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Allelopathic potential of buckwheat

Zahida Iqbal, Anna Golisz, Akihiro Furubayashi, Habib Nasir and Yoshiharu Fujii

Department of Biological Safety Science, National Institute for Agro-Environmental Sciences, 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan. E-mail: zahida@affrc.go.jp

Abstract

The search for sustainable and environmentally friendly farming methods has renewed interest in use of cover crops. Bioactivity guided solvent extraction, chromatographic purification and identification by advanced instrumentation (Mass and NMR spectroscopy) have yielded various allelochemicals from the aerial parts of tartary buckwheat (Fagopyrum tataricum), which possess catechol moiety in their structures. All of these compounds inhibited the root and shoot growth of test species at less than 100 ppm. Although these are reported in literature as natural product yet it is the first time that these are being investigated for their plant growth inhibitory activity.

These findings suggest that buckwheat can be used as cover crop as the allelochemicals released from the living or decaying plant material can reduce the growth of various co-occurring weeds effectively.

Media Summary

Allelopathic activity of tartary buckwheat was investigated using total activity (biological activity per unit weight of the organism) approach. The findings suggest that buckwheat can be used as ground cover crop to control weeds.

Key Words

Allelopathy; plant growth inhibitors; rutin; quercitrin; Fagopyrum tataricum; cover crop.

Introduction

In plant-plant interactions, allelopathy denotes the process by which plants release phytotoxic compounds (allelochemicals) in the soil environment, resulting in a harmful/beneficial effect on neighbouring plants. Both crops and weeds contain compounds that can be considered allelopathic. Crop species with allelopathic potential have been given greater attention during the last two decades (Chou 1999; Shilling et al. 1985; Weston 1996). However, only few crops have been studied thoroughly in terms of their weed suppressing activity (Einhellig 1996; Rice 1979). Crops with allelopathic properties may suppress subsequent crop growth. Recently, utilization of the allelopathic potential of plants for weed control instead of herbicide application is given great emphasis, because it would reduce the risk of environmental toxicity.

Buckwheat is an agronomic species of polygonaceae family that belongs to genus Fagopyrum. There are about 15 species in the genus Fagopyrum, which occur in temperate areas of Euro-Asia. Plants in this family produce a wide array of biologically active constituents. As fast-growing cover crop, buckwheat is most useful for weed suppression. Fagopyrum esculentum Moench (common buckwheat) is known to Japanese farmers as cover crop since long for limiting soil erosion. There are some reports about its allelopathic activity in field and green house. Experiments conducted under field conditions have shown that buckwheat shades and smothers weeds, or out competes them for soil moisture and nutrients (Tominaga and Uezu 1995). Fagopyrum esculentum is an economically important forage, grain and cover crop and has shown allelopathic activity in field and green house (Isojima et al. 2000). Previously we reported several allelochemicals from F. esculentum (Iqbal et al. 2002; 2003). In continuation to our work another member of this genus F. tataricum Gaertner (tartary buckwheat) was investigated for its allelopathic potential. In this investigation, we isolated two flavonol glycosides and a flavonoid from the upper part of F. tataricum and determined their allelopathic effects on seed germination and seedling growth of lettuce (Lactuca sativa L.).

Methods

Instruments

1H NMR spectra were recorded at 600 MHz, and 13C NMR spectra at 150 MHz on JEOL JNM α 600 spectrometer in CD3OD. Tetramethylsilane (TMS) was used as an internal reference. Silica gel GF254, analytical chromatoplates, Sephadex-LH 20, and silica gel grade 60, 70-230 mesh for column chromatography were purchased from Wako, Japan and Merck, Germany.

Isolation and identification of allelopathic substances

In this experiment, we focused on determination of the allelopathic potential of crude extract of F. tataricum shoot, along with all of its organic solvent fractions using seedling elongation test with pre germinated lettuce seeds. Freshly cut aerial parts of F. tataricum were extracted with 80 % aqueous ethanol. The water suspension of the original aqueous ethanolic extract was subjected to a liquid–liquid partition to obtain hexane, ethyl acetate, and butanol sub-fractions. Various chromatographic purification techniques (silica gel, Sephadex gel, HPLC and thin layer chromatography) and advanced instrumentation (mass and NMR spectroscopy) were used to isolate, identify and quantify the allelochemicals from the aqueous ethanolic extract of aerial parts of F. tataricum following the same procedure as described earlier (Iqbal et al. 2003). Three phytotoxins implicated as allelochemicals were identified from the most active fractions (ethyl acetate fraction). The allelopathic potential of all the isolated compounds was examined by testing their biological activities on lettuce.

Results

Comparison of the inhibitory activity on lettuce root growth revealed that the major activity of the original extract was fractionated into the EtOAc fraction (Figure-1).

Figure-1. Inhibitory activity of crude extract of buckwheat and its organic solvent fractions on root and shoot growth of lettuce seedlings. Plotted values are mean of three experiments. Each replication comprises 15 seeds. Error bars represent standard deviation.

