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Chapter 10 Allelopathy in Subtropical Vegetation and Soils in Taiwan

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Chang-Hung Chou Institute of Botany, Academia Sinica, Taipei, Taiwan 11529, Republic of China Allelopathy plays an important role in subtropical vegetation and soils, regulating the formation of plant dominance, succession, population dynamics of understory plants, and the productivity of many crops in Taiwan. This paper describes research findings on autointoxication phenomena of rice plants, sugar cane plantation, asparagus plants, and pangola grass (Digitaria decumbens), and on allelopathy in relation to agricultural practice, forest plantation, and environmental stresses. Allelopathy even plays an appreciable role in plant adaptation in many natural vegetation and plantations. The responsible phytotoxins reported here are phenolics, flavonoids, alkaloids, and other unidentified compounds. Since the 1960s allelopathy has been increasingly recognized as one of the important ecological factors i n plant interactions and has been regarded as impossible to single out from an environmental complex (1 ). Koeppe et a l . (2, 3) also reported several a l l e l o p a t h i c studies, i n which the tested plants were placed under conditions of environmental stresses. Duke and Putnam (4) introduced the concept into a g r i c u l t u r a l practice to select a crop variety with high phytotoxic potential i n order to avoid using herbicides. In the l a s t decade, a tremendous growth of publication on allelopathy has occurred i n the world (5, 6, 7, 8, 9 ) . Wang and his associates described methods of extraction and i d e n t i f i c a t i o n of phytotoxins i n s o i l (10, 11, 12), and subsequently studied the behaviors of phytotoxic phenolics i n s o i l (13, 14, 15). Since 1972, Chou and his co-workers have conducted such research i n a subtropical humid zone of Taiwan and accumulated substantial information concerning allelopathic interactions i n vegetation and s o i l s (5). These findings of a l l e l o p a t h i c studies are of great significance to understand the role of allelopathy i n the natural and a g r i c u l t u r a l ecosystems i n Taiwan. 0097-6156/87/0330-0102$06.00/0 © 1987 American Chemical Society

Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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A g r i c u l t u r a l Productivity

Autointoxication, i n which an organism releases a toxic chemical that suppresses i t s own growth, i s one phase of allelopathy. Autointoxication can also be important in i n t r a s p e c i f i c interactions, such as the regulation of population size by self-thinning. Several case studies conducted i n Taiwan are described below. Autointoxication as the cause of low y i e l d of the second crop of r i c e . Rice (Oryza sativa), the most important crop i n Taiwan, i s planted twice a year by a continuous monoculture system. For nearly a century, the y i e l d of the second crop there has been generally lower by 25% than that of the f i r s t crop (a reduction of about 1000 kg/ha). This reduction of r i c e productivity has been p a r t i c u l a r l y pronounced i n areas of poor water drainage. The cropping system of r i c e i n Taiwan i s different from that of other countries. For example, the fallowing period between the f i r s t crop and the second crop i s only 3 weeks, as compared with a 10week period elsewhere. In growth of the f i r s t crop (from March to July) the temperature increases gradually from 15 "C to 30 "C but for the second crop (August to December) i t decreases from 30 C to 15 C. Between these two crops, the farmers always leave r i c e stubble i n the f i e l d after harvesting, and submerge these residues in the s o i l for decomposition during the fallowing time. During the second crop season, the typhoon (or monsoon) brings a great amount of r a i n f a l l , leading to a high water table i n some areas where water drainage i s rather poor. Chou and h i s associates therefore conducted a series of experiments to elucidate the reason f o r the low y i e l d of r i c e i n the second crop season. Aqueous extracts of paddy s o i l collected i n Nankang were bioassayed and found to be phytotoxic. In pot experiments, a r i c e straw-soil mixture (100 g: 3 kg) was saturated with d i s t i l l e d water and allowed to decompose for 1, 2, and 4 weeks under greenhouse conditions. S o i l alone was treated i n the same manner, as a control. At the end of each decomposition time, 5 r i c e seedlings (3 weeks old) were transplanted into a pot containing straw-soil mixture or into the control s o i l . After one month, r i c e seedlings grown under control conditions were normal and usually over 66 cm t a l l , while the seedlings grew poorly (about 36 cm t a l l ) i n the straw-soil mixture. The roots of retarded plants were dark brown and the root c e l l s were abnormal and enlarged. Further experimental results showed that when the amount of r i c e straw mixed was increased to 100 g/3 kg s o i l , the phytotoxicity increased with the increase of straw added. The t o x i c i t y was s t i l l persistent after 16 weeks of decomposition. The r i c e straws o i l mixture with d i f f e r e n t i n t e r v a l s of decomposition was extracted with ethanol, the ethanol evaporated, and the residue reextracted with ethyl ether; then the phytotoxins present i n the ether extract were i d e n t i f i e d by chromatography. The compounds i d e n t i f i e d were p-coumaric, p-hydroxybenzoic, syringic, v a n i l l i c , o-hydroxyphenylacetic, and T e r u l i c acids (.16), and propionic, acetic, and butyric acids (17). P a r t i c u l a r l y , o-hydroxyphenylacetic acid, f i r s t reported to be a phytotoxin by us, was toxic to


