact that both of these weeds have developed resistance to several herbicide groups made them optimal candidates as target organisms for ongoing research about the potential application of allelochemicals and analogue compounds as natural herbicide models. Benzoxazinones, a family of natural allelochemicals present in corn, wheat, and rye, including 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one and 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one, together with some degradation products, found in crop soils as well as in other systems, and some synthetic analogues of them were tested on wild oat and rigid ryegrass seeds; the results were statistically treated, and some structure-activity relationships, useful in further development of natural herbicide models, were elucidated. The most active compounds were the synthetic benzoxazinone 2-acetoxy-(2H)-1,4-benzoxazin-3(4H)-one and the degradation product 2-aminophenoxazin-3-one, with highly significant inhibition on the development of both weeds. The ecological role of these compounds is discussed by considering both degradability and phytotoxicity. The bioactivity of aminophenoxazines has been correlated by their aqueous solubility-lipophilicity predicted by means of computational methods.

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