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Allelopathy in South China agroecosystems

Luo Shiming

Institute of Tropical and Subtropical Ecology, South China Agricultural University,
Guangzhou 510642, People’s Republic of China


This paper reports the major progress of allelopathic research on South China Agroecosystems in the past 15 years in South China Agricultural University. Allelopathy is a weapon for many exotic plant species to survive and growth in a new area. These species included Eucalyptus, Mikania micrantha, Chromolaena odoratum(L), Ageratum conyzoides. Autotoxicity could be found in Casuarnia equisetifoliam, Wedelia chinensis, tea and tomato. Special peaks from HPLC of rice plant can be used for rapid identification of rice species with allelopathic potential. Evident from Bidens pilosa, Eucalyptus citriodora, and Ageratum conyzoides showed that stress caused by water, soil fertility, or disease could stimulate the active production of allelochemicals. The temperature stress of the receiver plant could enhance the inhibitory effect of water extracts from Casuarnia equisetifolia. The mode of action on receiver plant was proposed. The mixture of allelochemicals from tomato or from Ageratum conyzoide had significant augment effect or diminishing effects on receiver plants when it was compared with single chemicals. Ageratochromene from Ageratum conyzoide have inhibitory effects not only on plants, but also on fungi and insects.

Media summary

Allelopathy existed in many exotic plants in South China. Stress environment can stimulate allelopathy. Allelochemicals have effect on plants, but also on pathogens and insects.

Key Words

South China, allelopathy, mix of allelochemicals, biochemistry, physiology


South China is located in tropical and subtropical monsoon area with average annual temperature around 20-26 °C and annual rainfall 1500-2000 mm. Except the flat plains along the lower reaches of rivers in this area, most of the landscape is covered by rolling hills and mountains. The major types of natural vegetation are evergreen broadleaf forest and tropical rain forest. The major cropping system is double rice with one winter crop. Because of the natural environment, bio-diversity here is rich. Allelopathic research in South China Agricultural University began in late 1980’s. In order to recover vegetation in hilly area, we found that building complex multi-layer vegetation was a good practice. However, not all combinations among woods, shrubs and grasses were workable. Besides the limitation set by factors like shading, nutrition, pathogens and insects, there were some factors that were not accounted for. Allelopathic phenomenon was one of these factors. Since then, we have explored many interesting aspects in allelopathy. These research results not only provide us opportunities for sustainable agro-ecosystems, but also for a better understanding of the natural ecosystems in the region.

Allelopathic plants in South China

We found that allelopathic effect existed between exotic plant species and local species, between two crops in cropping systems, between microorganisms and higher plants and between rice and weeds in South China agro-ecosystems (Luo et al. 1995).

Allelopathy in exotic plant species

Many plant species were introduced to South China artificially like Eucalyptus since 1890 (Wu 1991), or naturally like Mikania micrantha about 15 years ago. Allelopathy is one “ weapon” which is used by exotic plants for their survival and flourish in a new region.

Eucalyptus is now one of the most important tree species for wood production in South China. It can be easily found that less vegetation exists under Eucalyptus canopy than local trees. Zeng and Li (1997) found that volatile compounds from Eucalytus exserta and E. urophylla had allelopathic effects on the seedling growth of Raphanus sativa, Lactuca sativa, Leucaena leucocephala and Acacia mangium. Cao and Luo (1996) reported that aqueous extract from bark, and leaf, and volatiles from leaf of Eucalytus citriodora showed allelopathic effect on the growth of 9 species including weeds like Bidens pilosa L. Digitarie pertenuis,Eragrostics cilianesis, Setaria geniculata, and crops like corn, rice, cucumber, bean and Stylosanthes guianensi. Table 1 shows the allelopathic effects of 4 species of Eucalyptus on radish seed germination and seedling growth. Recent study by Lin et al. (2003) showed that water, ethanol, or acetone extracts from Eucalyptus urophylla also have allelopathic effect on Pisolithus tinctorius, a common fungus in South China.

