Ann. appl. Biol. (2001), 139:1-9 Printed in Great Britain
Allelopathy in wheat (Triticum aestivum) By H WU1*, J PRATLEY1, D LEMERLE2 and T HAIG1 1
Farrer Centre for Conservation Farming, Charles Sturt University, Wagga Wagga, NSW 2678, Australia 2 NSW Agriculture, Wagga Agricultural Institute, Wagga Wagga, NSW 2650, Australia (Accepted 27 March 2001; Received 9 November 2000) Summary
Wheat (Triticum aestivum) allelopathy has potential for the management of weeds, pests and diseases. Both wheat residue allelopathy and wheat seedling allelopathy can be exploited for managing weeds, including resistant biotypes. Wheat varieties differ in allelopathic potential against weeds, indicating that selection of allelopathic varieties might be a useful strategy in integrated weed management. Several categories of allelochemicals for wheat allelopathy have been identified, namely, phenolic acids, hydroxamic acids and short-chain fatty acids. Wheat allelopathic activity is genetically controlled and a multigenic model has been proposed. Research is underway to identify genetic markers associated with wheat allelopathy. Once allelopathic genes have been located, a breeding programme could be initiated to transfer the genes into modern varieties for weed suppression. The negative impacts of wheat autotoxicity on agricultural production systems have also been identified when wheat straws are retained on the soil surface for conservation farming purposes. A management package to avoid such deleterious effects is discussed. Wheat allelopathy requires further study in order to maximise its allelopathic potential for the control of weeds, pests and diseases, and to minimise its detrimental effects on the growth of wheat and other crops. Key words: Allelopathy, Triticum aestivum, allelopathic potential, weed control, allelochemicals, genetics Introduction The term allelopathy, first coined by Molisch (1937), refers to both detrimental and beneficial biochemical interactions among all classes of plants, including those mediated by microorganisms. The allelopathic effects are due to inhibitory substances that are released directly from living plants into the environment through root exudation, leaching and volatilisation, and through the decomposition of plant residues (Rice, 1984). Whittaker & Feeny (1971) termed these phytotoxic substances allelochemicals. The potential of allelopathy for weed control has been a particularly intense area of study during the past several decades (Rice, 1984; An et al., 1998; Wu et al., 1999a). Wheat (Triticum aestivum L.) seedlings, straw and aqueous extracts of residues have allelopathic effects on the growth of a number of agricultural weeds (Steinsiek et al., 1980, 1982; Shilling et al., 1985; Muminovic, 1991; Wu et al., 1998, 2000a,b). Wheat allelopathy has also been associated with the reduced incidence of pests and diseases (Bohidar et al., 1986; Givovich et al. 1994; Leszczynski, et al., 1995). Autotoxicity is an intraspecific type of allelopathy that occurs when a plant species releases chemical substances that inhibit or delay germination and growth of the same plant species (Putnam, 1985). *Corresponding Author E-mail: [email protected]
© 2001 Association of Applied Biologists
Schreiner & Reed (1907) claimed that the roots of wheat, oats (Avena sativa L.) and other crop plants exude chemicals inhibitory to their own seedlings. The rapid development of conservation farming where stubbles are retained has intensified research on wheat autotoxicity. Poor early growth of wheat and a yield reduction associated with direct drilling have often been observed (Kimber, 1967; Tang & Waiss, 1978). The cause of the yield reduction is unclear, but studies have shown the involvement of phytotoxins released from decomposing wheat residues (Kimber, 1967; Purvis, 1990). Research on wheat allelopathy has examined wheat allelopathy against other crops, weeds, pests and diseases, the isolation and identification of allelopathic agents, wheat autotoxicity, and the management of wheat residues. Screening of wheat varieties for differences in allelopathic potential for weed suppression, and studies on the genetic behaviour of the allelopathic trait, have also been undertaken. The beneficial effects of wheat allelopathy Wheat residue allelopathy for weed suppression Wheat has been successfully used as a cover crop for weed control in various cropping systems (Putnam
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et al., 1983). Cover crops of wheat, rye (Secale cereale L.), sorghum [Sorghum bicolor (L.) Moench], or barley (Hordeum vulgare L.) were allowed to grow to a height of 40-50 cm and then desiccated by overwintering with a contact herbicide. Putnam et al. (1983) reported that the remaining crop residues on the soil surface exhibited up to 95% control of weeds for 30 to 60 days following desiccation of the cover crop. Among the nine cover crops of Italian ryegrass (Lolium multiflorum L.), perennial ryegrass (Lolium perenne L.), creeping red fescue (Festuca rubra L. spp. commutata), tall fescue (Festuca arundinacea Schreb.), Dutch white clover (Trifolium repens L.), rye, wheat, barley, and oats, wheat was particularly useful because it was easy to control chemically, provided reasonable weed supression, and was least inhibitory to the seedling establishment of cucumber (Cucumis sativus L.), soybean [Glycine max (L.) Merr.], snapbean (Phaseolus vulgaris L.), pea (Pisum sativum L.), and corn maize (Zea mays L.) (Weston, 1990). It has been found that the aqueous extract of wheat residues is allelopathic to a number of weeds, and has consistently reduced weed emergence and growth. Under laboratory conditions, aqueous extracts from wheat straw are allelopathic against a broad spectrum of weed species (Steinsiek et al., 1980, 1982; Liebl & Worsham, 1983; Rambakudzibga, 1991). Steinsiek et al. (1980, 1982) reported that weed species differed