Agronomy Journal 95:1063-1070 (2003)
© 2003 American Society of Agronomy
ALLELOPATHY
Comparison of Allelopathic Potential of Rice Leaves, Straw, and Hull Extracts on Barnyardgrass
I. M. Chunga,
K. H. Kima,
J. K. Ahn*,a,
S. B. Leeb,
S. H. Kima and
S. J. Hahna
a Dep. of Crop Sci., Konkuk Univ., Seoul, South Korea, 143-701
b Div. of Life and Environ. Sci., Natl. Crop Exp. Stn., RDA, Suwon, South Korea, 137-030
* Corresponding author (jkahn{at}konkuk.ac.kr)
Received for publication December 2, 2002.
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ABSTRACT
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The use of rice (Oryza sativa L.) allelopathy for weed control is a new technology in agronomy. A laboratory bioassay using water extracts was conducted to determine the allelopathic potential of rice body parts on seed germination and growth of barnyardgrass (Echinochloa crus-galli P. Beauv. var. oryzicola Ohwi) and to determine rapid and simple methods for selecting allelopathic rice varieties using genetic characters and phenotypes. In this study, the highest inhibition rate was for ‘Danganeuibangju’ (76.9%) in straw extracts, ‘Dongobyeo’ (74.1%) in the leaves, and ‘Baek’ (31.7%) in the hull. ‘CUBA 65-v-58’ (38.6%) had the highest inhibition as a whole (average of leaves, straw, and hull), and there was a higher average inhibitory effect for straw extracts (21.6%) than for hulls (8.2%) and leaves (12.4%). With regard to classification by phenotypic and genetic characteristics, these groups showed a higher inhibitory effect in domestic varieties (14.2%), middle-maturing varieties (15.3%), varieties of hull color (15.1%), and varieties of awn color (16.0%). These results suggest that rice body parts may be a source of natural herbicides and that it is necessary to develop acceptable selection standards. There may also be genetic variation in rice varieties for their allelopathic potential on barnyardgrass. In the future, it might be possible to develop rice varieties with high allelopathic potential.
Abbreviations: GP, germination percentage • GR, germination rate • HPLC, high-performance liquid chromatography • TDW, total dry weight
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INTRODUCTION
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WEED MANAGEMENT during the crop season has been a serious problem for many years. Worldwide, a 10% loss of agricultural production can be attributed to the competitive effect of weeds, despite their intensive control. Indeed, three million tonnes of herbicides per year are used in most agricultural systems (Stephenson, 2000). Potential yield reductions caused by uncontrolled weed growth throughout a crop season have an estimated range of 45 to 95%, depending on ecological and climatic conditions (Ampong-Nyarko and De Datta, 1991; Moody, 1991). Therefore, weed management is a key element of most agricultural systems. The application of herbicides was a major factor enabling the intensification of agriculture in past decades, but there has been increasing herbicide resistance in weeds and widespread concern about adverse environmental effects from herbicide use. For this reason, the use of allelopathic rice varieties may provide an alternative to minimize the risk to agroecosystems by serving in a complementary fashion with herbicides.
The term allelopathy was coined by Molisch in 1937 and as a new method of weed control could lead to reduced labor costs and increased efficiency. His definition referred to both the detrimental and beneficial biochemical interactions among all plant species, including microorganisms. Since Dilday et al. (1991) found allelopathic potential in rice accessions in the USA, several scientists have suggested similar allelopathic activity (Hassan et al., 1994; Olofsdotter et al., 1995). Since then, the International Rice Research Institute (IRRI) and countries such as Japan, Egypt, and Korea have been actively studying the phenomenon. Putnam and Weston (1986) noted that substances with allelopathic potential are present in virtually all plant tissues, including stems, leaves, roots, and seeds. These substances are released through processes such as volatilization, root exudation, leaching, and decomposition of plant residues. It is also known that allelochemicals can affect mineral uptake by altering the cellular membrane function in plant roots. Phenolic acids depolarize the electrical field across membranes and thereby inhibit the active absorption of mineral ions (Balke, 1985). However, the mere presence of allelopathic substances in the plant is not sufficient to prove allelopathy. Rather, allelopathic activity is believed to be the joint action of several secondary metabolites that may act synergistically (Chou et al., 1991; Olofsdotter et al., 1995; Geally et al., 2000; Chung et al., 2001a, 2001b). Most allelochemicals are released during germination and early growth (Dekker and Meggitt, 1983). However, barley (Hordeum vulgare L.) releases the largest amount of allelopathic alkaloids (
2 g plant-1 d-1) 36 d after germination in hydroponic culture (Liu and Lovett, 1993b). Among the known allelochemicals are phenolic acids, flavonoids, terpenoids, alkaloids, and quinones. Of these substances, terpenoids have an approximate activity range of 0.3 to 10.5 µg kg-1, which are at much lower concentrations than the traditionally considered phenolics, quinones, or alkaloids (Macias, 1993). Most plant chemicals associated with allelopathic activity are secondary metabolites from shikimic acid or acetate pathways (Rice, 1984, 1985; Rizvi and Rizvi, 1992). However, Macias (1993) suggested the pathway of terpenoids as another promising allelochemical synthetic method.