An exotic weed species Chromolaena odoratum(L) originated in South America, is now a common weed in hilly plantation and upland fields in South China. He et al. (2002) found that the 0.1g/ml alcohol extracts from Chromolaena odoratum (L) has inhibitory effects on the germination, seedling growth and root growth of Brassica parachinsis, B. chinensis, and B. perkinensis, but 0.01 and 0.002g/ml alcohol extracts showed stimulatory effects on the seedling growth.

Table 1 The influence of four species of Eucalyptus on the growth of radish. (Data from Luo et al. 1995)
RI=Respond Index=( result from treatment with allelochemical -Control)/ Control


Aqueous extracts from leaf (fresh weight:water=1:8)

E. citriodora


E. exerta

E. camaldolensia

E. robusta

E. citriodora












Root length








Bud length

















Mikania micrantha is a relatively new exotic plant species in South China, but it expanded very quickly and destroyed many existing vegetations. Seedling growth of barnyard grass (Echinochioa colonum L.), cucumber (Cucumis sativus Linn.), radish (Raphanus sativus L.), rice (Oryza sativa L.), Brassica parachinensis, B. chinensis, B. alboglabra, Lolium multiflorum decreased when exposed to the increasing concentration (200, 400, 800, 1600 mg/l) of the volatile oil from Mikania micrantha. The fresh weight of all test plants decreased and the emergence of all test plants delayed for 1-2 days under oil treatment (2500g / hm2) (Zhang et al. 2002).

A pure single species community is usually a clue for the existence of allelopathic effect. It is true for Chromolaena odoratum(L), Mikania micrantha, Wedelia trilobata and Ageratum conyzoides. There are exceptions, however. Molasses grass (Melinis minutiflora) was introduced to South China in 1980’s. It can easily spread and suppress other grasses, and form a pure community. However, it did not show allelopathic effects on germination and seedling growth of rice and cucumber in the presence of 1:4 (fresh weight of upper plant part: water) extracts (Zeng 1991). Its competition advantage may rely on seed dispersal, root growth and tolerance of low nutrition rather than allelopathy.

Autotoxicity and continuous cropping

Autotoxicity is one of the factors affecting continuous cropping of the same crop. It was found that tea, tomato, Casuarnia equisetifolia,and Wedelia chinensis all have autotoxicity effects on its own seedlings in South China ( Luo 1995, Deng 1996).

Casuarnia equisetifolia is adaptable to high salinity sandy soil along coastal beach in South China and has become a major tree species in shelterbelt system since 1960s’. However, the wilting disease spread from early 1980s’ and the plant community degraded gradually. Deng et al.(1996) reported that allelochemicals produced by C.equisetifolia could inhibit the growth of its own seedlings (Table 2). Allelochemicals from C.equisetifolia branchlet were isolated and identified by HPLC, IR and NMR. It was found that kaempferol-3-α-rhmanoside, quercetin-3-α-araboside and luteolin-3′,4′-dimethoxy-7-β-rhamnoside could inhibit the seedling growth, especially root growth.

Tea was originated in China and is one of the most important crops in hilly and mountainous area in South China. Luo (1995) and Cao (1992) reported that the aqueous extracts (3.5 dry weight:100 water) from root, stem, fruit and leaves of tea plant had inhibitory effect on the germination of tea seed. The respond index (RI)

Table 2 Effect of aqueous extracts from C. equisetifolia branchlet on the root growth of its seedlings (Data from Deng et al. 1996) RI=Respond Index =( result from treatment with aqueous extracts - Control) / Control

(g FW/ml)
















t: t test result, t0.05=4.303, t0.01=9.925

were -0.521**,-0.269**,-0.607**,and -0.378** respectively. The extracts also inhibited the growth of tea seedlings. For example RI for the height of tea seedling inhibited by aqueous extracts from tea fruit was -0.549** and from leaves was -0.605**. The polyphenol and caffeine in tea leaves could be accounted for at least part of this autotoxicity. RI of polyphenol was -0.258** and RI of caffeine was -0.113**.

