Department of Bioscience and Biotechnology, Faculty of Agriculture, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan E-mail: email@example.com
The allelopathic properties of plants can be exploited successfully as a tool for pathogen and weed reduction. Over a decade, preliminary surveys of some hundred allelopathic plants in Southeast Asian and Japanese ecosystems were conducted. More than 30 species, including crops, which exhibited the greatest allelopathic activity were selected and examined for their impacts on emergence of pathogens and weeds. Application of these plant materials at 1–2átons/ha can reduce weed biomass by about 70%, and increase rice yield by about 20%. Incorporation of the allelopathic plants to rice fields at 1-5 days after transplanting gave the greatest weed reduction. The selective impacts of these plants on major noxious paddy weeds (such as Echinochloa crus-galli, Monochoria vaginalis, Rotala indica, Eleocharis acicularis, Scirpus juncoides, Doparium juncencum, Lindernia pyxidaria, and Cyperus difformis) were demonstrated. Some species (Alpinia zerumbet, Ageratum conyzoides, Azadirachta indica, Piper methysticum, Leucaena leucocephala, and Melia azedarach) showed strong inhibition on major plant pathogens (such as Corticium rolfsii, Fusarium solani, Pyricularia grisea, Pythium spp., Rhizopus stolonifer, Taphrina deformans, and Thanatephorus cucumeris), and they may become effective tools in reducing plant pathogens and weeds. However, application of 1–2átons/ha of plant material to the field makes heavy fieldwork. Numerous growth inhibitors (alkaloids, phenolics, fatty acids, lactones, and flavonoids) identified from these allelopathic plants are responsible for their allelopathic properties and may be a useful source for the future development of bio-herbicides and fungicides.
Plants with strong allelopathic potential selected from the plant ecosystems in Southeast Asia showed promising paddy weed reduction and rice yield increase. Some species were effective for plant pathogen suppression.
Allelopathy, allelochemicals, inhibition, pathogen, rice, paddy weed
The current trend in agriculture production is to find a biological solution to reduce the perceived hazardous impacts from herbicides and insecticides (Khanh et al. 2005). Plant allelopathy may be an effective tool to help resolve this critical issue. The detrimental effect of allelopathy can be useful for weed and pest control (Rice 1984). Much research has documented the potential of allelopathic plants to reduce pathogens and affect weed emergence. An explanation of the allelopathy mechanism has been attempted and the influence from a complex of phytochemicals synthesized in plants as secondary metabolites has been proposed. The allelochemicals can be broadly classified as plant phenolics and terpenoids, which show great chemical diversity and are involved in a number of metabolic and ecological processes (Xuan et al. 2005). The most common effects of allelochemicals may occur through leaching, volatilization, root exudation, and the death and decay of the fallen plant parts are either via biotic or abiotic means (Anaya 1990).
The question of what allelopathic plants should be selected, how they are applied, and their benefits should be seen as a requisite before introducing them to the farmers for field usage. This presentation reviews our research carried out in a period of 10 years (1993-2003) on the use of allelopathic plants for pathogen and weed control in paddy rice.
Screening for plants with strong allelopathic potential
The preliminary requirements to determine plants collected for screening should included (a) their invasiveness and area in the plant ecosystems, (b) the plants have less natural weed density in their canopy and surroundings than other plants in their ecosystem, and (c) are traditionally used as green manure, weed or pest management by local farmers. Their plant materials including leaves, stems and roots were collected, dried, and tested for their impacts on indicator plants in bioassays and their magnitude of allelopathic potential was evaluated. Species which exhibited strong allelopathic potential were selected and examined for their impacts on paddy weed emergences in bioassay and greenhouse trials. The influence on major plant fungi was also conducted in laboratory experiment.
In laboratory and greenhouse experiments, plant species which had significant suppression on emergences of major noxious paddy weeds and plant fungi, as well as showed promotion on rice yield, were selected. Their different applied doses and treated times as well as frequency of treatment which bring the most effective impacts to minimize weed biomass and plant fungi growth and promote rice yield were determined. Plant materials were dried, chopped to be at maximum of 2 cm length were incorporated to paddy field. Trials of hand-weeding, herbicides, and control (without any weed management) were also conducted.
Plant species which caused strong reduction of weed emergence, fungi growth, and promotion of rice yield were used for chemical analysis to search for strong bioactive allelochemicals. Analytical methods using HPLC, TLC, GC-MS, LC-MS, IR, and NMR were applied to identify and isolate allelochemicals with strong herbicidal and fungicidal action.
Table 1. Allelopathic potential of some crops and plants selected from the plant ecosystem (in decreasing order of activity).