A field experiment was conducted at the University of Arkansas to screen allelopathic effects of 10000 rice accessions against several paddy weeds; about 4% of these accessions had allelopathic effects with one or more weed species. In particular, the allelopathic effect of rice on paddy weeds had a higher growth inhibition on the root than the shoot (He, 2000). Allelopathic accessions had six to nine times heavier root dry weights than nonallelopathic accessions (Dilday et al., 1989, 1991). In addition, the japonica type had higher allelopathic activity than the javanica type. Red rice accessions (O. sativa L.) and African rice varieties (O. glaberrima L.) species had high activity while the highest activity was found in the Deng Mak Tek accessions (Fujii, 1993).
Allelopathy can be used in weed management in two ways. The first is by selecting an appropriate crop variety or incorporating an allelopathic character into a desired crop variety. The second way is by applying residues and straw as mulches or growing an allelopathic variety in a rotational sequence that allows residues to remain in the field (Rice, 1995).
Resource competition is difficult to separate from allelopathy under field conditions. To overcome this problem, various laboratory screening techniques have been developed to measure allelopathy without the interference of resource competition. For example, several methods such as the stair-step method (Bonner, 1950; Liu and Lovett, 1993a), hydroponic culture test (Einhellig et al., 1985), relay seeding technique (Navarez and Olofsdotter, 1996), cluster analysis using high-performance liquid chromatography (HPLC) (Mattice et al., 1999), water extraction method (Ahn and Chung, 2000; Kaworu et al., 2001), and agar medium selection (Fujii, 1992; Wu et al., 1999) have been reported and tested for bioassays. These methods gave a similar result in both the laboratory and the field.
The objective of this study was to evaluate the rice germplasm collection for allelopathic activity and, on this basis, establish the genetic differences of allelopathic effects in the laboratory.
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MATERIALS AND METHODS
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Preparation of Rice Extracts
One hundred and fourteen rice (O. sativa L.) varieties, including ‘AC 1423’, were grown in a field at Konkuk University and harvested and separated in October 2001. The harvested plants (leaves plus straw) were dried at room temperature (24°C), and then their separated body parts (hull, leaf, and straw) were ground through a 40-mesh screen and stored at 5°C until needed. Aqueous extracts (w/v) were prepared by extracting 5-g ground samples with 100 mL of distilled water and stirred for 24 h at room temperature with a slight modification of the method developed by Ahn and Chung (2000). The solutions were filtered through four layers of cheesecloth to remove fiber debris and centrifuged at low speed (3000 rpm) for 4 h, and the supernatant was filtered through one layer of filter paper (Whatman no. 42). To prevent microorganism growth, the solutions were then filtered again using a 0.2-mm Nalgene filter unit (Becton Dickinson Labware, Lincoln Park, NJ). Fungal contamination was not observed during these experiments.
Bioassay in the Laboratory
Barnyardgrass seeds were purchased from Seminis, Korea, in 2001, and 100 were placed on a filter paper (Whatman no. 42) in sterilized 9-cm Petri dishes. Ten milliliters of each extract solution, made from the rice body parts, was added, and a control containing 10 mL of distilled water was also used. The Petri dishes were placed in an illuminated growth chamber at 24°C. A germination test was conducted after 7 d, the rate of which was calculated by dividing the number of germinating seeds each day by the number of days and summing the values (Maguire, 1962). After the germination test, all seeds that had germinated over the 7 d were dried at 65°C for average dry weight determination. The inhibitory percentage was calculated by the following equation:
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Statistical Analysis
The laboratory bioassay was conducted with three replications using a completely randomized design and the data combined as there were no significant differences among cultivars over the 2 yr of the experiment. The analysis of variance for all data was conducted using the general linear model procedure of the Statistical Analysis System (SAS) program. Pooled mean values were separated based on least significant difference (LSD) at the 0.05 probability level (SAS Inst., 1988).