Since more and more farmers for vegetable production in South China adapt hydroponics method, it is important to understand the allelopathic effect within the system. Zhou et al. (1997) found that hydroponics medium and volatile components of tomato had allelopathic effects on cucumber, but did not have significant effect on lettuce and cabbage. Tomato also showed autotoxicity. Both aqueous extracts of upper plant part and hydroponics medium from tomato culture had autotoxic effect (Table 3). It suggests that the growth medium from tomato production should not be used for another crop of tomato or cucumber.

Table 3 Allelopathic effect (RI) of aqueous extract from tomato on its seedling growth (Data from Zhou et al. 1997). RI=Respond Index =(treatment -CK) /CK


Seedling height

Root length










Wedelia chinensis is a common herb for landscaping in South China. It can form a good coverage with yellow flower and prevent the invasion of other weeds. It had inhibitory effect on weed species like Eragrostics cilianesis, Alternanthera philoxerides Cypers difformis L, Paspalum thunbergu kunth Exsteud, Alternantherase sssilis DC. Cynodon dacylon Pers.(Nie et al. 2002). It also showed toxicity effect on its own

Table 4 Allelopathic effect (RI) of aqueous extracts from upper plant part of Wedelia chinensis on the germination of stem cuts of three specis ( Data from Luo et al. 1995)

(g FW/ml)

Eragrostics cilianesis

Alternanthera philoxerides

Wedelia chinensis

















growth (autotoxin). However the effective concentration for autotoxicity is much higher than that for other species (Table 4). When the germination of E. cilianesis and A. philoxerides had already suffered in low concentration (0.1 g FW/ml), the germination of W. chinensis was still be stimulated (Luo et al. 1995). This phenomenon is important for the competition of W. chinensis with other species grown in the same habitat.

Identification of allelopathic rice resources

Rice (Oryza sativa) is the most important daily food in South China. An effective way to identify allelopathic rice varieties can benefit the breeding program to incorporate allelopathic trait. Bioassay method and field screening methods are time consuming. Kong et al. (2002) found that there were 9 peaks in HPLC within retention time between 11-15 minutes under set conditions. Five of these peaks did not depend on varieties, and the other 4 peaks depended heavily on allelopathic potential of varieties. An Allelopathic Index (AI), where AI= area sum of the 4 special peaks / area sum of all 9 peaks, was proposed for identification of allelopathic potential of rice variety and single rice plant. When AI > 0.45, it indicates great potential of allelopathy. When AI < 0.35, it indicates low potential for allelopathy. To compare the AI result from HPLC and Influence Index (I I) from sandy culture and aqueous culture using barnyard grass as an acceptor species, highly negative correlation existed between AI and I I. Field test results also supported AI as an indicator for allelopathic potential (Zhu et al., 2002). This HPLC method can save a lot of time for resources screening. It is not only able to screen varieties, but also able to screen single plant. It is especially useful to identify single potential offspring during the separation stage of a breeding program.

Environmental effects on allelopathy

The environmental factors on the one hand affect the allelopathic potential of donor plants and the respond of receiver plants, on another hand affect the fate of released allelochemicals.

Environmental factors on donor plants and receiver plants

Environmental factors such as light, water and nutrient are essential for competition between plants. It is not surprised that these environmental factors also affect the chemical relations between plants. Allelopathic effect of Bidens pilosa fresh weight : water= 1:4 to radish seed germination (RI) was negatively correlated (R=-0.674**) with rainfall one month before harvest. Correlation of allelopathy effect of volatile compounds from leave of Eucalyptus citriodora to radish seed germination was found negative (R=-0.99**) to the rainfall of the month (Luo et al. 1995). The amount of volatile oil in Ageratum conyzoides was affected by soil nutrient condition. The poorer the soil, the more volatile oil was produced by the plant (Table 5).

