Locating Genes Controlling Allelopathic Effects against Barnyardgrass in Upland Rice

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 plant, and the percentage reduction in the number of confirmed, these QTLs may be useful for genetic improvement of weed plants in a tested plot relative to a nonallelopathic allelopathy in rice using marker-assisted selection. control. These methods have also been used by others (Hassan et al., 1994, 1998; Kim and Shin, 1998). Dilday et al. (1998) identified 412 rice accessions as having T increasing population in rice (Oryza sativa L.)evident allelopathic potential against ducksalad, 145 consuming countries and changes in farming pracagainst red stem, and 16 against both weed species. The tices call for new ways to sustainably enhance producaccessions with allelopathic activity originated from 31 tion. Weeds are one of the most important causes of different countries and were genetically very diverse, yield losses in rice. In the upland rice ecosystem, yield showing that allelopathic potential is widespread within loss estimates from weed infestation have ranged from rice germplasm. 30 to 100% (Hassan et al., 1994; Pandey, 1996). MoreThe main problem in conducting allelopathy field exover, weeds increase production costs and lower rice periments is that competition and allelopathy cannot be quality. separated in the field. To overcome this problem, variIn the upland rice ecosystem, hand weeding is still ous laboratory screening techniques have been develthe most common practice to control weeds. Although oped to measure allelopathy without the interference hand weeding is an effective remedy, it is extremely of competition (Leather and Einhellig, 1986; Dilday et laborious and time-consuming. The time required for al., 1991; Fujii, 1992; Kawaguchi et al., 1997). Using a hand weeding varies from 45 to 455 d/ha, corresponding laboratory bioassay called the relay seeding technique, to 40 to 50% of the total crop labor input (Roder et al., rice varieties suppressing barnyardgrass in the field have 1997). In all rice ecosystems, an increase in wage rates significantly reduced the root length of barnyardgrass has led to a replacement of manual weeding by chemical under laboratory conditions (Olofsdotter and Navarez, control and increased concerns regarding environmen1996). This technique has been routinely used in screental and health effects (Bhuiyan and Castañeda, 1995; ing hundreds of rice varieties at the International Rice Pingali et al., 1995). Consequently, there is an urgent Research Institute (IRRI) for allelopathy, resulting in need to identify sustainable weed management systems the identification of several accessions with strong allelfor rice production that will reduce the use of herbicides opathic potential. and the burden of hand weeding. The varieties now identified as allelopathic belong to Allelopathy is defined as “any direct or indirect harmdifferent isozymic groups and to different ecosystems. ful or beneficial effect by one plant on another through There appears to be a higher frequency of allelopathic the production of chemical compounds that escape into varieties among tropical japonicas within Oryza sativa and among O. glaberrima accessions than in other variInternational Rice Research Inst., PBGB and APPA Division, MCPO etal groups (Fujii, 1992; Courtois and Olofsdotter, 1998). Box 3127, 1271 Makati City, Philippines. Brigitte Courtois, seconded from CIRAD-CA, BP 5035, 34032 Montpellier Cedex, France. ReAbbreviations: NRC, no-rice check; RFLP, restriction fragment length ceived 29 Nov. 1999. *Corresponding author ([email protected]). polymorphism; RIL, recombinant inbred lines; QTL, quantitative trait loci. Published in Agron. J. 93:21–26 (2001).