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f i x e growth at a concentration of 1.64 χ 10 M. We found tha£ the concentration of o-hydroxyphenylacetic acid reached about 10 M i n the s o i l containing decomposing r i c e residues. The additional evidences of phytotoxic effects a r i s i n g from the study w i l l be described l a t e r i n this paper. Inter- and i n t r a - s p e c i f i c interactions between Oryza perennis and Leersia hexandra. The wild r i c e , Oryza perennis Moench, distributed throughout the humid tropics, i s considered to be a progenitor of cultivated 0^_ sativa (18). The Asian race shows a perennial-annual continuum, varying greatly i n various lifehistory t r a i t s among i t s v a r i e t i e s (19, 20). Leersia hexandra Sw. i s a perennial grass with short rhizomes, commonly found i n marshy habitats i n Taiwan and other t r o p i c a l Asian countries. It i s a companion of (h_ perennis i n about 40% of the habitats observed i n India and Thailand (21). In Taiwan, three small populations of 0. perennis, hybrid with 0^ sativa (18), had existed i n marshes along natural streams at Patu, Taoyuan Hsien, but they became extinct around 1975, displaced by L^ hexandra. An experimental introduction of CL_ perennis populations into different habi.tats indicated that L^_ hexandra was a key determining the b i o t i c enviionment of the former. To look into the interaction mechanisms of the two species, their a l l e l o p a t h i c interrelations were examined by several methods, such as bioassay of the effects of aqueous leachates and extracts of the two species on the radicle growth of r i c e and lettuce and on the growth of adventitious roots from nodes of cuttings of the two species, and the effects of powdered plant material added to s o i l s on the root development of cuttings. Both grass species showed phytotoxic effects on the radicle growth of r i c e and lettuce, i n t r a - and inter-specifically. L,^ hexandra showed i n many cases higher phytotoxicity than Oryza although the pattern of variations was complex. The concentrations of many phytotoxins i d e n t i f i e d were higher i n Leersia extract than i n that from Oryza. Observation of plants growing from buried seed pool i n s o i l s to which powdered plan,: materials were added also showed higher phytotoxicity of Leersia than Oryza. Probably, allelopathy plays an appreciable role in the successional replacement of the two species (22). Autointoxication of sugar cane plantation. Inadequate germination and growth of ratoon cane have been found to be the two major problems i n the farms of Taiwan Sugarcane Corporation (TSC). The y i e l d of monoculture sugar cane has declined i n many sugar cane f i e l d s . The causes of t h i s y i e l d reduction have been investigated, but no single factor causing the reduction can be found. Wang et al. (23J demonstrated by f i e l d and laboratory experiments that phytotoxic effects are one of the important factors involved. Five phenolic acids (p-hydroxybenzoic, ferulic, p-coumaric, syringic, and v a n i l l i c ) and formic, acetic, oxalic, malonic, t a r t a r i c , and malic acids were i d e n t i f i e d i n t^e decomposing sugar cane leaves i n water-logged s o i l . At 3 χ 10 M solution of these phenolic acids i n water culture, the growth of young sugar cane root was i n h i b i t e d . The a l i p h a t i c acids w^ere also found to i n h i b i t the growth of ratoon sugar cane at 10 M. Furthermore, Wu et


