Abstract

Media summary

Key Words

Introduction

The allelopathic effects are due to inhibitory substances that are released directly or indirectly from living plants (donor) into the environment through root exudation, leaching, volatilization and residual decomposition of the donor plant (Rice 1984). These phytotoxic substances were termed as allelochemicals. The potential of allelopathy has been a particularly intense study area of ecology and environment. Rice allelopathy has received a great deal of attention after Dilday and his workshop screened 10000 accessions of rice cultivars and found 3.5% accessions showing higher allelopathic potential against ducksalad (Dilday et al 1991). But allelopathy and competition occur simultaneously in the field where crop plants often grow together with the weeds, and it is extraordinarily difficult to distinguish allelopathic effect from resource competition. The evaluation of allelopathy activity depends on the choice of an appropriate bioassay (Romeo et al. 1999). Several bioassays have been developed to distinguish allelopathy from competition in order to manipulate the allelopathic potential of a crop species. But resource competition and allelopathic effect still remain obscure especially in field condition. Although relay seeding technique distinguished allelopathy from competition in laboratory conditions (Navarez et al, 1996), the way is not available in field test (Lin et al 2002). On this point, the bioassay for allelopathy and competition separation described by Weidenhamer (1996) was the better way to follow. However it was effective only when the control was the isogene line of the donor plant (Lin et al 2002). It is time-consuming in breeding for this isogene line. This therefore affected the use of the method in the research work on allelopathy. In present study, a new method of allelopathy and competition separation based (ACS) was proposed and employed to effectively assess the allelopathy potential of donor rice without the influence of biointerference raised by resource competition of barnyardgrass to demonstrate the mechanism of changes in rice allelopathic potential under different nitrogen supplies in the mixture trial.

Materials and Methods

Strong allelopathic rice line PI312777 (USA) and non-allelopathic rice accession Lemont (USA) were employed as test materials, the barnyardgrass (Echinochloa crus-galli L.) (BYG) served as a receptor. The experiment was conducted in a greenhouse under natural conditions in Fujian Agriculture and Forestry University in July, 2004.

Experiment 1(Mixture trial )

The seeds of the rice accessions were surface-sterilized with NaClO for 30 minutes and germinated in Petri dishes in a temperature-controlled growth chamber. The seedlings grown in the field up to 3-leaf stage were wrapped with foam and transplanted into a styrofoam float. The styrofoam float with seedlings was placed into plastic pot (45cm×35cm×15cm) containing 10 L Hoagland solution prepared in hydroponic culture. Thirty five rice seedlings were grown at 5cm apart in the pot. Five barnyardgrass plants were placed in the center of all lines, which were used: (1) to determine rice allelopathy on the target weeds as receptors in the mixture. (2) to act as an induced factor of rice allelopathy. The pot was painted to black on the outside of the wall to inhibit algae growth. The solution was maintained at pH 5.5 throughout the whole time courses in the mixture. The Hoagland solution was kept at full strength until 7 days after transplanting, then changed to that only containing nitrogen (N) phosphorus (P) potassium (K) elements for the sake of regulation in the monoculture of experiment 2. The specific solutions contained 5.65 mg/L in P element and 42.72mg/L in K content but different N levels. The nitrogen treatment was set at three concentrations of 5, 10 and 20mg/L, which are 1/4, 1/2 and 1 time nitrogen concentration in Hoagland solution respectively. pH and the residual amounts of nitrogen(N) phosphorus (P) potassium (K) were measured every week to investigate any changes in the solution. It showed that rice accessions and BYG could grow normally in the hydroponic culture in which the solution only contained the three elements which were made from ammonium (NH₄NO₃) potassium (K₂SO₄) and potassium dihydrogen phosphate (KH₂PO₄) compounds based on the design. Rice and BYG plants were sampled at 21 days after co-culturing, and dried in an oven at 70⁰C for 5 days to determine the inhibition rate over control (IR) according to the equation: IR=(treatment-ck)/ck×100%, it means promotion when IR>0, and inhibition when IR<0. The used solutions under the different treatments were collected respectively for the monoculture in experiment 2.