Table 5. Volatile oils contents (mg/g) from Ageratum conyzoides at different growth stages under different soil fertility conditions (Data from Xu 2000)
Means±(SD)followed by unlike letter of the same column indicates that the values are significant different at 0.05 determined by ANOVA and Duncan multiple range tests

Soil fertility


Vegetative stage

Blooming stage

Ripening stage


1.04±0.30 a

1.41±0.11 a

1.32±0.13 a

1.13±0.10 a


1.55±0.19 a

2.70±0.19 b

2.45±0.29 b

1.85±0.28 b

It seems reasonable to suggest that plant can produce more allelochemicals when competition for water and nutrition resources is intense and under optimum condition they save energy by producing less allelochemicals. Xu (2000) also found that A. conyzoides infected by powdery mildew disease produced more volatile oil than the healthy plants. It increased from 1.59 mg/g in healthy plants up to 2.80 mg/g in the infected plants. The infection of disease stimulated the allelochemical production system of A. conyzoides. It is a clue for one common defence mechanism to deal with multiple bio-competitions.

Allelopathic effects on receiver plant are also affected by environmental factors. The allelopathic effect of water extracts from leaflet of Casuarnia equisetifolia on root length of radish, lettuce and barnyard grass were significantly influenced by temperature. Two-way ANOVA analysis showed that temperature had additive effect with concentration. The dosage effect was larger in non-optimum temperature range than in optimum temperature range. In Figure 1, 28°C was optimum for both radish and lettuce and 30°C were not the optimum temperature for lettuce (see the 0 concentration). With concentration increase, the root growth of lettuce at 30°C or radish at 24°C suffered more than at 28°C. It is interesting to notice that environmental stress affects allelopathy both on donor plant and receiver plant. On the one hand, it increases allelochemical production in donor plant; on the other hand it increases the dosage effect on the receiver plant.

Figure 1. Temperature effect on water extracts from leaflet of Casuarnia equisetifolia on root length of (a) radish and (b) lettuce (Luo et al. 1995).

Environmental factors on the fate of released allelochemicals

In general, allelochemicals in soil will be absorbed by soil particles, decomposed by microorganisms and move with water. Luo et al. (1995) found that volatile oils from Eucalyptus citriodora were more difficult to be washed away than aqueous extracts (Figure 2).

The decreasing inhibitory effect of soil added with eucalyptus leaf on germination of radish seed (Y) over time (t) followed the negative exponential curve:

Y=0.1-0.3731×exp (-0.06) (F=12.2*).

The same rule existed in the inhibitory effect of soil added with volatile oil of eucalyptus leaf on the germination of radish seed (Y) over time (t) :

Y=0.1-0.4458×exp (-0.0702 t ) (F=63.8**).

Figure 2. The allelopathic effect of soil added with water extract or volatile oil of Eucalyptus citriodora and then washed by different amount of water, on the growth of radish (Luo et al. 1995).

However, this negative exponential rule was broken in the decomposition process of ageratochromene from Ageratum conyzoides The amount of ageratochromene reached its second peak after 30 days of the decomposition (Figure 3). It was because two dimmer of ageratochromene formed in soil through microbiological processes in the first 22 days and then decomposed back to ageratochromene before it was further decomposed (Kong et al. 2002). Microorganisms also play an important role on allelopathy of rice. Kong et al. (2002) found that only glycosides could be detected in hydroponics rice culture under sterilized condition. After it was mixed with contaminated rice growing medium for 6 hours, saccharides appeared. After 12 h, phenolic acid, and fatty acid appeared. Thus the decomposition products of allelochemicals by microbial activities influence allelopathic effect of rice.

Figure 3. Amount of ageratochromene and its dimers over time in soil with high organic matter and nutrition (Kong et al. 2002).

Allelopathic effect on receiver plants

The mode of allelochemical effect on receiver plants is quite complicated. Usually the released chemicals are a mixture of many organic compounds. We found that the result of combined materials was quite different from that of a single compound. The released chemical not only attacked other plants, but might also attack other competitors, like pathogens of diseases and insects as well. Let’s first have a look at the biochemical mechanism of single allelochemical on receiver plant.

