Variation in the Allelopathic Effect of Rice with Water Soluble Extracts

growth of ducksalad (Dilday et al., 1994), which is a major weed in the southern USA and causes a 21% The allelopathic effect of rice (Oryza sativa L.) on lettuce (Lactuca reduction in the yields of direct-seeded rice (Smith, sativa L.) and ducksalad [Heteranthera limosa (Sw.) Willd.] was investigated with water soluble extracts. Ducksalad is a major weed 1988). In field experiments, some rice cultivars produced in southern USA rice fields. ‘PI312777’ and ‘Rexmont’, which are a weed-free radius of 10 to 15 cm around an individual suppressive and nonsuppressive to ducksalad in the field, were used plant while nonsuppressive cultivars were densely surto establish the bioassay of allelopathic activity in rice. Water soluble rounded by ducksalad. Rice cultivars with an allelocompounds, which were extracted from rice seedlings and adult plants, pathic effect to barnyardgrass (Echinochloa crus-galli were applied to lettuce seeds. Leaf extract of PI312777 inhibited the (L.) Beauv.) were also screened by assessment in the root growth of lettuce more strongly than those of Rexmont. Extracts field and laboratory (Olofsdotter and Navarez, 1996). from the leaves of rice seedlings at the six-leaf stage inhibited the Varietal differences in the allelopathic effect were degrowth of ducksalad and lettuce, and a close relationship existed tected using the plant box method, in which lettuce between the inhibitory effect and the two test plants. A wide range seeds were placed together with rice seedlings in agar of variation in allelopathic activity among rice cultivars was assessed using water soluble extracts from their leaves and lettuce as a test plant. media at the fourto seven-leaf stages (Fujii, 1992). Aqueous extracts of decomposing rice residues in soil inhibited the root growth of rice and lettuce seedlings (Chou and Lin, 1976). Six phenolic acids (p-salicyclic, A plants release compounds into the p-coumaric, vannilic, syringic, ferulic, and mandelic environment through root exudation, leaching by acid) were isolated from decomposing rice straw and dews and rains, and volatilization or decaying plant tispaddy soil (Chou, 1980). A total of 16 potential allelosue (Rice, 1984). In most cases, the compounds inhibit chemicals, including the above-mentioned compounds, the germination or growth of neighboring plants alhave been found in rice (Olofsdotter et al., 1995). Allelothough sometimes the compounds stimulate their chemicals, which are completely responsible for varietal growth. differences in the allelopathic effect of rice observed in Allelopathy in crops may act as a biological weed field or laboratory experiments, have not been identicontrol in the agroecosystem. The genetic improvement fied. The utilization of allelopathy might be straight of the allelopathic effect in crops is a strategy for biologiforward if allelopathic cultivars can be identified by cal weed control in breeding programs. In the 1970s, the amount of allelochemicals, but this seems difficult. germplasm assessment was extensively undertaken to Genes for an allelopathic effect or production of allelodetect allelopathic accessions of crops. Accessions with chemicals have not been detected although quantitative an allelopathic effect have been found in crops such as trait loci (QTL) for an allelopathic effect against barnbeet (Beta vulgaris L.), lupine (Lupinus lutens L.), maize yardgrass were reported (Bach Jensen et al., 1999). A (Zea mays L.), wheat (Triticum aestivum L.), oat (Avena successful breeding strategy to incorporate allelopathic sativa L.), pea (Pisum sativum L.), barley (Hordeum activity into advanced cultivars would include a largevulgare L.), rye (Secale cereale L.), and cucumber (Cuscale evaluation of a broad array of germplasm, the cumis sativus L.) reviewed by Rice (1984). A total of identification of allelochemicals, and the mapping of 538 accessions of cultivated and wild cucumber were genes on the chromosomes. screened by the pot and field test, and several accessions In genetic analysis for rice allelopathy to ducksalad, inhibited the growth of weeds (Putnam and Duke, 1974). we first focused on bioassay improvement using a water Out of more than 3000 assessions of oat, several were soluble extract because it should be more simple and found with a fluorescent microscope to exude a large reproducible. This study aims to establish an improved amount of an allelochemical, scopoletin (Fay and bioassay using a water soluble extract to quantify genetic Duke, 1977). variation in the allelopathic effect among rice germResearch on rice allelopathy has been undertaken to plasm collections. The second objective is to select the select allelopathic cultivars in the field or laboratory parental lines for genetic analysis. and to identify allelochemicals. Several accessions of rice germplasm in the field were found to decrease the MATERIALS AND METHODS K. Ebana and K. Okuno, Natl. Inst. of Agrobiol. Resources, KannonPlant Materials dai 2-1-2, Tsukuba, Ibaraki, 305-8602 Japan. K. Ebana and H. Namai, Two cultivars or lines, PI312777 and Rexmont, were seInst. of Agric. and Forestry, Univ. of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8572 Japan. W.G. Yan and R.H. Dilday, Dale lected to represent suppressive and nonsuppressive rice Bumpers Natl. Rice Res. Cent., USDA-ARS, Stuttgart, AR 72160. against ducksalad, based on earlier germplasm evaluation K. Okuno, current address: Hokkaido Natl. Agric. Exp. Stn., Hitsuji(Dilday et al., 1994). PI312777 is a progeny line derived from gaoka 1, Toyohira, Sapporo, 062-8555 Japan. Received 29 Nov. 1999. the cross combination 2* ‘Taichung 65’/‘Taichung Native 1’ *Corresponding author ([email protected]). Abbreviations: QTL, quantitative trait loci; TN1, Taichung Native 1. Published in Agron. J. 93:12–16 (2001).