Experiment II (Monoculture trial)

The treated solution1, 2, 3 collected from the solutions of the three treatments (i.e. the nitrogen treatment was set at three concentrations of 5, 10 and 20 mg/L, as mentioned above), and adjusted to the same level as the contents of N.P.K in Hoagland solution, were used to culture BYG to detect its growth response mediated by the exudates from the roots of the two accessions used in experiment 1. 9 BYG plants at 3-leaf stage were transplanted in each pot with treated solution. For control treatment (CK), Hoagland solution was used to monoculture the target weed with the same density in each pot. That means there were no significant differences in the nutrient levels of the two culture solutions except that the treated solutions(1, 2, 3) may contain different compounds released from the roots of the two rice accessions under different nitrogen supplies in the mixture trial(experiment 1). The experiments were conducted in three replications Dry weight of barnyardgrass was transformed and expressed as inhibition rate (IR) as mentioned above. All data were subjected to analysis of variance followed by the least significant difference (LSD) test to determine the significant differences among the mean values at the P<0.05 and P<0.01 probability levels, using a general linear model procedure of the Statistical Analysis System Program (SPSS).

Results

The dry weight of BYG co-cultured with allelopathic rice PI312777 in the mixture varied in the range of 0.539-0.639g/plant. The inhibition rate over control was 64.88%-72.67%. No marked difference was found in different nitrogen supplies. In contrast, DW of BYG cocultured with Lemont was strongly affected and the inhibition rate was in the range of 67.14%-12.72%, showing significantly declined tendency as the nitrogen concentration increased. Further analysis indicated that DW of BYG co-cultured with Lemont was much higher than that with PI 31277 in the mixture under different nitrogen supplies especially in lower N supply (5 mg/L) (Figure 1). This implied that Lemont performed its lower ability in resource competition under normal or rich nitrogen conditions (10-20mg/L), but higher ability in lower nitrogen supply (5mg/L), which was considered to be the result from econiche competition happened in rice-weed system. However it is needed to know how resource competition and allelopathic effect happen in the co-cultured system.

To address the effects of resource competition and allelopathy in rice–weed system, further study was conducted in the monoculture. The result confirmed that PI312777 had higher allelopathic ability to suppress on the target weed than the counterpart, Lemont under different used solutions derived from the mixture of the two rice accessions with barnyardgrass under different nitrogen treatments in experiment 2. (Figure 2).

Figure 1. Comparison of dry weight (DW) and inhibition rate(IR) of barnyardgrass(BYG) cocultured with different rice accessions exposed to the same nitrogen supplies

Figure.2 Effect of different rice solution treatments on dry weight (DW) and inhibition rate (IR) of barnyardgrass (BYG) under the same condition (Note:1,2,3 and CK refer to the meaning as described in experiment 2.)

Combining the results obtained in the two trials as shown in Figure 3, you are able to see what is the resource competition and allelopathy. In fact, the inhibition rate on BYG was caused by biointerference including resource competition and allelopathic effect mediated by the two rice accessions in the mixture trial under different nitrogen treatments, and it resulted from allelopathy in the monoculture trial. The deviation of the two curves referring to biointerference (A+R) and allelopathy (A) was the values of resource competition in the co-culturing system. It was showed that the resource competition of PI312777 caused 26.34%-29.14% reduction in IR and its allelopathic effect on the target weed was 38.5 %-51.4% in terms of inhibition rate (IR) under different nitrogen treatments. However, the small deviation of the two curves in Lemont was found under normal or rich nitrogen conditions (10-20mg/L) in hydroponic culture, showing the lower values of resource competition and allelopathy as well as insignificantly different between them. Reverse was true in lower N supply, performing the stronger resource competition in Lemont. These findings suggested that rice accession PI312777 had higher ability in the interference with the target weed, barnyardgrass, performing stable and higher ability in resource competition as well as the increasing tendency in allelopathic potential in the suppression on the companied weed, BYG as N supplies decreased in the cocultured system. The reverse was true in the case of Lemont, rice accession, exhibiting its lower allelopathic potential in all nitrogen treatments.

