Weed Suppression by Buckwheat

Transcription

1 98 Weed Suppression by Buckwheat Current Advances in Buckwheat Research (1995) : Tohru Tominaga and Takako Uezu Faculty of Agriculture, Shinshu University, Ina, Nagano, Japan Abstract To clarify the allelopathic effects of buckwheat on weeds, buckwheat was cultivated alone and with weeds under field condition. Weed plot, where weeds were left as they grew, was also established. Each treatment had three replications. Buckwheat biomass and yield were not influenced by weeds, but total weed biomass in buckwheat-weed plot was only 28% of that in weed plot. The biomass of Amaranthus lividus, Chenopodium album, Echinochloa crus-galli var. crus-galli and Portulaca oleracea was severely reduced by buckwheat. Bioassay showed that the root elongation ofthe seedlings of E. crus-galli var. crus-galli and P. oleracea was significantly inhibited by water extracts from the soil where buckwheat was cultivated. These results indicate that buckwheat is a successful competitor against weeds and this is partly due to the allelopathic effects ofbuckwheat. Key words : Allelopathy, Buckwheat, Root growth inhibition, Smother crop, Weed suppression Introduction Buckwheat (Fagopyrum esculentum Moench) is a successful competitor against weeds in fields as empirically known. This may be partly due to its rapid growth at the early growth stage. Besides this growth habit, buckwheat seems to have some inhibiting effects on the growth of other plants. The growth of crops cultivated at the fields where buckwheat was planted previously is sometimes suppressed. These suggest the allelopathic potential of buckwheat and it will be very useful for biological control ofweeds. Allelopathy is inferred to be important in agroecosystems (Putnam, 1978). Crop losses by weeds are partially caused by allelopathic effects of weeds (Rice, 1984), though it is very difficult to separate the allelopathic effects from competition for resources in fields. On the other hand, breeding programs to select strains that have allelopathic effects on weeds are in progress in some crops (Putnam and Duke, 1974 ; Fay and Duke, 1977 ; Lockerman and Putnam; 1979). Cultivation of such strains may lead to decrease herbicide use and will be very helpful for sustainable agriculture. The reports on the allelopathy of buckwheat were scarcely

2 694 found out. Zabyalyendzik (1973) found that root exudates of buckwheat stimulated the growth of oats (Avena sativa L.) but inhibited that of lupine (Lupinus albus L.). The effects of allelochemicals seem to be different among receiver species. It is very important to clarify the allelopathic potential of buckwheat, for the crop is one of the candidates as a smother crop (Overland, 1966). This study was conducted to clarify the allelopathic effects of buckwheat on major weed species in Japan. It consisted of two experiments. One was conducted to clarify the effects ofbuckwheat on weed growth under field condition and another was done to clarify the allelopathic effects of buckwheat root exudates on weed seedling growth. Materials and Methods Effects of buckwheat on weed growth under field condition This field study was conducted in 1994 at the Research Fann of Shinshu University. Before the start ofthe experiment, the field was plowed up and compound fertilizer was applied at the rate ofn 1.2, P20S 4.8 and K kg / loa. No herbicide was applied. Three treatments consisting ofbuckwheat alone, buckwheat and weeds grown together, and weeds were established with three replications of each treatment. Each plot was arranged in a randomized complete block design. In buckwheat plots and buckwheat-weed plots, buckwheat was seeded at the rate of 6 kg / loa on August 1. In buckwheat plots, only buckwheat was grown and weeds were completely removed. On the other hand, in buckwheat-weed plots and weed plots, no weeding practice was made and weeds were left as they grew. Both buckwheat and weeds grown in I_m 2 of each plot were harvested late in October and weeds were identified by species and counted. Dry matter weight of each plant was measured after drying at 80 C for at least 48 hours. Effects ofwater extract from buckwheat cultivated soil on weed seedling growth After the harvest of buckwheat and weeds late in October, soil was sampled in the field where buckwheat was cultivated and in the field free from vegetation during the above mentioned field study. Soil was put in a flask and the same volume ofdistilled water as that ofthe soil was added into the flask. The flask was agitated on a reciprocating shaker for 12 hours at room temperature (12 C). The upper layer of water extract solution was used in the experiment. Water extract solution from the soil free from vegetation was prepared as the same way as described above and used as a control ofthis experiment. Weed seeds were collected in 1992 to 1994 at the same field as the field experiment was conducted and stored at 4 C until the start ofthis experiment. Among them, Amaranthus lividus L., Digitaria ciliaris (Retz.) Koeler, Echinochloa crus-galli (L.) Beauv. var. crus-galli,

