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Agronomy Journal 92:1162-1167 (2000)
© 2000 American Society of Agronomy

ALLELOPATHY

Allelopathic Potential of Rice Hulls on Germination and Seedling Growth of Barnyardgrass

Joung Kuk Ahn and Ill Min Chung

Department of Crop Science, College of Agric. and Life Science, Konkuk Univ., KwangJinKu MoJinDong, Seoul, South Korea 143-701

imcim{at}kkucc.konkuk.ac.kr


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Hull extracts from 91 cultivated rice cultivars (Oryza sativa L.) were used to determine their allelopathic potential on seed germination and seedling growth of barnyardgrass (Echinochloa crusgalli P. Beauv. var. oryzicola Ohwi). The allelopathic effects of various concentrations of hot and warm water hull extracts from selected cultivars were also investigated. In the initial screening the `SR31' extract inhibited germination 59%. The length and dry weight of roots were more affected by hull extract than the shoots. The greatest total seedling length and dry weight inhibition was from the `Janganbyeo' warm extracts and was 75 and 96%, respectively. Rice cultivars demonstrating significant allelopathic potential were compared using varying concentrations with a hot or warm extraction procedure. The two extraction procedures displayed different ultra violet (UV) absorption, pH, electrical conductivity (EC), and osmotic potential. Warm extracts exhibited a lower percentage absorbance in the UV range, higher percentage absorbance in the visible range, higher EC (S m-1) and osmotic potential (kPa), and showed lower pH than those of hot extracts. As concentration increased, the warm extracts had a greater inhibitory effect on barnyardgrass germination, seedling growth, weight, and caloric content than the hot extracts. The greatest inhibition occurred when the highest concentration (8 g L-1) warm water extract was applied. These results suggest that rice hull extracts may be a source of natural herbicide, and warm water may extract more allelochemicals than hot water. There may be genetic differences among rice cultivars for allelopathic potential on barnyardgrass. The breeding of rice cultivars with greater allelopathic potential may be possible.

Abbreviations: UV, ultraviolet • EC, electrical conductivity • LSD, least significant difference


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
MUCH ATTENTION has focused on the allelopathic effects of rice (Oryza sativa L.) as a potential tool for ecologically sound weed control (Chou and Lin, 1976; Chung et al., 1997; Dilday et al., 1994; Fujii, 1992, 1993; Hassan et al., 1994; Lin et al., 1993; Olofsdotter et al., 1995; Tamak et al., 1994; Yang and Futsuhara, 1991; Young et al., 1989). These researchers evaluated the allelopathic potential of rice cultivars by analyzing leaf and straw extracts, decomposing straw, and the soil where rice was grown.

In field experiments, Dilday et al. (1994) evaluated about 10000 accessions of different origin from a worldwide rice germplasm collection (including the USA) for allelopathic effects on ducksalad [Heteranthera limosa (Sw.) Willd.], and other annual broadleaf weeds. The authors evaluated allelopathic potential by measuring the weed-free radius surrounding the base of rice plants within a hill. Chung et al. (1997) compared the phytotoxicity of leaf, stem, and hull extracts and their mixture for 47 rice cultivars on barnyardgrass (Echinochloa crus-galli P. Beauv. var. oryzicola Ohwi). They reported significant variability for allelopathic effects among cultivars that rice hull extracts contained more water-soluble substance toxic to barnyardgrass than leaf or stem extract. These studies suggested potential for developing allelopathic weed suppression through plant breeding.

Little information is available on breeding rice cultivars with higher allelopathic potential. Rice, one of the main food crops in Korea, is grown during the summer on about 1066000 ha. Heavy use of fertilizers, herbicides, and other pesticides may pollute water and soil in the paddy ecosystems (Chung et al., 1997). As cultural practices for rice change from transplanted to direct-seeded to reduce production costs, weed problems like barnyardgrass will be more serious because rice and weeds can emerge together. Barnyardgrass, one of the greatest yield limiting (57–95%) weeds in the irrigated rice system of Korea, is expected to cause a greater problem in direct-seeded because it is better adapted to emerge under aerobic than wet anaerobic conditions. Direct-seeded rice is expected to have a greater reliance on herbicides to control weeds. Therefore, with an increase in direct-seeded rice and the implementation of conservation tillage practices, the development of rice cultivars with proven allelopathic characteristics could provide an environmentally acceptable and low-cost approach for barnyardgrass control.