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al.(17) found that the population of Fusarium oxysporum associated with the rhizosphere s o i l of poor ratoon cane roots was much greater than that of good growing ratoon or of newly planted sugar cane roots. They found that fusaric acid, a secondary metabolite of the organism, was toxic to the growth of young sugar cane plants i n v i t r o (17). Autointoxication of Asparagus officinalis L. Asparagus o f f i c i n a l i s i s a perennial ratoon crop widely planted i n many plantations of Taiwan. A s i g n i f i c a n t reduction of y i e l d and quality of asparagus often occurs i n old plantation s o i l . The wilting of asparagus plants has been found to be due to monoculture of the crop. Young (2Λ) indicated that there was about 40% of asparagus seedlings missing from the plantation. Young further showed that the root exudates of asparagus retarded the seedling growth of asparagus c u l t i v a r s , namely Mary Washington, C a l i f o r n i a 309 and C a l i f o r n i a 711 (24). Exudate collected by use of the c i r c u l a r trapping with a XAD-4 resin s i g n i f i c a n t l y retarded r a d i c l e and shoot growth of asparagus seedlings (25). Six phytotoxic phenolics, namely 3,4-dihydroxybenzoic, 3,4-dimethoxybenzoic, 2,5-dihydroxybenzoic, 3,4dihydroxyphenylacetic, and p-(m-hydroxyphenyl)propionic acid, and 3,4-dimethoxyacetophenone were found i n the extracts and exudates of asparagus plant parts. The amount of phytotoxins i d e n t i f i e d was s i g n i f i c a n t l y higher i n the stem than i n the root, and was well correlated to phytotoxicity (25). I t i s concluded that the reduction of asparagus productivity i n old asparagus f i e l d s i s due primarily to phytotoxins released from the plant parts and those produced from the decomposition of residues remaining i n s o i l . Allelopathy And A g r i c u l t u r a l Practice Allelopathy of native and pasture grasses. Miscanthus floridulus, widely distributed i n Taiwan, i s a native and predominant grass and often occurs i n poor s o i l on h i l l s i d e s and /or channels. A f i e l d experiment conducted at a Nankang h i l l s i d e showed that the botanical composition i n Miscanthus stands i s about 65% for M. f l o r i d u l u s , 17% for Lactuca indica, and less than 4% for Eupatorium formosanum, Brachiaria distachys, Sporobolus fertilis, Pouderia scandens, Cyperus pilosus, Digitaria violascens, and three unknown grasses (26). A successional trend of botanical composition was caused by the aggressive nature of Miscanthus f l o r i d u l u s . For example, i n an experiment i n which M. f l o r i d u l u s was cleared, the dominance of Miscanthus recurred after three years. The associated species found i n the Miscanthus stands were again found to be suppressed by the Miscanthus (5,). Furthermore, aqueous extracts and leachate of Miscanthus leaves caused a s i g n i f i c a n t reduction of radicle growth of tested species (26). Additionally, the extracts of s o i l s collected from the Miscanthus rhizosphere, between stands, under the canopy of Miscanthus, and i n open ground control area adjacent to the Miscanthus stands were also bioassayed for their phytotoxicity. Of these, the extract of root s o i l of Miscanthus exhibited the highest i n h i b i t i o n of the tested plants (26).


