The objective of this paper is to review the possibilities for using allelopathy to improve overall crop competitive ability against weeds, using rice, Oryza sativa, as an example. Laboratory, greenhouse and ®eld screenings for allelopathy and overall weed suppression in rice have been made and allelopathic rice germplasm has been identi®ed in laboratory and greenhouse screening. Field experiments revealed that allelopathy accounted for 34% of overall competitive ability in rice. For strongly allelopathic cultivars, allelopathy was the dominant factor determining competitive ability. Based on the results of the screenings, recombinant inbred line populations were developed for identi®cation of quantitative trait loci (QTL) controlling allelopathy. Populations of recombinant inbred lines (RILs) were derived through single-seed descent from crosses between varieties with contrasting behaviour and QTL controlling allelopathy were identi®ed. For rice and most probably also for other cereal crops, the ®ndings presented can explain the limited success in previous breeding programmes for weed competition, as allelopathy has never before been acknowledged as an important factor. The ®ndings in allelopathy indicate that it is possible to improve allelopathy in rice using marker-assisted selection. Optimizing allelopathy in combination with breeding for competitive plant types could result in crop cultivars with superior weed-suppressive ability.
the environment" (Rice, 1984). Rice plants with an allelopathic effect on weeds can mean lower production To understand the genetic control of allelopathy in rice (Oryza costs because the need for herbicide application and/or sativa L.), quantitative trait loci (QTL) mapping was performed using a population of 142 recombinant inbred lines derived from a cross hand weeding is reduced. Thus, incorporating allelobetween cultivar IAC 165 ( japonica upland variety) and cultivar CO pathic genes into rice varieties while maintaining grain 39 (indica irrigated variety). The map contained 140 DNA markers. yield and quality could benefit farmers and consumers The relay seeding technique, which is a laboratory bioassay measuring as well as the environment. the inhibition in weed root growth due to the presence of rice seedlings, Research on allelopathy in rice started in the USA was used to evaluate the allelopathic effect of the rice lines. Cultivar in the late 1980s. During several years, 12 000 accessions IAC 165 showed strong and consistent allelopathic activity against from the USDA-ARS rice germplasm were examined in barnyardgrass [Echinochloa crus-galli (L.) Beauv.], whereas CO 39 field experiments for their allelopathic potential toward was weakly allelopathic. Transgressive segregation for allelopathic ducksalad [Heteranthera limosa (Sw.) Willd.], redstem activity in both directions was observed in the population. No significant correlation was found between root morphology of the lines and (Ammannia coccinea Rottb.), broadleaf signalgrass their allelopathic potential, suggesting that allelopathy in rice was [Brachiaria platyphylla (Griseb.) Nash], rice flatsedge under genetic control independent from root morphology. Four main-(Cyperus iria L.), sprangletop (Leptochloa spp.), and effect QTLs located on three chromosomes were identified, which barnyardgrass [Echinochloa crus-galli (L.) Beauv.]. Two collectively explained 35% of the total phenotypic variation of the methods were used to record allelopathic activity: the allelopathic activity in the population. One pair of digenic epistatic weed-free radial area (cm) from the base of the rice loci, not involving any of the main-effect loci, was also detected. Once
house and field experiments. Research to accomplish the above goals cannot be carried out in one laboratory, Rice (Oryza sativa L.) allelopathy has been on the research agenda and needs active collaboration among a wide range of for a decade. Now it is important to step back and look at its progress to enable priority setting for future research. This paper aims to do scientists, including biologists, ecologists, agronomists, so primarily using the following five-step protocol for allelopathy natural product chemists, plant physiologists, and genetresearch: (i) carrying out laboratory, greenhouse, and field studies to
Summary Rice cultivars resistant to broad‐spectrum herbicides have been developed and their commercial release is imminent, especially for imidazolinone and glufosinate resistant varieties in the USA and Latin America. Glyphosate‐resistant rice should follow within a few years. Rice growers throughout the world could benefit from the introduction of herbicide‐resistant rice cultivars that would allow in‐crop, selective control of weedy Oryza species. Other perceived benefits are the possibility to control ‘hard‐to‐kill’ weed species and weed populations that have already evolved resistance to herbicides currently used in rice production, especially those of the Echinochloa species complex. Weed management could also be improved by more efficient post‐emergence control. Introduction of herbicide resistant rice could also bring areas heavily infested with weedy rice that have been abandoned back to rice production, allow longer term crop rotations, reduce consumption of fossil fuels, promote the replacement of traditional chemicals by more environmentally benign products, and provide more rice grain without adding new land to production. There are also concerns, however, about the impact of releasing herbicide‐resistant rice on weed problems. Of most concern is the possibility of rapid transfer of the resistance trait to compatible weedy Oryza species. Development of such herbicide resistant weedy rice populations would substantially limit the chemical weed management options for farmers. Herbicide‐resistant rice volunteers also could become problematic, and added selection pressure to weed populations could aggravate already serious weed resistance problems. Because of the risk of weedy Oryza species becoming resistant to broad‐spectrum herbicides, mitigating measures to prevent gene flow, eventually attainable by both conventional breeding and molecular genetics, have been proposed. With commercialisation of the first herbicide resistant varieties planned for 2001, these mitigating measures will not be available for use with this first generation of herbicide resistant rice products. Release of herbicide resistant rice should depend on a thorough risk assessment especially in areas infested with con‐specific weedy rice or intercrossing weedy Oryza species. Regulators will have to balance risks and benefits based on local needs and conditions before allowing commercialisation of herbicide‐resistant rice varieties. If accepted, these varieties should be considered as components of integrated weed management, and a rational herbicide use and weedy rice control should be promoted to prevent losing this novel tool.
A bioactivity‐guided isolation method was developed with the objective of isolating the allelochemicals in rice (Oryza sativa L.). Roots of the allelopathic rice cultivar Taichung Native 1, grown hydroponically, were extracted and fractionated, with the activity of the fractions followed using a 24‐well culture plate microbioassay. Some of the fractions obtained consisted of pure compounds, but none inhibited the growth of barnyardgrass [Echinochloa crusgalli (L.) Beauv.] at the lower concentration at which they were tested. Identified compounds were azelaic acid; ρ‐coumaric acid; 1H‐indole‐3‐carboxaldehyde; 1H‐indole‐3‐carboxylic acid; 1H‐indole‐5‐carboxylic acid; and 1,2‐benzenedicarboxylic acid bis(2‐ethylhexyl)ester. ρ‐Coumaric acid, a known allelochemical, inhibited the germination of lettuce (Lactuca sativa L.) seedlings at 1 mM However, ρ‐coumaric acid was active against barnyardgrass only at concentrations higher than 3 mM The two most active fractions obtained from the bioassay‐guided isolation were still a mixture of compounds as analyzed by gas chromatography–mass spectrometry (GC‐MS). Further fractionation is being done to isolate and identify the allelochemical(s) in these active fractions. This work has demonstrated the use of bioassay‐guided isolation in identifying allelochemicals in rice and has correlated observed field activity with laboratory experiments.
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