The main purpose of this research was to study the effect of aqueous hull extracts from 91 cultivated rice cultivars on barnyardgrass germination and seedling growth and to compare the allelopathic potential of various concentrations of hot and warm water rice hull extracts. The results of this study will propose a method to evaluate the allelopathic potential of parental lines breeding.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Allelopathic Activity of Hull Extracts
Ninety-one rice cultivars were grown at the Konkuk University farm, and harvested in October 1996. Since aqueous hull extracts had shown more activity than leaf extracts in a previous study (Chung et al., 1997), hulls from these cultivars were dried (24°C) and ground in a Wiley mill through a 40-mesh screen. Ground hulls (5 g) were soaked in 100 mL distilled water for 24 h at 24°C in a lighted room. The solutions were filtered through four layers of cheese cloth to remove debris and then centrifuged at 3000 rpm for 4 h. The supernatant was filtered through one layer of Whatman no. 42 filter paper. To prevent microorganism growth, the solutions were filtered again through a 0.2-mm Nalgene filter (Becton Dickinson Labware, Lincoln Park, NJ).

Barnyardgrass seed was collected in October 1996 and stored at -35°C. Trash was removed from the seeds by floating them in distilled water. Before the bioassy, seeds were surface sterilized in a 1:10 (v/v) dilution of commercial hypochlorite bleach for 10 min and rinsed several times with distilled water. Seeds were placed on moistened paper towels for 2 h. One hundred barnyardgrass seeds were placed on filter paper in a sterilized 9-cm petri dish. Ten mL of the extract solution was added to each petri dish and distilled water was used as a control. All petri dishes were placed in a lighted growth chamber at 24°C. Percent germination was determined after 5 d, and seedling root and shoot lenghts were measured after 9 d. Seedlings were separated into roots and shoots and oven-dried at 65°C for 4 h. Percent inhibition was calculated as [(Control-Aqueous extracts)/Control] x 100.

Effect of Hull Extract Concentration and Extraction Temperature on Allelopathic Activity
Various concentrations of hull extracts from `Janganbyeo', the rice cultivar with the strongest allelopathic activity in the above bioassay, were obtained at two different water temperatures by shaking for 4 h in the light 2, 4, 6, and 8 g of dried ground hulls in a 250-mL flask with 100 mL double distilled water immersed in a water bath at either 24°C (warm temperature extract) or 80°C (hot temperature extract). Extract processing, bioassay conditions, and plant measurements were conducted as in the above bioassay. After 9 d of growth, the caloric content of the dried seedling was determined with a calorimeter (Shimadzu, CA-4). For each measured variable, percent inhibition was defined as [(Control-Aqueous extracts)/Control] x 100.

Hot and warm extracts were characterized by measuring their light absorbance (200–800 nm), pH, electrical conductivity (EC), and osmotic potential. Extract absorbance values at each wavelength were expressed as percent of the total absorbance added across all wavelengths. Electrical conductivity was measured using a conductivity bridge (YS, Yellow Springs, OH, Model 3100). Also, the values of osmotic potential was measured using model C-51 with peltier type thermocouple psychrometers (Wescor, Logan, UT).

Statistical Analysis
All experiments were conducted twice in a completely randomized design with six replications. Analysis of variance was performed for all data using a general linear model procedure (SAS Inst., 1985). Data from two experiments were pooled and mean values were separated on the basis of least significant difference (LSD) at the 0.05 probability level.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Seed Germination and Seedling Length
Extracts from `SR31' and `Nonglimna 1' were the most (59%) and least (6%) inhibitory of barnyardgrass germination, respectively (Table 1) . Rice cultivars were grouped into five categories according to their allelopathic activity. The first group (germination inhibition >40%) consisted of 7 cultivars including `Jinbubyeo'. The second group (31–40% germination inhibition) consisted of 14 cultivars including `Seoanbyeo'. The third group (21–30% germination inhibition) consisted of 31 cultivars including `Chengchengbyeo'. The fourth group (11–20% germination inhibition) consisted of 38 cultivars including `M202'. `Nonglimna 1' was in the fifth group (<10% germination inhibition). Germination inhibition within maturity group (Table 2) was 36% for early cultivars, 30% for extra early, 23% for intermediate, 23% for late, and 19% for intermediate late cultivars. Seed germination, seedling length, and dry weights were not statistically different among domestic and foreign cultivars (Table 3) .