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RESULTS AND DISCUSSION
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Allelopathic Potential in Rice Body Parts
The average inhibition rate in barnyardgrass from extracts of 114 rice varieties, including ‘Philippine 2’, was highest for CUBA 65-v-58 (38.6%) and lowest for ‘DOU-U-LAN’ (0.4%). On average, straw extracts (21.6%) had a higher inhibitory effect than hulls (8.2%) and leaves (12.4%) (Table 1). Fifty-eight rice varieties, including ‘Duchungjong’, had inhibitory effects of about 10% while six varieties, including CUBA 65-v-58, had inhibitory effects greater than 30% (Fig. 1). Rice varieties were fractioned in three groups by the allelopathic activity of their body parts. The first group was leaf extracts, of which nine varieties had average inhibition rates greater than 40%. Among this group, Dongobyeo (74.1%) had the highest inhibition rate while DOU-U-LAN (-6.6%) had the lowest (Fig. 1). CUBA 65-v-58 (79.8%) had the highest inhibition of GR, and 15 varieties, including Dongobyeo, had inhibition percentages greater than 40%. Dongobyeo also had the highest inhibition of GP (73.3%) and TDW (74.5%) (Table 1). The second group was straw extracts, of which seven varieties had average inhibition rates greater than 40%. The highest inhibition percentage (76.9%) was for ‘Dangneunbangju’ (Fig. 1), which also had the highest inhibition of GR (86.9%), GP (71.4%), and TDW (72.3%) (Table 1). ‘Olbyeo’ (0.1%) had the lowest inhibition percentage of this group (Table 1). The third group was rice hull extracts, which included six varieties, including ‘Dadajo’, with average inhibition rates greater than 20%. Baek (31.7%) had the highest inhibition percentage while DOU-U-LAN (-8.7%) had the lowest (Fig. 1). ‘Rikuu 132’ had the highest inhibition of GR (32.6%) and GP (24.5%), and Baek had the highest inhibition of TDW (45.4%) (Table 1).
This bioassay with water-soluble extracts using barnyardgrass as a test plant was easier to conduct than a field test for allelopathic activity, which would have required several years of replication. In this study, the inhibition of germination and dry weight of barnyardgrass using rice body parts may reflect the allelopathic potential of individual rice varieties. Results of this study are in agreement with Dilday et al. (1994), Chung et al. (1997), Ahn and Chung (2000), and Olofsdotter et al. (1995), who concluded that there was variation in allelopathic effects among varieties. Chung et al. (1997) reported that allelopathic potentials were higher in rice hull extracts than in leaf extracts in a study on the control of barnyardgrass (hull: 46%, leaves: 76%). However, rice hulls had low inhibition rates compared with the leaf and straw extracts in this study. It is believed that different results among studies are due to different genetic factors and climatic and soil environments of the cropping year. Additionally, among the three body parts, hulls only occur in small quantities, and theoretically basing this bioassay on the field, 14850 kg hulls ha-1 would have been required. Inhibitory chemicals of hull extracts are actually greater in the field than in this study, and it is possible to understand why farmers in Korea generally leave a large amount of rice hulls in the field (Ahn and Chung, 2000).
Generally, the allelopathic effect was known to involve many secondary metabolites, which reacted with one another, and allelochemicals were synthesized by either the shikimic acid or acetate pathways (Rice, 1984, 1985; Rizvi and Rizvi, 1992). Olofsdotter et al. (1995) and Geally et al. (2000) reported that allelopathic chemicals, including ferulic acid, are abundant in rice straw. Chung et al. (2001a) also isolated phenolic acids, including o-hydroxyphenylacetic acid, from rice straw and nine phenolic acids from rice hulls. These chemicals inhibited seed germination and seedling growth of barnyardgrass at concentrations of 1 x 10-3 M and sometimes even lower (Chung et al., 2002).
Other than the hulls, extract solutions of the plant body parts in this study were slightly acidic in solution (pH 6.7–7.0). As such, the allelopathic potential can be regarded as being related to the pH value, which steadily obstructs the growth of barnyardgrass over time. Additionally, in acidic solution, it is believed that a low GR would potentially lead to a low GP and TDW (Ballester et al., 1979; Chung et al., 1997; Ahn and Chung, 2000).
The main purposes of this study were to evaluate rice varieties for allelopathic potential by looking at three body parts (hull, leaf, and straw) and to provide basic information for breeding allelopathic varieties through the extension of databases.