Purification of the ethyl acetate soluble fraction by repeated chromatographic procedures led to the isolation of allelochemicals, which were identified by NMR spectroscopic techniques as rutin (1, 600 mg), quercetrin (2, 22 mg) and quercetin (3, 18 mg). The structures of these known compounds were confirmed by co-chromatography with authentic samples and comparison of their spectroscopic data with those reported in literature, (Figure-2). Although these are reported in literature as natural product yet it is the first time that some of these are being investigated for their plant growth inhibitory activity.

Figure-2. Structures of isolated allelochemicals from F. tataricum

The bioactivity of all these compounds was tested on lettuce seedlings and was found to be significantly inhibitory. The EC50 was found to be in the order of fresh weight (fw), 98 μg g-1, 45 μg g-1, and 90 μg g-1 fw for rutin, quercetrin and quercetin, respectively. All of these compounds inhibited the root and shoot growth of lettuce seedling and root growth was more sensitive to these compounds than the shoot. Among these compounds, quercetrin was found to be the most active, the concentration required for 50 % inhibition on lettuce roots was 45 μg g-1 as interpolated from the concentration- response curve.

The analysis of variance for all data was measured using the general linear model procedure of SAS/STAT (1990). Effective concentration required for 50% inhibition (EC50) of test species was calculated based on fitted regression equation.

Figure-3. Inhibitory activity of rutin (A), quercitrin (B) and quercetin (C) on the root () and shoot (▲) growth of lettuce seedlings.

On the other hand, EC50 value of rutin was not as low as the other isolated compounds but it was found in much higher amount. This makes it evident that the purified compound of low specific inhibitory activity may play a greater role in the expressed inhibitory potential than those of high specific inhibitory activity by virtue of being present in larger quantity (Lehle and Putnam 1983). In the present study total activity of all the inhibitory compounds was also calculated and among these, total activity of rutin was found to be the highest. Recently it has been reported that a compound with high total activity might acts as communication device among organisms in the natural ecosystem (Hiradate et al. 2004).

Among buckwheat species, F. tataricum is a species to note because it contains 50-130 times much higher amount of rutin. All the flavonoids reported here, especially rutin are known to have antibacterial, antiviral, antihypertensive and antioxidant activities. These studies show that rutin is the potential allelochemical of F. tataricum, which along with other active constituents is responsible for the allelopathic activity of buckwheat. It is therefore possible that in future buckwheat could be used as ground cover crop, this will allow long-term weed suppression in agro-economic system through the release of allelochemicals.

References

Chou CH (1999) Roles of allelopathy in plant diversity and sustainable agriculture. Crit. Rev. Plant Sci. 18, 609-636.

Einhellig FA (1996) Interactions involving allelopathy in cropping system. Agronomy Journal 88, 886-893.

Iqbal Z, Hiradate S, Noda A, Isojima S, Fujii Y (2002) Allelopathy of buckwheat: assessment of allelopathic potential of extract of aerial parts and identification of fagomine and other related alkaloids as allelochemicals. Weed Biology and Management 2, 110-115.

Hiradate S, Morita S, Sugie H, Fujii Y (2004) Phytotoxic cis-cinnamoyl glucosides from Spiraea thunbergii. Phytochemistry 65, 731-739.

Iqbal Z, Hiradate S, Noda A, Isojima S, Fujii Y (2003) Allelopathic activity of buckwheat: Isolation and characterization of phenolics. Weed Sci. 51, 657-662.

Isojima S, Iqbal Z, Koizumi A, Fujii Y (2000) Allelopathy of Fagopyrum esculentum: Analysis of allelochemicals. J. of Weed Sci. Technol. (supplements in Japanese) 45, 92-93.

Lehle FR and Putnam AR (1983) Allelopathic potential of sorghum (Sorghum bicolor): Isolation of seed germination inhibitors. J. Chem. Ecol. 9, 1223-1235.

Rice EL (1974) Role of allelopathy in patterning of vegetation and creation of bare areas. Pages 126-173 in Allelopathy. New York, Academic Press.

Rice EL (1979) Allelopathy. An update. Bot. Rev. 45, 15-109.

Shilling DG, Libel RA, Worsham AD (1985) Biochemical interaction among plants. In: The chemistry of allelopathy: (ed. by A.C. Thompson) ACS Symposium Series 268 American Chemical Society, Washington DC, 243-271.

Tominaga T and Uezu T (1995) Weed suppression by buckwheat: Pages 693–697 in T. Matano and A. Ujihasa, eds. Current Advances in Buckwheat Research. Volume 2. Proceedings of the 6th International Symposium of Buckwheat. Nagano, Japan: Shinshu University Press.

Weston LA (1996) Utilization of allelopathy for weed management in agro ecosystems. Agronomy Journal 88, 860-866.

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