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  		Table 1 Inhibitory effect of rice hull extracts on barnyardgrass germination, seedling length, and dry weight.{dagger}

 

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  		Table 2 Effect of rice hull on barnyardgrass germination, seedling length, and dry weight by maturity grouping.{dagger}

 

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  		Table 3 Effect of domestic and foreign origin rice cultivar hull extraction barnyardgrass germination, seedling length, and dry weight{dagger}

 
`Janganbyeo' extracts reduced barnyardgrass shoot growth by 73% and root length by 77% and inhibited total seedling length by 75% (Table 1). Seventy-eight hull extracts including `Jinbuolbyeo' and `Labella' either stimulated or did not affect shoot and root length. Thirty-eight extracts including `Unbongbyeo' had no effect on total seedling length, while extracts from `Labelle' resulted in the highest (50%) stimulation (Table 1).

Seedling Dry Weight
Inhibition of root dry weight was greater than that of shoot dry weight (Table 1). `Janganbyeo' extracts inhibited barnyardgrass shoot growth by 96% and root dry weight by 92%. Thirty-one extracts (including `Baekambyeo' and `Cheonmabyeo') showed no inhibition or stimulated root dry weight accumulation. Twenty-eight extracts (including `Gayabyeo') stimulated dry matter accumulation, and `Baekambyeo' extracts resulted in the highest stimulation (34%) (Table 1).

Effect of Extract Concentration and Extraction Temperature on Allelopathic Activity
The extraction method affected the toxicity of the Janganbyeo hull extracts (Table 4) . Warm temperature extracts were more inhibition of barnyardgrass seed germination than hot extract. The phytotoxicity of extracts was significantly increased as the hull rate increased. When compared with the control, the maximum hull rate of the warm extract inhitibed germination, seedling length, dry weight, and caloric content by 80, 75, 69, and 99%, respectively. The maxiumum hull rate of the hot extract inhibited germination, seedling length, dry weight, and caloric content by 68, 61, 39, and 56%, respectively. The slopes of the linear regression equations for seed germination, total seedling length, dry weight, and caloric content varied for each extraction method. Warm water extracts had steeper slope than hot water extracts (Table 4).


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  		Table 4 Comparison of regression equation for barnyardgrass germination and seedling growth as a function of increasing rice hull concentration of warm and hot water extracts of `Janganbyeo'

 
Warm hull aqueous extracts were more acidic than hot extract (pH 5.43 and 6.28, respectively). Light absorbance by warm extracts was higher in the UV range and lower in the visible range than that of hot extracts (Fig. 1) . Warm extracts had higher EC and osmotic potential than hot extracts (Table 5) .



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  		Fig. 1 Absorbance profiles of `Janganbyeo' hull extracts by warm and hot water extractions

 

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  		Table 5 The EC, pH, and osmotic potential of warm and hot `Janganbyeo' hull extract at 8% rice hull concentration

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
The inhibition of barnyardgrass germination and seedling growth by rice hull extracts may reflect the allelopathic potential of individual rice cultivars. The magnitude of allelopathic effects varied amont the rice cultivars studied. Results of this study are in agreement with those of Dilday et al. (1994), Chung et al. (1997), and Olofsdotter et al. (1995), who concluded that variation in allelopathic activity existed among cultivars. Although it has not been determined if this difference in allelopathic activity is a result of higher concentrations of the same chemical or a result of different chemicals between cultivars, response differences in the current study were attributed to genetic differences between cultivars, since comparisons were made among extracts with the same hull rates. It is possible that cultivars may produce different amounts of one of more allelopathic substances at a given extract concentration or hull rate.