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Some 12 subtropical introduced species of forage grasses gave aqueous leaf extracts evaluated for their phytotoxicity on tested species. Acroceras macrum, Cynodon dactylon, Chloris gayana,· D i g i t a r i a decumbens, Eragrostis curvula, Panicum repens, and P. maximum always caused s i g n i f i c a n t i n h i b i t i o n of radicle growth of test plants. Of them, D i g i t a r i a decumbens had the highest phytotoxicity upon the tested species at 10 milliosmols, i n which the osmotic i n h i b i t i o n i s zero (27, 22). Chou furthermore found that D^ decumbens was also an autotoxic species, and the productivity was s i g n i f i c a n t l y depressed after several years of planting (Chou, unpublished data). The aqueous leachate and exudates of D i g i t a r i a plants showed a s i g n i f i c a n t reduction of growth of this species. Selection of weed control grass for pasture. An increased amount of a l l e l o p a t h i c research on grassland species has been conducted i n many parts of the world during recent decades (5, 9.). Most of the studies have been concerned with the interpretation of a l l e l o p a t h i c phenomena i n the f i e l d . Only a few studies have employed the a l l e l o p a t h i c effect as a p r a c t i c a l means of d i r e c t l y c o n t r o l l i n g weeds. In Taiwan, many grasses have been introduced into pasture but only a few v a r i e t i e s can be established as forage pasture. As already mentioned, among 12 species studied (28), pangola ( D i g i t a r i a decumbens) exhibited the highest toxic effect on test species. Under sufficient nitrogen fertilizer application, pangola grass forms a pure stand where almost no other weeds can grow. We also found that d i f f e r e n t v a r i e t i e s of pangola had different growth performance and competitive ability. Liang et a l . (29) thus selected eight v a r i e t i e s of pangola for f i e l d t r i a l s and laboratory assays. These showed that the invasion a b i l i t y of c u l t i v a r s A65, A255, and A254 were highest i n Hsinhwa, Hengchun, and Hwalien station, respectively; while c u l t i v a r s A79 and A80 were i n f e r i o r i n a l l stations. Cultivars A84, A254, and A255 possessed the highest t o x i c i t y , which was due to phytotoxins, of which nine phytotoxic phenolics were identified. The interference of grasses i n the f i e l d i s very complicated, and allelopathy alone cannot account for the complicated phenomena. Further f i e l d and laboratory experiments thus need to be performed i n order to c l a r i f y the role of allelopathy i n grassland ecosystems. Phytotoxic e f f e c t of cover crops on orchard plants. Wu et a l . (30) compared the phytotoxic e f f e c t s of some cover crops, namely Centrocema sp., Indigofera sp., and Paspalum notatum (Bahia grass), on the growth of pea, mustard, cucumber, cauliflower, rape, Chinese cabbage, mungbean, watermelon, tomato, and r i c e . They found that rape was most sensitive to the extracts of these cover crops. Among them, Centrocema and Indigofera exhibited the greater phytotoxic e f f e c t ; moreover, the leachate of Centrocema inhibited the growth of banana. More recently, several cover crops including Bromus catharticus, Pennisetum cladestinum, Lolium multiflorum (both chromosome 4X and 2X c u l t i v a r s ) , Paspalum notatum, and white clover are now under investigation for a l l e l o pathic effects on the productivity of apple and peach plantations


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in the Lishan area of central Taiwan. A vast area of apple plantations has been situated on the h i l l s i d e s of the Central mountain since the 1950s. The productivity of these plantations was exceedingly high i n the f i r s t decade after planting but has gradually decreased i n recent years. In fact, t h i s problem has been encountered i n many European countries and Northern America as well. Forest-pasture intercropping system. Taiwan i s an island, with two thirds of the land occupied by mountains, and i t s forests are extremely important for water conservation. The limited amount of a g r i c u l t u r a l land for crops and pasture forces farming a c t i v i t i e s to move upward to h i l l s i d e s and higher elevations. A forestpasture intercropping system has been thought to be a possible way to increase livestock production. Recently we have conducted several experiments i n the forest area of Hoshe Experiment Station of National Taiwan University located at an elevation of about 1200 meters. An area of about one hectare was deforested, part was cleaned by removing the leaf l i t t e r of the conifer tree 'Cunninghamia lanceolata), and part was l e f t unchanged to serve as control. The cleaned and unchanged plots were planted with kikuyu grass (Pennisetum cladestinum) or l e f t open. The experiment was designed to determine the reciprocal interaction of f i r l i t t e r and kikuyu grass, and to evaluate the a l l e l o p a t h i c potential of the two plants on weed growth under natural condition. Results indicated that the biomass of kikuyu grass i n the cleaned plot was signifcantly higher than that i n the control plot. In addition, the number of weeds that grew i n the plot planted with kikuyu grass was lower than that i n the control plot, indicating that the kikuyu grass may compete with and suppress weeds. The seedlings of f i r regenerated i n the deforested area grew well and seemed to not be affected by the neighoring newly planted kikuyu grass. However, the growth of kikuyu grass was inhibited by the f i r l i t t e r l e f t on the unchanged plot i n the f i r s t three months after deforestation. Furthermore, bioassay of aqueous extracts showed that the f i r l i t t e r extract exhibited higher phytotoxicity than the kikuyu grass. Nevertheless, four months after deforestation the kikuyu grass growth i n the f i e l d was luxuriant, indicating that the phytotoxicity of f i r l i t t e r disappeared (Chou et a l . , 1985 unpublished data). Forest intercropping system. On the h i l l s i d e s of mountainous d i s t r i c t i n Taiwan, there i s an increasing area of deforestation. Forest regeneration of the area i s very important to ecological conservation. Many highly valuable forest species have been planted i n a forest intercropping system, such as bamboo, conifers, Acacia confusa, Leucaena leucocephala, Liquidambar formosana, Casuarina glauca, Alnus formosana, and Pinus taiwanensis. We have evaluated the s u i t a b i l i t y of intercropping systems among the aforementioned species. The f i r s t experiment was conducted with leucocephala, an a l l e l o p a t h i c plant, intercropped with other species mentioned above. Pinus taiwanesis grew very well and could tolerate the leachate of leucocephala, but the remaining species were damaged by the leachate to some


