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Allelopathy in riceBook is availableEdited by M. Olofsdotter Allelopathic activity in rice for controlling major aquatic weeds R.H. Dilday, W.G. Yan, K.A. K. Moldenhauer, and K.A. Gravois Of the more than 16,000 rice accessions or varieties from 99 countries in the germplasm collection of the U.S. Department of Agriculture's Agricultural Research Service, a substantial number have been evaluated for allelopathic effects on aquatic weeds-about 12,000 for ducksalad [Heteranthera limosa (Sw.) Willd] and around 5,000 for redstem (Ammannia coccinea Rottb.). In field tests during 1988-90, researchers identified 412 rice accessions that produced an area of allelopathic activity around the rice plant greater than 10 cm for ducksalad and 145 accessions produced the same area of activity for redstem. The accessions demonstrating allelopathic effects on ducksalad originated in 31 countries. A hybrid between PI 338046 (allelopathic) and Katy (nonallelopathic) had superior agronomic characteristics in field tests, and significantly fewer ducksalad plants were found growing with this hybrid in greenhouse tests. Preliminary genetic data indicate that allelopathic activity in rice is quantitatively inherited. Weed management in rice using allelopathic rice varieties in Egypt S.M. Hassan, I.R. Aidy, A.O. Bastawisi, and A.E. Draz During 1993-96, approximately 1,000 rice varieties were screened and evaluated in the field, greenhouse, and laboratory to identify those with allelopathic effects on Echinochloa crus-galli (L.) P. Beauv. (barnyardgrass) and Cyperus difformis L. These are the most troublesome weeds of rice in Egypt. More than 30 varieties, including RP 2269-424, LD 183-3, LDS 183-7, IET 1444, Dular, CI selection-63, UPR 82-1-7, GZ 1368-5-2, and OR 131-58, provided 50 to 90% control of E. crus-galli in the field. Incorporating the residues of some of these plants into the soil reduced seed germination of E. crus-galli. More than 10 varieties, including RP 2271-433-231, IET 11754, Dular, and OR 131-5-8, suppressed C. difformis by from 50 to 75%. The varieties expressed allelopathic properties at the 3- to 4-leaf stage. They inhibited root development and emergence of the first or second leaf of both weeds. Residues of these varieties did not affect subsequent winter crops. Ammannia sp. stimulated rice growth and reduced weed growth. Rice allelopathy research in Korea K.U. Kim and D.H. Shin In Korea, direct seeding of rice is receiving great attention because of its low-input nature. Farmers who shift to direct seeding are experiencing problems with the weed Echinochloa crus-galli. Development of allelopathic rice would be an alternative to chemicals and other practices in reducing occurrence of this serious weed that thrives in direct seeded rice. Studies on screening of allelopathic rice have been conducted recently under Korean conditions, using accessions obtained from IRRI and improved in Korea as well as traditional cultivars. Seven cultivars--AC 1423, Tang Gan, Kouketsumuchi, Musashikogane, Takanenishiki, PSDRC-10, and Dandura--showed promise under Korean conditions. Isolation and identification of allelochemicals from rice through bioassay-directed isolation and chemical analyses are in progress. The inheritance of allelopathic potential in rice is being investigated to provide information needed to develop an allelopathic rice for Korea as well as to generate isogenic lines. Using and improving laboratory bioassays in rice allelopathy research Inderjit and M. Olofsdotter Weeds are a major biological constraint to rice production worldwide. One option to reduce herbicide dependency is to utilize allelopathic effects that rice may have on certain weeds. To prove the existence of allelopathy in rice, compounds produced by rice varieties or their residues must be shown to interfere with the associated weed species. Laboratory bioassays permit researchers to screen large amounts of plant material in a short time. By design, they also allow researchers to eliminate interference parameters other than the one under study. However, laboratory experiments conducted under controlled environments, including the use of bioassays, must always be validated in the field to render convincing evidence of the allelopathic effect of rice on weeds. The main objectives of these bioassays are to demonstrate allelopathic interactions and to validate allelopathic strength in a large plant collection. These objectives create a different set of quality criteria and, as such, different sets of bioassays. This paper reviews some of the concerns about laboratory bioassays and makes some modest proposals for improving the bioassay as a tool in rice allelopathy research. Incorporating the allelopathy trait in upland rice breeding programs B. Courtois and M. Olofsdotter Promising results in rice allelopathy research offer the possibility to use this phenomenon in breeding programs to enhance weed control in upland rice. To incorporate the allelopathy trait in improved rice varieties destined for the uplands, several elements are necessary: 1) a good screening technique, 2) the existence of genetic variability within Oryza sativa, O. glaberrima, and the close wild relatives of rice, and 3) an understanding of the structure and genetic control of the trait. This paper outlines current gaps in knowledge, ways to fill in these gaps, and a perspective on possible breeding strategies. F.A. Macías, R.M. Oliva, A.M. Simonet, and J.C.G. Galindo In this paper, we clarify and unify the