Among the 91 rice cultivars used in this study, mostly bred from native and introduced cultivars, `Janganbyeo' yielded extracts that reduced barnyardgrass germination by 39%, total seedling length by 75%, and total seedling dry weight 96%. This cultivar may provide one of the most important gene resources for breeding rice cultivars with highly allelopathic hulls. Allelopathic potential would be a valuable trait to incorporate in rice cultivars for improved weed control.

Hull extracts either stimulated or inhibited barnyardgrass growth. Rice (1984) reported stimulatory effects at low concentrations of allelopathic substances, but inhibitory effects at higher concentrations. Since measuring seedling length is often complicated due to curling and other morphological alterations, seedling dry weight may be the better selection criteria for allelopathic potential. The higher inhibition of roots compared with shoots (Table 1) may be due to their more intimate contact with the treated filter paper.

When using similar bioassay to screen cultivars for allelopathic potential without authentic allelochemical separation and identification, it is advisable to ensure that extract concentrations are similar to those found under natural conditions, since the concentration of inhibitory substances is probably greater in aqueous extracts than in the field. Based on the bioassay hull rates (50 g/L) used in this study, 14850 kg hulls ha-1 would be theoretically required for allelopathy to occur in the field. However, since factors other than extract concentration are involved in allelopathic activity, it is more appropriate to generalize that the more rice hulls remaining in the paddy soil, the greater the concentration of allelopathic compounds released during the decomposition. Farmers in Korea generally leave a large amount of rice hulls in the field.

The allelopathic effects of rice hull extract on barnyardgrass varied with the extraction method (Table 4 and Fig. 1). As concentration of hull extract increased, warm water extracts inhibited germination, seedling growth, dry weight, and caloric content more than hot water extracts. The caloric content of the organic matter in barnyardgrass seedlings was significantly inhibited as extract concentrations increasted (Table 4).

The studies of UV absorption (Fig. 1), pH, EC, and osmotic potential (Table 5) suggest that there is a different chemical composition between hot and warm water extracts. The differences in UV reading suggest that the two extracts are chemically different, which provides an indirect explanation for the differential growth responses observed (Fig. 1). The low pH of the warm water extract partially contributed to the inhibitory effect as observed by Chung et al. (1997). Higher EC and osmotic potential of warm vs. hot extracts suggests higher salt or chemical release from hulls. Besides possible allelochemicals, higher salts or chemical concentration in the extracts might possibly cause an osmotic stress during seed germination and seedling growth.

Some investigators have suggested that heating by autoclaving or boiling increased (Guenzi and McCalla, 1962; Ohman and Kommendahl, 1964; Jensen et al., 1984), decreased (Siegel, 1950; Jensen et al., 1984), or had no effect on extract activity (Peters et al., 1986). Our results indicate that warm water extracts are generally more phytotoxic on barnyardgrass than hot extracts. Hot water extraction may result in allelochemical degradation or binding, thus reducing its inhibitory effects on barnyardgrass.

The objective of this study was to screen rice cultivars for allelopathic potential using hull extracts. This would provide a new selection method for development of allelopathic cultivars. This study demonstrates that inhibitory substances in rice cultivars should be extracted by warm water and specific inhibitory substances identified if water extracts are to be used as natural herbicide against barnyardgrass in the field.

One limitation of this study is that the concentration of allelopathic substances in extracts may be greater than in nature and subject to positive or negative effects from soil components, which were not evaluated. Further investigations are needed to select allelopathic cultivars under field conditions.Ministry of Agriculture and Forestry 1999; SAS Institute 1985


   ACKNOWLEDGMENTS
 
Earlier drafts of this manuscript were greatly improved by comments and criticism from Dr. Miller, Crop Science, University of Illinois, USA. authors are also grateful to Ms. J. Miller, English Language Institute, Konkuk University, for her kind proofreading and helpful suggestions during the preparation of the manuscript. The authors acknowledge the financial support of the Korea Research Foundation made in the program year of 1997.

Received for publication October 4, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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