Screening for plants with strong allelopathic potential
Some hundred plant species in plant ecosystems of Southeast Asia and Japan were surveyed and determined for their allelopathic potential. Their allelopathic magnitudes were averaged from the inhibition of leaves, stems, and roots on the indicator plants. Plants with alleloapthic activity greater than 20% were tested for their impacts on growth of major paddy weeds and plant fungi. The effects of selected plant species are presented in Table 1.
Results from laboratory and greenhouse trials indicated that at 1-2 tons/ha, applied plant species can suppress major paddy weed species such as Echinochloa crus-galli, Monochoria vaginalis, Rotala indica, Eleocharis acicularis, and Scirpus juncoide. The incorporation of plant material to paddy fields should be within 1 week after transplanting. Frequency of treatment caused greater reduction of weed biomass than an individual application. However a dose greater than 2 tons/ha and the frequency of treatment may require more fieldwork and becomes more costly. Plant species which reduced paddy weed biomass by greater than 70% and increased rice yield by greater than 20% are listed in Table 2.
Table 2. Effects of allelopathic plants on paddy weeds and rice in paddy fields as compared with the respective control.
Effects on plant fungi
We examined the impacts of Piper methysticum L.(kava) root extracts on the growth of five fungal species including rhizopus rot (Rhizopus stolonifer MAFF305786), pearl leaf curl (Taphrina deformans MAFF305614), rice blast (Pyricularia grisea MAFF101002), rice shealth blight (Thanatephrorus cucumeris MAFF305844), and papaya dry rot (Fusarium solani MAFF306358). Kava root had a strong allelopathic effect and exhibited strong suppression of paddy weed emergence. In addition, the extract of kava root also significantly inhibited the five noxious fungi studied. R. stolonifer was the most inhibited, and other species were also strongly suppressed by kava. Application of kava root at 200 g L-1 concentration gave 70-100% control of these plant fungi. The magnitude of kava inhibition on individual fungi appeared to be species dependent and proportional to the applied dose.
Allelochemicals isolated from Alpinia zerumbet, and Ananas comosus var. Cayenne (pine apple) were also examined for their biological impact on fungi. 7,8-dihydro-5,6-dehydrokavain (DDK) isolated from Alpinia and its derivative dimethyl phosphorothionate exhibited the greatest antifungal activitity; 91% and 72% against Corticium rolfsii and Pythium spp., respectively. Cinamic, p-coumaric, and ferulic acids were isolated from A. comosus and twenty-four kinds of esters were made from these acids, alcohols, and the components of Alpinia. Among these compounds, isopropyl 4-hydroxycinnamate and butyl 4-hydroxy-cinnamate were observed to have similar antifungal activity on Pythium spp. at 10 ppm.
Thus, the use of plants with strong allelopathic effect may give beneficial effects on both weed and fungi suppression. Our research examining the extracts of plant with strong allelopathic properties on major pests and plant fungi is continuing.
We found various phenolic acids and fatty acids from different plant parts from A. conyzoides, A. indica, Fagopyrum spp., M. indica, M. sativa, and P. methysticum. Two allelochemicals which showed strong herbicidal and fungicial activities and account for great amounts in the plant leaves are mimosine and DDK found in Leucaena and Alpinia, respecitvely. These two compounds may be utilized as leading substances for bioactive herbicides and fungicides.
Amendment of plant materials with strong allelopathic properties brings important agronomic impacts: (i) a 70% reduction in weed biomass and a 20% increase in rice yield, (ii) soil improvement: additional nutrients from plant materials to soil may minimize the quantity of synthetic fertilizers needed for rice or crop growth and development, (iii) the application of allelopathic plants does not provide the same level of weed control as synthetic herbicides, but the remaining of 30% weeds left after application of plant materials can be controlled successfully by a lower dose of herbicides and by rice shading. This leads to a reduction of applied herbicides. Hence, the soil environment may be kept from detrimental effects caused by synthetic agrochemicals, but this point needs further evaluation. In addition, the relative quality of crops grown in the presence of allelopathic plants or agrochemicals also requires detailed analysis. And (iv) the infestation of pests and pathogens may be reduced simultaneously with the reduction of weeds. For instance, A. conyzoides L.(billy goat weed), F. esculentum Moench (buckwheat), M. azedarach L.(Chinaberry), P. methysticum L.(kava), and A. indica A. Juss (neem) gave growth reduction of pests and plant diseases. Further determination of whether allelochemicals or the other compounds available in the allelopathic plants are responsible for pest and pathogenic suppression is needed. Identified strong bioactive allelochemicals are a useful source for the development of biological herbicides and fungicides.
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