terminology used by researchers who study allelopathy. In particular, we use the term "allelopathy" as defined at the 1996 meeting of the International Allelopathy Society (IAS) and propose definitions for the terms "allelochemical", "biocommunicator", and "plant growth regulator". Also, we look at the structural relations between plant hormones and allelochemicals, their bioactivity, and their mechanisms of action. And we discuss possible applications of allelopathy in weed control, pointing out the benefits that this environmentally friendly technology should contribute to agriculture and public health. A summary of the preliminary results we obtained with sunflower varieties are presented as an example of the weed control potential that allelopathy can offer to farmers. Searching for allelochemicals in rice that control ducksalad [Heteranthera limosa (Sw.) Willd] R. Mattice, T. Lavy, B. Skulman, and R. Dilday Forty-one compounds were analyzed by gas chromatography mass spectrometry (GC/MS). Single allelopthic or nonallelopathic rice plants were grown in either 500 or 50 g of soil and then transplanted to deionized water for 48 h. GC/MS analysis of extracts of the water showed statistically higher levels of 3-hydroxybenzoic acid (3HBA), 4-hydroxybenzoic acid (4HBA), 4-hydroxyhydrocinnamic acid (4HHCA), 3,4-dihydroxyhydrocinnamic acid (34DHHCA), and tentatively identified 4-hydroxyphenylacetic acid (4HPAA) in water coming from soil growing allelopathic rice lines PI# 294400 and/or PI# 277414 than in water from soil growing the nonallelopathic rice variety Rexmont. The aquatic weed ducksalad [Heteranthera limosa (Sw.) Willd.] was better controlled in soil where the two allelopathic lines were grown than in soil where the nonallelopathic variety was grown. When rice was grown in 5 g of soil that was analyzed before flooding, statistically larger amounts of 4-hydroxybenzaldehyde (4HB), 4HBA, 3-hydroxy-4-methoxybenzoic acid (3H4MBA), 4-hydroxycinnamic acid (4HCA), stearic acid (STEARIC), tetradecanoic acid (TETRA), and a compound tentatively identified as n-valeric acid were found in allelopathic line PI# 312777 than in Rexmont or the blank control soil. The use of ethyl ether for the extraction of known and potential acidic allelochemicals was found to be superior to solid-phase extraction methods using C18, charcoal, styrene divinylbenzene, or XAD-4 resin. Adaptive autointoxication mechanisms in rice C.-H. Chou Crop debris has always been considered to be beneficial to succeeding crops. However, evidence now indicates that crop residues can sometimes inhibit plant growth. In what is called adaptive autointoxication, an aspect of allelopathy, phytotoxins produced by a plant can limit the growth and/or population size of not only other plant species but of its own species as well. For example, a 25% reduction in rice yield for a second crop in Taiwan was attributed primarily to the phytotoxins produced during the decomposition of rice residues left in the soil. Bioassays of phytotoxic substances obtained from the rice residues decomposing in the soil were done in laboratory and greenhouse experiments. The aqueous extracts of decomposing rice residues in the soil significantly reduced the growth of rice seedlings and other test plants. Major phytotoxic phenolics found included p-coumaric, o-hydroxyphenylacetic, vanillic, ferulic, and syringic acids. The phytotoxicity of these extracts persisted for up to 4 mo. N availability also appears to be affected. The amounts of NH4+-N and NO3--N in the soil were significantly higher in the first crop than those in the second crop. Incorporating rice straw into the soil may decrease the availability of both forms of N. Autointoxication in rice appears to be a problem particularly in continuous monoculture ricefields and especially pronounced in fields with poor water drainage. Allelopathic strategies for weed management in the rice-wheat rotation in northwestern India S.S. Narwal, M.K. Sarmah, and J.C. Tamak Within the last 25-30 yr, rice cultivation in northwestern India has grown in importance where irrigation is ample during the summer rainy season. Rotating the summer rice crop with a winter wheat crop has become very popular because of the high yield potential of both crops. Unfortunately, both crops are heavily infested with weeds, forcing farmers to use large amounts of herbicides. The heavy use of herbicides in the rice-wheat rotation has contributed to several problems in northwestern India: 1) a change in natural weed flora, 2) development of herbicide-resistant weeds, 3) human health hazards, and 4) contamination of ground water resources. Allelopathic strategies may provide an alternative to overcome the serious ecological problems associated with herbicide use. Field, pot culture, and laboratory studies show that introducing weed-smothering crops to the rotation can suppress weed populations considerably in current and succeeding crops. For example, early summer (April-June) fodder crops such as sorghum, pearl millet, and maize drastically reduced the weed population and its biomass. The suppression effects of pearl millet also persisted as much as 45 d into the next sorghum crop. Including such summer fodder crops before the rice crop in the rice-wheat rotation may provide enough weed control in the rice crop to reduce considerably the need for herbicides. In addition, substituting winter fodder crops of oat and berseem (Trifolium alexandrimum) in place of wheat also controlled weeds during the winter season. Additional allelopathic studies are