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extent. As mentioned e a r l i e r , we found several phytotoxic phenolics and mimosine produced by L^ leucocephala. It i s notable that the growth of Mimosa pudica was suppressed by Leucaena leaf leachate, even though the leaf juice of M^_ pudica contains a r e l a t i v e l y high amount of mimosine. Among 84 seedlings of M^_ pudica tested only 2 seedlings survived, showing that mimosine can be p r a c t i c a l l y useful to control a notorious weed such as M. pudica i n the f i e l d . Allelopathy And Forestry Plantation Allelopathic nature of some bamboos. On many h i l l s i d e s of mountainous d i s t r i c t s i n Taiwan, there i s a vast area of bamboo plantations, and i n the Chitou area we often found Cryptomeria japonica (conifer) and Phyllostachys edulis (bamboo) growing adjacent to one another. However, the P. edulis often encroaches on the C_j_ japonica area, resulting i n the gradual decline of productivity and ultimately the death of the l a t t e r trees. Chou and Yang (31) found that the l i t t e r of Phyllostachys edulis possesses phytotoxic phenolics, which suppress the growth of i t s understory. The f l o r i s t i c composition of the two vegetations showed that the understory species are d i f f e r e n t . For example, f i v e predominant species of the understory i n the ί\_ edulis community are Ageratum conyzoides, Cornmelina undulata, P i l e a funkikensis, Pratia nummuaria, and Tetrastigma formosana; while i n the C^ japonica community, they are Ficus pumila, Pellionia scabra, Pilea funkikensis, Piper arboriola and Urtica thunbergiana. These species respond d i f f e r e n t l y either to l i g h t intensity or to the phytotoxic leachates, so that there i s a d i f f e r e n t d i s t r i b u t i o n of species density and biomass under the canopy of the two tree species. The t o t a l number and dry weight of seedlings per square meter were much higher i n the conifer community than i n the bamboo forest, although the l i g h t intensity, s o i l moisture, and nutrient contents were s i g n i f i c a n t l y higher i n the bamboo habitat than i n the conifer. Further experimental r e s u l t s indicated that the aqueous extracts and leachates of bamboo leaves were more phytotoxic than those of conifer leaves, r e f l e c t i n g that allelopathy plays a s i g n i f i c a n t role i n the regulation of species diversity and production under the canopy of at least these two forests. Nevertheless, the difference i n potential for species exclusion between the two forests may be due partly to an anatomic factor, such as the rhizome. There are two types of rhizomes, sympodial rhizocauls and horizontal rhizomes with l a t e r a l culms. edulis has the l a t t e r type, which grow rapidly. Thus, the invasion of P_;_ edulis to t e r r i t o r y of C. japonica may be due to (a) the fast-growing rhizomes, which may possibly release phytotoxic root exudates, and (b) a l l e l o p a t h i c substances produced by the bamboo leaves and decomposing l i t t e r . The continuous release of water-soluble phytotoxins from F\_ edulis and accumulation of these i n the s o i l may result i n suppression of the growth of understory or i n elimination of neighboring plants. In addition, the aqueous leaf extracts of 14 bamboo species were evaluated for a l l e l o p a t h i c potential. The bioassay results showed that Sinocalamus l a t i f l o r u s possessed the highest phytotoxicity