Biochemical mechanism of allelochemicals on receiver plants

Aspergillus japonicus is a common fungus, found in soil and on the surface of plant seeds. Shi (1999) and Zeng et al. (2001, 2004) reported that secalonic acid F (SAF) is responsible for the allelopathic effect of A. japonicus on higher plants. The biochemical mechanism of SAF on receiver plants was summarized as Figure 4.The allelochemicals reduce the activity of superoxide dismutase (SOD) and peroxidase (POD) of receiver plant; hence increase free radicals in cell. The accumulation of free radicals will cause more lipids to become peroxides and the basic structure of the cell membrane system to be destroyed. Cell electronic conductivity increases because of the release of ion and organic compound through the damaged cell membrane. The structure of organelles such as chloroplasts and mitochondria are destroyed. The decrease of photosynthesis and the increase of respiration lead to the deficit in energy balance. Finally, it influences the growth and physical appearance of the plant. After rice seedling was treated with 0.3 mmol/l SAF for six days, Shi (1999) found that SOD and POD decreased 71.4% and 18.2% respectively, and at the same time methane dicarboxylic aldehyde (MDA) was increased by 110%, membrane conductivity increased 30.1%. At this stage the swelling of mitochondria of root cell can be detected under electronic microscope. The TTC (2,3,5-triphenyl tetrazolium chloride) reduction ability of root system was decreased 54.5%, photosynthetic rate decreased from 10.6±0.7 mmol/l to only2.6±0.6 mmol/l, respiration rate increased from 1.2±0.6 mmlo/l up to 4.6±0.8 mmol/l. The root length was decreased from 42.7 mm to 1.6 mm and seedling height from 24.1 mm to only 4.9 mm. Similar effect can be observed in corn, sorghum, rape seed, cucumber and Bidens pilosa treated with SAF. Nie et al. (2002) conducted an experiment of allelopathic effect of Wedelia tribobata on peanut. After treated with aqueous extracts of 0.4 g fresh weigh/ml for about 15 d, similar biochemical and physiological responses were observed (Table 6).

Figure 4. The biochemical and physiological mechanism of Secalonic acid F on receiver plant ( Zeng et al., 2001. Shi et al., 1999)

Table 6. Effects of aqueous extracts of W. trilobata on physiological activities and enzyme activities of the geminating seeds of peanut ( Data from Nie et al. 2002).
Mean values fallowed by different letters in the same column indicates the significant difference at 1% level determined by ANOVA and Duncan multiple range test.


Lipase (U.g-1.h-1)

(OD470. g-1.min-1)

Membrane permeability

Respiration rate
(mgCO2. g-1.h-1)

Seed activity
(μg. .g-1.h-1)


23.6 a

0.69 a

100.0 a

1.68 a

68.2 a

Treated plant

18.2 b

0.45 b

127.7 b

1.16 b

48.3 b

Effects of mixing allelochemicals

In most cases, donor plants release a mixture of allelochemicals, rather than single chemicals. The effect of mixture is quite different from its individual components. Zhou et al. (1998) used a series of allelochemicals from tomato to test the effect of different mixture on the acceptor plants including lettuce, cabbage, and radish. Figure 5 shows a few typical curves. If there was no augment effect in the mixture of ester and organic acid, there should be a straight line between 1\0 to 0\1. Because the existing of augment effect in the B+E mixture (di-iso-butyl-phthalate mixed with tannic acid), the lines of B+E are above the straight line. Because of the existing diminishing effect of A+D, the line of A+D is below the straight line.

Figure 5. Mixed allelochemicals from tomato had augment effect or diminishing effects on lettuce root or shoot length (Zhou et al. 1998).A: di-iso-octyl-phthalate, B: di-iso-butyl-phthalate, E: tannic acid D: salicylic acid. RI stands for Respond Index, equals the length difference between treatment and control divided by the length of control.