needed before a satisfactory weed management strategy can be put in place for the rice-wheat rotation. Using allelopathic plants for weed control in rice M.N.V. Prasad Recently, allelopathy has gained attention for its potential role in integrated pest management and sustainable agricultural production. In the future, agriculture may benefit through exploiting this trait for controlling weeds and microbes, optimizing crop productivity, and safeguarding the environment. Two important crop-weed interactions in ricefields that influence yield are 1) allelopathy that is mediated through allelochemical action and 2) competition between weeds and the rice crop for common resources such as water and nutrients. Although allelochemicals may decrease agricultural yields, some of that occur in the phytomass may also be used to control weeds and pests. A few of them have already been purified and characterized. Higher plants have evolved biosynthetic pathways through which secondary metabolites are synthesized and accumulated in or released from plants. These compounds reach the environment in appreciable quantities via root exudation and leachates during litter decomposition, and they produce allelochemicals. Plants and crop residue can also be exploited for weed control through intercropping and mulching. This paper examines the potential use of allelopathic plants for weed management in ricefields and suggests possible strategies. G.L. Bansal Lantana camara L., one of the world's 10 worst weeds introduced in India during early part of the nineteenth century, has become a major intractable problem in the state of Himachal Pradesh. The fast growth of the weed has been responsible for substantial economic losses in the state's agricultural, animal husbandry, and forest industries. However detrimental the weed itself is to agriculture, it has been found that aqueous extracts from L. camara twigs inhibit, to various degrees, the germination and seedling growth in rice-associated weeds, e.g., Echinochloa colonum, Digitaria sanguinalis, Panicum psilopodium, and Commelina benghalensis. This inhibitory effect was substantially relieved when activated charcoal was added to the soil medium in laboratory tests, which indicates that allelopathy is most likely involved. In four of five rice varieties tested, these extracts did not affect germination. L. camara twigs incorporated into the soil under field conditions significantly increased chlorophyll content in transplanted rice and subsequently grain yield while at the same time significantly reducing the abundance of two weeds, P. psilopodium and C. benghalensis. It would appear that L. camara has some useful allelopathic effects that warrant more study. Potential of allelopathy for weed management in wet-seeded rice cultivation in Sri Lanka B. Marambe Experiments conducted in Egypt and the USA have shown that several rice varieties collected from Sri Lanka are allelopathic to major weeds. Despite the potential significance of allelopathy to the agricultural sector, evidence is lacking on rice allelopathy in Sri Lanka, where wet-seeded cultivation is the major practice of peasant farmers. Experiments have now been carried out using four rice varieties, BG 1165-2, BG 34-11, LD 183-3, and LD 183-7, to elucidate their allelopathic potential in controlling Echinochloa crus-galli (L) Beauv. Knowledge of rice allelopathy and its allelochemical functions offer several attractive possibilities for agricultural practices in Sri Lanka. C. Premasthira and S. Zungsontiporn Because gooseweed (Sphenoclea zeylanica Gaertn.) has been reported to have allelopathic potential, we studied the effects of gooseweed extracts on weed growth in the laboratory. The seeds of various grasses, broad-leaved weed species, and rice were treated with three concentrations of gooseweed extracts, equivalent to 0.1, 1.0, and 5.0 g of fresh weight. To determine the growth of tested plants as affected by the gooseweed extract solutions, the lengths of the longest root and shoot of the weed seedlings were measured 7 d after treatment. The tested plants were differentially inhibited by a given concentration of gooseweed extract. Seedlings of the following species were more significantly different on growth inhibition than those of the untreated controls at the lowest concentration of extract solution: Leptochloa chinensis (L.) Nees, Chloris barbata Sw. Dactyloctenium aegyptium (L.) B.P., Pennisetum pedicllatum Trin., Pennisetum setosum L.C. Rich., Hogrophila erecta Hochr., Mimosa invisa var. inermis Adelb., Hyptis suaveolens Poit., and Scirpus articulatus L. When the concentration of extract solution was increased, the growth of the weed seedlings decreased; however, the growth of rice seedlings was less inhibited than the weeds. |
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Allelopathic activity in rice for controlling major aquatic weeds Weed management in rice using allelopathic rice varieties in Egypt Rice allelopathy research in Korea Using and improving laboratory bioassays in rice allelopathy research Incorporating the allelopathy trait in upland rice breeding programs Searching for allelochemicals in rice that control ducksalad [Heteranthera limosa (Sw.) Willd] Adaptive autointoxication mechanisms in rice Allelopathic strategies for weed management in the rice-wheat rotation in northwestern India Using allelopathic plants for weed control in rice Potential of allelopathy for weed management in wet-seeded rice cultivation in Sri Lanka |
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