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for lettuce, rye grass, and r i c e plants, but Bambusa oldhami, B. pachinensis, B. ventricosa, Phyllostachys edulis, and ί\_ makinoi also showed s i g n i f i c a n t phytotoxicity. Aqueous extracts obtained from the associated bamboo s o i l s also exhibited some i n h i b i t i o n , which i n most extracts was correlated to that of leaf extracts (32). Allelopathic e f f e c t of Leucaena leucocephala. Leucaena leucocephala trees have been widely planted in Taiwan because of i t s high economic value for producing n u t r i t i o u s forage, firewood, and timber. Generally, after a few years of growth, the f l o o r s of these plantations are r e l a t i v e l y bare of understory plants, except Leucaena seedlings. This pattern of weed exclusion beneath Leucaena trees i s p a r t i c u l a r l y pronounced i n areas having a drought season. Chou and Kuo (33) therefore undertook a series of experiments conducted i n f i e l d s , greenhouse, and laboratory. F i e l d data showed that the phenomenon was not due primarily to physical competition, involving l i g h t , s o i l moisture, pH, and nutrients. Instead, aqueous extracts of Leucaena fresh leaves, litter, s o i l , and seed exudate showed s i g n i f i c a n t l y phytotoxic effects on many test species, including r i c e , l e t t u c e , Acacia confusa, Alnus formosana, Casuarina glauca, Liquidambar formosana, and Mimosa pudica. However, the extracts were not toxic to Leucaena seedlings. Decomposing leaves of Leucaena also suppressed the growth of the aforementioned plants grown i n pots but did not i n h i b i t that of Leucaena plants. 3y means of paper and thin-layer chromatography, UV-visible spectrophotometry, and high performance l i q u i d chromatography, 10 phytotoxins were identified. They included mimosine, quercetin, and gallic, protocatechuic, p-hydroxybenzoic, p-hydroxyphenylacetic, v a n i l l i c , f e r u l i c , c a f f e i c , and p-coumaric acids. The mature leaves of Leucaena contain about 5% (dry weight) of mimosine, the amount varying with v a r i e t i e s . Seed germination and r a d i c l e growth of lettuce, r i c e , and rye grass were s i g n i f i c a n t l y i n h i b i t e d by aqueous mimosine solutions at a concentration of 20 ppm while that of the forest species mentioned was suppressed by mimosine solution at 50 ppm or above. Hov/ever, the growth of Miscanthus f l o r i d u l u s and Pinus taiwanensis was not suppressed by a mimosine solution at 200 ppm. Seedlings of Ageratum conyzoides died i n mimosine solution at 50 ppm within 7 days and wilted at 300 ppm within 3 days. It i s concluded that the exclusion of understory plants i s due to the a l l e l o p a t h i c effect of compounds produced by Leucaena. The a l l e l o p a t h i c pattern was most c l e a r l y shown i n the area with a heavy accumulation of Leucaena leaf l i t t e r , which was a r e s u l t of drought and heavy winds. Allelopathic potential of V i t e x negundo. Vitex negundo i s a dominant component of coastal vegetation and widely distributed i n the southern parts of Taiwan. Chou and Yao (34) found that the biomass and density of i t s associated understories are relatively lower than i n adjacent pasture. F i e l d results showed that the natural leachate of V_^_ negundo s i g n i f i c a n t l y retarded the growth of D i g i t a r i a decuabens but stimulated the growth of Andropogon nodosus as compared to the r a i n f a l l control. The growth of


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decumbens grown i n pots under greenhouse conditions was s i g n i f i c a n t l y retarded by watering with a 1% aqueous extract of V. negundo, but the growth of Andropogon nodosus and Mimosa pudica was stimulated. The aqueous extract was phytotoxic to lettuce and rye grass seeds. The aqueous effluents obtained from a polyamide column chromatograph were also bioassayed. Some fractions inhibited r a d i c l e growth of lettuce and r i c e seedlings, whereas other f r a c t i o n s had a stimulatory e f f e c t . The responsible substances were isolated and i d e n t i f i e d . These included phenolic acids, p-hydroxybenzoic, f e r u l i c , p-coumaric, v a n i l l i c , and syringic acids, and 10 flavonoidsT One flavonoid, 3hydroxyvitexin, and nine other flavonoids were i d e n t i f i e d (34). f

Allelopathy And Environment Relationship The actions of many a l l e l o p a t h i c compounds produced by plants are often affected by environmental factors, such as water potential of the environment, temperature, l i g h t i n t e n s i t y , s o i l moisture, nutrient, and s o i l microorganisms. The compounds are released to the environment by means of v o l a t i l i z a t i o n , leaching, decomposition of residues, and root exudation (_1, 5, 7_, 9). Firstly, the terpenoids, such as 0

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