3 695 Galinsoga ciliata (Raf.) Blake, Portulaca oleracea L. were used as test plants. Weed seeds were placed on wet filter papers in plastic trays kept in an incubator at 25 C to germinate before the experiment. Germinated seeds ofeach species were placed on a filter paper wet with 2.5 ml water extract solution in a 9-cm diameter plastic petri dish. Plastic petri dishes were kept in an incubator at constant 25 C under continuous light. The root length of each seedling was measured after three-day incubation. Fifteen seedlings of each weed species were used in each treatment of this bioassay. This bioassay experiment was conducted twice and data were combined. Results and Discussion A total of 13 weed species listed below appeared in the experimental plots during the study; A. lividus, A. viridis L., Chenopodium album L., D. ciliaris, E. crus-galli var. crus-galli. G. ciliata, P. oleracea, Erigeron canadensis L., Equisetum arvense L., Lamium amplexicaule L., Panicum dichotomiflorum Michx., Poa annua L. and Stellaria media (L.) Villars. Among them, the former seven species were predominant, but the number ofthe latter six species was very few. The total number of weeds that appeared in a plot was not different between buckwheat-weed plots and weed plots, but the total weed biomass was significantly less in buckwheat-weed plots than in weed plots (Fig. I). On the other hand, buckwheat biomass and yield were not significantly reduced in buckwheat-weed plots as compared with buckwheat plots. Buckwheat was a successful competitor against weeds and effectively suppressed weed growth, but may not influence the number ofweeds that emerged. Total weed biomass (dry matter weight / m 2 ) in buckwheat-weed plot was only 28% of that in weed plot and the reduction rate in biomass ofeach weed was different among species. The biomass of D. ciliaris and G. ciliata was scarcely decreased by buckwheat. Tsuzuki et al. (1977) reported that the growth of Digitaria adscendens Hem. (synonym of D. ciliaris (Retz.) Koeler) was remarkably inhibited when it was planted near F. cymosum Meissn, a perennial buckwheat. Different Fagopyrum species seem to show different effects on D. ciliaris. The biomass of E. crus-galli var. crus-galli, P. oleracea, C. album and A. lividus was severely reduced by buckwheat to 32.8, 31.9, 13.1 and 10.3% of that in weed plot, respectively (Fig. I). Different weed species appear to interfere with buckwheat in a different way. Similar buckwheat effects on weed growth as shown in the field experiment were found in the bioassay experiment of water extract from the soil (Fig. 2). Water extract solution from the buckwheat cultivated soil significantly inhibited the root elongation of seedlings of E. crusgalli var. crus-galli and P. oleracea as compared with that from the soil free from vegetation. This result strongly supports the allelopathic effects of buckwheat on the two weed species. The reduction of the biomass of the two weed species partially results from the allelopathic

4 696 effects of buckwheat. On the other hand, the root elongation of seedlings of D. ciliaris and G. ciliata was not influenced by the water extract solution from the buckwheat cultivated soil as shown in the field experiment. As for the root growth ofa. lividus, the different result from the field experiment was found. A. lividus was severely suppressed by buckwheat under the field condition, but not influenced by the water extract from the soil. A. lividus may be suppressed by the competition with buckwheat for light, nutrition, water and other resources as shown in the suppression of Cirsium arvense (L.) Scop. by buckwheat (Eskelsen and Crabtree, 1991), but in the case ofa. lividus, allelochemicals other than water soluble root exudates of buckwheat may influence the interference between them under the field condition. As we failed to break dormancy ofthe seeds ofa. viridis and C. album, the effects of water extract on these two species could not be clarified. Total weed biomass Digitaria ciliaris Galinsoga ciliaris Amaranrhus viridis &hinochloa crus - gam Portulaca oleracea Chenopodium album ~.~.E, E!!!g8!!",~... :m:!'«e ""2l,~" Amaranrhus lividus o % of Control Fig. 1. Effects ofbuckwheat on weed biomass in field. Digitario ci/iaris Galinsoga ciliaris Echinochloa crus - galli Portulaca oleracea Amaranthus lividus o % of Control Fig. 2. Effects ofwater extract from buckwheat cultivated soil on weed seedling root growth.

5 697 It is very difficult to distinguish allelopathy from competition for resources and the mode of action of allelochemicals is very complicated under field condition. The results obtained here show that buckwheat is a successful competitor against weeds and has allelopathic effects on the growth of some weeds, though its effects are different among weed species. In this study, only water extract from the buckwheat cultivated soil was bioassayed, but allelochemicals presumably are also included within a buckwheat plant. The crop is probably useful as a smother plant. The bioassay on the extracts from buckwheat plant, isolation and identification ofthe allelochemicals will be necessary in the future. References Eskelsen, S. R. and G. D. Crabtree (1991) Interaction between buckwheat and Canada thistle. Proc. West. Soc. Weed Sci. 44, Fay, R. K. and W. B. Duke (1977) An assessment ofallelopathic potential in Avena germplasm. Weed Sci. 25, Lockerman, R. H. and A. R. Putnam (1979) Evaluation of allelopathic cucumbers (Cucumis sativus) as an aid to weed control. Weed Sci. 27, Overland, L. (1966) The role of allelopathic substances in the "smother crop" barley. Am. J. Bot. 53, Putnam, A. R. (1978) Allelopathy in agroecosystems. Ann. Rev. Phytopathol. 16, Putnam, A. R. and W. B. Duke (1974) Biological suppression of weeds: evidence for allelopathy in accessions ofcucumber. Science 18, Rice, E. L. (1984) Allelopathy, 2nd ed. 422 pp. Acad. Press, New York. Tsuzuki, E., A. Katsuki, S. Shida and T. Nagatomo (1977) On the growth inhibitors contained in buckwheat plants. II. The effects of water and organic solvent extracts on the growth of rice seedlings. Bull. Fac. Agri. Miyazaki Univ., 24, Zabyalyendzik, S. F. (1973) Allelopathic interaction of buckwheat and its components through root excretions. Vyestsi Akad. Navuk BSSR Syer Biyal Navuk 5, Cited from "Allelopathy" written by Rice, E. L. in Acad. Press, New York.