Comparison of Allelopathic Rice Varieties using Genetic Characters and Phenological Types
As allelopathic potential consists of complex reactions between the plant and conditions such as water stress, temperature, light, and plant age, it is difficult to select for allelopathic varieties in rice germplasm. Therefore, many methods have been developed to find a test plant that could be used to assess allelopathic activity. Most of these methods were observations of the inhibition of germination or seedling growth of barnyardgrass in vitro or in the field (Chung et al., 1997). In this study, selection for allelopathic varieties was classified into four groups: time to maturity, existence or nonexistence of the awn, colored or colorless hulls, and plant origin.
Comparison of Allelopathic Rice Varieties from Different Origins
The average inhibition percentage with the region of origin was 14.2% for domestic varieties and 12.9% for foreign varieties. However, the differences were not significant (Fig. 2). Straw extracts had the highest inhibitory effects, followed by the hull and leaves for the domestic and foreign varieties. Straw extracts also produced lower GR in foreign varieties (30.5%) in the first 3 d compared with domestic varieties. Hull extracts significantly inhibited GR and TDW for domestic and foreign varieties but not GP. This inhibition is believed to be more related to GR over the first 3 d than TDW. Leaf extracts significantly affected GR, GP, and TDW for the domestic and foreign varieties, and domestic varieties had a high inhibitory effect on GR (21.7%), GP (12.8%), and TDW (9.4%). In particular, inhibition of GR and GP of the domestic varieties was double that of the foreign varieties (Table 2). The average inhibition percentage was highly correlated with GR (r2 = 0.93***, where *** = significant at the 0.001 level), GP (r2 = 0.96***), and TDW (r2 = 0.93***) for the domestic varieties (data not shown). Many studies have indicated that allelopathic potential can be found in both traditional and improved varieties of different origins (Dilday et al., 1991; Fujii, 1992; Hassan et al., 1994). Fujii (1992) showed that improved cultivars showed less allelopathic activity than domestic varieties with the japonica rice varieties. In this study, even if it was not significant, domestic varieties (14.2%) had higher inhibitory effects than foreign varieties (12.9%). Consequently, the selection of allelopathic varieties within different origins was not substantiated by these results.
Comparison of Allelopathic Rice Varieties with Different Maturity Times
The average inhibition percentage with maturity was 12.8% for early maturing varieties, 15.2% for middle-maturing varieties, and 13.2% for late-maturing varieties. Although these values were not significant (Fig. 3), the average GP and TDW were significant for early, middle-, and late-maturing varieties, and the middle-maturing varieties were most highly inhibited (GP: 15.6%, TDW: 12.5%). Straw extracts of these three varieties had a higher inhibitory effect than hull and leaf extracts, but it was only significant for TDW, which was especially low for the early maturing varieties. Hull extracts significantly inhibited GR, GP, and TDW, and early maturing varieties had the highest inhibitory effects (GR: 11.4%, GP: 10.2%, TDW: 8.4%). Leaf extracts significantly inhibited GR, GP, and TDW, which was high compared with hull extracts for the middle-maturing varieties (GR: 22%, GP: 14.3%, TDW: 12.9%). These results assumed that the inhibition of GR was closely related to a high inhibitory effect of the GP and TDW (Table 3). Wu et al. (1976) noted that plants produce different allelochemicals at different developmental stages and that the amount released also varies. In particular, allelochemicals that released the largest amounts of allelopathic alkaloids are released during germination and early growth (Dekker and Meggitt, 1983). Although the three different maturing varieties used in this study did not show significant average inhibition percentages, it is still worth conducting further studies on allelochemicals because the GR and GP (within the first 7 d) of the middle-maturing varieties were highly inhibited.
Comparison of Allelopathic Rice Varieties with the Existence or Nonexistence of Hull Color
The average inhibition percentage with the existence or nonexistence of hull color was 15.1% for the colored hull and 13.5% for the colorless hull. Straw extracts had a higher inhibition percentage than hull extracts (Fig. 4). Straw extracts inhibited the GR and GP in the colored and colorless hull varieties, respectively, but it was not significant. Hull extracts produced significant differences in the GR and GP between the colored and colorless hulls, but differences in the TDW were not significant. These results suggested that germination was lower in seeds with colorless hulls, but variations in weight after 7 d were not statistically different between the colored and colorless hulls. Leaf extracts had higher inhibitory rates on GR than GP and TDW, and GR was especially inhibited in the colored varieties (26%). Other than for the TDW, GR and GP were also significantly affected, and these results indicated that inhibition of seed germinated for 7 d was greater for colorless than colored hulls. However, increasing weight was another statistically different factor (Table 4). In general, colored hulls had higher inhibition rates (GR: 19.8%, GP: 14.8%, TDW: 10.7%) than colorless hulls (GR: 17.7%, GP: 13.9%, TDW: 9.0%) although the differences were not significant (Table 4).