Figure 3 Deviation curve of biointerference and allelopathy in rice cocultured with barnyardgrass(BYG).
a refers to the biointerference curve R+A of PI312777 rice accession acting on BYD.b refers to the allelopathic curve A of PI312777 rice accession acting on BYD .c refers to the biointerference curve R+A of Lemont rice accession acting on BYD. d refers to the allelopathic curve A of Lemont rice accession on BYD

Discussion and conclusion

From the present study, some important information could be obtained. First, PI312777 rice is confirmed to have stronger allelopathic potential in the suppression on barnyardgrass, while Lemont appeared to be lower ability to suppress on the target weed which was consistent with the previous studies (Dilday et al. 2000; Lin et al. 2001; Lin et al. 2002). Second, biointerference on BYG increased as N concentration declined both in PI312777 and Lemont rice accessions, but showing different mechanisms in it. Third, ACS method was recommended and successfully used to distinguish allelopathy from resource competition and to evaluate the properties in the responses to different N concentrations in rice-weed mixtured system, by which different survival strategies in allelopathic rice and counterpart was revealed. It was suggested that under limited resource supply, allelopathy activity trended to dominate in biointerference for allelopathic rice line but resource competition appeared to manipulate in it for non-allelopathic rice accession because of enhanced econiche competition in the mixed system (Wu 1986), and in sufficient resource supply, the interspecific competition tended to be weak for non-allelopathic rice accession. Allelopathic rice PI312777 showed constant the higher ability of biointerference and remained dominant in interspecific competition (Xiong, 2005). However, it is needed to further understand the mechanism of enhanced allelopathy activity in unfavorable external environment.

Acknowledgements

We thank Professor Lin for critical instructor, and his work was supported by grants (30471028,20020F012) from National Natural Science Foundation of China, and the key Scientific Technological Program of Fujian Province, China.

References

Dilday RH, Mattice JD and Moldemhauer KR (2000) An Overview of Rice Allelopathy in the USA. In ‘Proceedings of workshop on rice allelopathy’. (Eds KU Kim, DH Shin) pp.15-26. (Chan Suk Park publishing: Korea).

Dilday RH. Nastasi P. Lin J. Smith RJ Jr. (1991) Allelopathic activity in rice (Oryza sativa L.) against ducksalad (Heteranthera limosa(Sw.) Wild).in:’Proceedings of sustainable agricultural for the great plains’ (Eds: JD Hansen. MJ.Shaffer DA Ball. and CV Cole.) pp193-201 (USDA, Agricultural Research Services,Bettsville,MD,USA)

Einhellig A (1995). Allelopathy:current status and future goals. In: ‘Allelopathy: Organisms, Processes, and Applications’. (Eds: Inderjit, KMM Dakshini., FA Einhellig.). 582, 1-24 (ACS Symposium Series).

Inderjit K, Olofsdotter M (1998). Using and improving laboratory bioassarys in rice allelopathy. In ‘Proceedings of workshop on allelopathy in rice’: pp45-55 (Ed: M.Olofsdotter). (International Rice Research Institute publishing, Philippines).

Lin WX, He HQ, GuoYC (2001). Rice allelopathy and its physiobiochemical characteristics. Chinese Journal of Applied. Ecology 12, 871-875 (in Chinese).

Lin WX, He HQ, Dong ZH. (2002). New advance of research on allelopathy in rice (Oryza sativa L.). Research Agricultural Modernization 23, 140-143 (in Chinese).

Navarez D, and Olofsdotter M. (1996).Relay seedling procedure as screening method in allelopathy research. In ‘Proceeding of the 2nd International weed control Conference’, (Eds B.H. Cussans, GW Devine and SO Duke et al) pp1285-1290 (Department of Weed Control and Pesticide Ecology, Slagelse, Denmark).

Romeo JT, Weidenhamer JD (1999). Bioassays for allelopathy in terrestrial plants.In: ‘Methods in chemical ecology’. (Eds: KF Haynes, JG Millar) pp179-211 (Kluwer Academic publishing).

Rice EL (1984). Allelopathy. New York. Academic Press Inc.130-188.

The 2^nd Edit Room of Standard Press, P. R.China (1983). Comprehensive collection of environment protection. Beijing: Standard Press of P. R.China. (in Chinese).

Weidnhamer JD (1996). Distinguishing resource competition and chemical interference: overcoming the methodological impasse. Agronomy Journal 88,866-875.

Wu ZQ (1986). Agroecological basis.Fuzhou: Fujian Scientific and Technological Press, Fujian China, 150-350(in Chinese).

Xiong J, Lin WX, Zhou JJ and Wu MH (2005) Allelopathy and resource competition of rice under different nitrogen supplies Chinese Journal of Apllied Ecology 16, 885-889 (in Chinese).