An interesting phenomenon was discovered in a research on the mixture of allelochemicals from volatile oil of Ageratum conyzoide on seedling growth of radish (Kong et al. 1998). Precoene I and Precoene II are the two major allelochemicals in Ageratum conyzoide. However the mixture of these two compounds did not have significant augment effect ( see A, B, and C in Figure 6). Bisabolene, caryophyllene, and fenchylacelate were not very important in term of allelopathy, no matter whether they acted alone or together (see H in Figure 6). However it caused significant augment effect when they were mixed with Precoene II alone or together (see D, E, F, G in Figure 6). It means that small amount of ingredient may enhance allelopathic effect dramatically. Exploring rules behind this phenomenon would be worthwhile in future research.

Figure 6. Mixture of allelochemicals from Ageratum conyzoide on seedling growth of radish (Kong et al. 1998). A:1/4saturated aqueous solution of precoene I; B: 1/2saturated aqueous solution of precoene II; C: precoene I + precoeneII; D: precoene II + bisabolene; E: precoene II + caryophyllene; F: precoene II + fenchylacelate; G: precoene II + bisabolene + caryophyllene + fenchylacelate; H: bisabolene + caryophyllene + fenchylacelate.

Multifunctional effects of allelochemicals

Ageratochromene of Ageratum conyzoides can act as anti-juvenile hormone on insect, at the same time had inhibition effects on the growth of radish, ryegrass, barnyard grass, and on the pathogens like rice banded sclerotial blight and phytothora foot rot of pepper (see Table 7, Lu 1999). This result was further confirmed by another experiment (Table 8).

Volatile oil from Mikania micrantha also had inhibitory effect on both plants and fungi. Besides its allelopathic effect on the seedling growth of barnyard grass, cucumber, radish, rice, Brassica parachinensis, B. chinensis, B. alboglabra, and Lolium multiflorum, it could also suppress the growth of rice blast disease ( Fusarium grisea) by 53.38 %, Fusarium oxysporum by 28.66 % and Phytophthora nicotianae by 18.69 % under concentration of 400mg/l (Zhang et al. 2002).

Table 7 Effect of ageratochromene and ageratochromene demethylate on the growth of pathogen of rice and pepper in medium (Data From Lu 1999)



Inhibition rate (%)


Volatile oil

Precocene II


Rhizoctonia solani




Carbendazim as 100

Botrytis cinerea




Carbendazim as 100

Sclrotinia sclerotiorum




Carbendazim as 100


Tribolimn confusum




Acetone as 0

Mythimna separata




Acetone as 0

Culex pipiens pallens




Acetone as 0


Echinochloa crusgalli




Soil only as 0

Brassica campesiris




Soil only as 0

Amarenthus retroflexus




Soil only as 0

Table 8 Effects of volatile oil and Precocene II from Ageratum conyzoides on Fungi, insects and plants (Data from Kong 2001)


(mg/ L)

rice sclerotial blight

root rot of pepper

diameter of conobium

inhibition rate

diameter of conobium

Inhibition rate (%)













demethylate ageratochromene






The production of allelochemicals consumes energy and materials. If the process of secondary metabolism can deal with multiple biological competitions and environmental stresses, it obviously has an evolutionary advantage. Further research on it will be very important, not only for the understanding of nature, but also for a revolutionary concept from pesticide to “plant protectant” which can have multiple protective functions.

In the tropical and subtropical environment, rich allelopathic phenomenon exists in the agroecosystem of South China, just as rich as biodiversity. More and more exiting phenomenon have been revealed and the rules behind have been discovered. Evident showed that allelochemicals are “wise weapon” of plants for competition and survival. In order to save energy, it has a common biochemical and physiological bases for different kinds of biological thread including insect, fungi, and weed. The combination of chemicals is unique and effective. Plant also makes good use of the environmental factors to win its battle. Microorganism helps it to generate allelochemicals and save some processes within plant. Stressful environment can enlarge the effect of allelopathy. Our latest research also began to reveal that the behaviour of insects and their natural enemies have also adapted to the allelopathic environment and form a very interesting and quite complicated relationship. The understanding of allelopathic relationship will provide a solid base for future sustainable agricultural practice in the region.


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