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Fig. 4. Allelopathic effect on barnyardgrass from rice body parts with the existence or nonexistence of hull color.
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Table 4. Inhibition percentage of extracts from rice body parts with the existence or nonexistence of hull color on barnyardgrass.
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Generally, hull extracts either stimulated or inhibited barnyardgrass growth. Various methods have been used to measure the allelopathic effects of rice hulls on weeds. These include the concentration of hull extracts as well as the fact that warm water extracts had higher inhibition rates than hot water extracts and that CHCl3–soluble portions extracted germination inhibitors that were not extracted by n-hexane (Tadahiro et al., 1977; Ahn and Chung, 2000). Rice (1984) reported stimulatory effects at low concentrations of allelopathic substances but inhibitory effects at higher concentrations. Recently, Chung et al. (2002) reported that preliminary identification by HPLC analysis resulted in peaks with retention times near those of standards, including p-hydroxybenzoic acid, which was confirmed with selection impact mass spectrometry (EI/MS). Hull extracts from the allelopathic cultivar Janganbyeo contained higher levels of p-hydroxybenzoic acid than those of the nonallelopathic cultivars Labele and Baekambyeo. These results suggested that the concentration and composition of potentially allelopathic compounds depended on the cultivars and especially on their hulls. Colorless hulls had higher inhibitory effects than colored hulls, and this factor will be used in further studies on allelochemicals.
Comparison of Allelopathic Rice Varieties with the Existence or Nonexistence of an Awn or Awn Color
The average inhibition percentages were 16.0% for the colored awn, 12.0% for the colorless awn, and 14.0% for awnless varieties, which were significant values (Fig. 5). Straw extracts had higher inhibitory effects than hull and leaf extracts and showed a high inhibition in GR and GP between the awn and awnless varieties, respectively. However, only the GR was significant, indicating that the awnless varieties effectively inhibited germination within the first 3 d for both the colored and colorless awns. Hull extracts had low inhibitory effects on GR, GP, and TDW among three groups, but there was one notable exception. Hull extracts of the awnless varieties had a high inhibition on the GR (11%), GP (9.9%), and TDW (9.5%) for both the colored and colorless awns. Leaf extracts also significantly inhibited the GR (24.4%), GP (16.4%), and TDW (13.2%) of the colored awn varieties compared with the colorless awn and awnless varieties. The high inhibitory effect within the first 3 d was correlated to the inhibition of the GP (r2 = 0.89***) and TDW (r2 = 0.77***) in the colored awn varieties. In general, the average inhibition percentage of the GR was higher than that of both the GP and TDW. Straw extracts, when compared with hull and leaf extracts, had a higher inhibition of GR, GP, and TDW (Table 5 and Fig. 5).
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Fig. 5. Allelopathic effect on barnyardgrass from rice body parts with the existence or nonexistence of an awn and color.
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Table 5. Inhibition percentage of extracts from rice body parts with the existence or nonexistence of an awn or awn color on barnyardgrass.
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In this study, the selection pressure for nonallelopathic varieties using the phenotype (existence or nonexistence of the awn and hull color) was weak. However, hull extracts, compared with straw and leaf extracts, inhibited GR and GP when the phenotypic index was used. Therefore, it is believed valuable to research allelopathic rice varieties based on phenotype.
The total inhibition percentage of the extract solution with plant parts was correlated more with GR, GP, TDW, and pH than with phenotypic and genetic characters such as time to maturity and existence or nonexistence of the awn. Among the plant parts, the straw extract solution had a higher total inhibition percentage than the hull and leaf extract solutions. In addition, among the high total inhibition percentages, the inhibition percentage of GR was highly correlated with GP (r2 = 0.8***), TDW (r2 = 0.73***), and total inhibition percentage (r2 = 0.92***). The pH value (r2 = -0.5***) was negatively correlated with the total inhibition percentage (data not shown).
This study suggests that the allelopathic compounds present in rice hulls, leaves, and straws may serve as a potential natural herbicide by inhibiting seed germination and growth of barnyardgrass, which has become a problem because of increased use of direct seeding of rice to reduce production costs. If these varieties are used to contribute to the control of barnyardgrass, they may also be used as genetic markers to identify allelopathic varieties.
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ACKNOWLEDGMENTS
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The authors acknowledge the financial support of the Rural Development Administration made in the Biogreen 21 project year of 2002.
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