in their responses to the extracts, with ivyleaf morning glory [Ipomoea hederacea (L.) Jacq.], velvetleaf (Abutilon theophrasti Medic.) being inhibited the most, and Japanese barnyard millet (Echinochloa crus-galli var. frumetaceae (Roxb.) Link], pitted morning glory (Ipomoea lacunosa L.), and sicklepod (Cassia obtusifolia L.) the least. These results suggest that allelochemicals in wheat straws might be leached into the soil to selectively influence the growth of certain weeds in the vicinity. Wheat residue allelopathy differs among varieties. In Australia, 38 wheat accessions were evaluated for residue allelopathy against annual ryegrass (Lolium rigidum Gaud.) by an aqueous extract bioassay (Wu et al., 1998). Results showed that both germination and root growth of ryegrass were significantly inhibited by aqueous extracts of wheat residues and that inhibition differed significantly between accessions. The inhibition for root growth ranged from 19.2% to 98.7%, and for seed germination from 4.2% to 73.2%. The same set of wheat accessions was also employed to test a biotype of annual ryegrass resistant to herbicides of acetyl CoA carboxylase inhibitors (group A), acetolactate synthase inhibitors (B), photosystem II inhibitors (C), and tubulin formation inhibitors (D) (Wu et al., 2001a). Results showed that wheat
aqueous extracts significantly inhibited the germination and root growth of this resistant biotype, with the germination being inhibited by 3.3% to 100% depending upon accession. The allelopathic effects on ryegrass root growth ranged from 12% stimulation to 100% inhibition, compared to a control. The results suggest that wheat allelopathy might also have potential in managing herbicide-resistant weed species. The allelopathic activity of wheat residues against weeds has been further investigated in greenhouse studies (Muminovic, 1991), and in the field (Banks & Robinson, 1980; Thilsted & Murray, 1980). Thilsted & Murray (1980) found that the inhibition of Amaranthus spp. in wheat straw-mulched plots was approximately equivalent to that obtained with herbicides in straw-mulched and bare-soil plots. Banks & Robinson (1980) also reported that a straw mulch suppressed the growth of spiny amaranth (Amaranthus spinosus L.), tall morningglory [Ipomoea purpurea (L.) Roth], and volunteer wheat more than herbicides used on non-mulched areas. Shilling et al. (1985) claimed that wheat mulches had an allelopathic suppressive effect on some broadleaved weeds. The allelochemicals in wheat residues could kill weeds in the next crop sown into the mulched residues under no-till systems (Worsham, 1984). The application of wheat straw for weed suppression has also been extended to forest plantations. Jobidon et al. (1989a) demonstrated that water extracts of wheat straw inhibited propagule growth of the common forest weed red raspberry (Rubus idaeus L.) by 44%. This allelopathic effect was further verified in field experiments (Jobidon et al. 1989b). Wheat seedling allelopathy for weed suppression Wheat has been evaluated for seedling allelopathy on certain weeds. Spruell (1984) screened 286 wheat accessions for allelopathic potential in the USA. Root exudates of each accession were compared with those of a commercial strain, T64, for inhibiting root and shoot growth of Japanese brome (Bromus japonicus L.) and fathen (Chenopodium album L.). Five accessions produced root exudates significantly more inhibitory to the root growth than the commercial strain. When accession CI13633 was grown with B. japonicus on a one-to-one basis in U-tubes containing aerated Hoaglands solution, growth of the weed was approximately 53% of that recorded when grown with T64. More recently, a new laboratory screening bioassay, the equal-compartment-agar-method (ECAM), was developed to assess wheat seedling allelopathy on L. rigidum (Wu et al. 2000a). ECAM was further employed to evaluate seedling allelopathy
against L. rigidum in a worldwide collection of 453 wheat accessions originating from 50 countries (Wu et al., 2000b). Results showed that wheat accessions differed significantly in their seedling allelopathy, inhibiting the root growth of ryegrass over a range from 10% to 91%. Of the 453 accessions screened, 63 accessions were strongly allelopathic, inhibiting the root growth of ryegrass by > 81%, while 21 accessions were weakly allelopathic, inhibiting L. rigidum by < 45%. Wheat seedling allelopathy varied significantly with country of origin, with accessions from Afghanistan, Canada and Poland being weakly allelopathic, and accessions from Germany, Mexico, and South Africa being strongly allelopathic. Wheat progenitors have also been screened for differential seedling allelopathy on the growth of wild oat (Avena fatua L.) and Indian hedge mustard (Sisymbrium orientale L.) (Hashem & Adkins, 1998). It was found that one out of 17 accessions of Triticum speltoides L. inhibited root length of wild oat, and two out of 19 accessions inhibited the radicle length of Indian hedge mustard. Wheat allelopathy on pests and diseases Several studies have examined the effects of wheat allelopathic compounds upon the incidence of pests and diseases. The chemical defensive functions of phenolic acids against pests and diseases have been well documented (Rice, 1984; Appel, 1993). Wheat plants are also known to accumulate several hydroxamic acid glycosides (Hx) during the early stages of development (Willard & Penner, 1976), with the most abundant compound being 2,4dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) (Bohidar et al., 1986). Wheat varieties with higher levels of hydroxamic acids are more resistant to stem rust (Puccinia graminis), and to aphids (Metopolophium dirhodum, Schizaphis gramineum and Sitobion avenae) (Niemeyer, 1988; Bohidar et al., 1986). The concentrations of hydroxamic acids were inversely correlated to aphid number, survival, feeding habit, and reproduction (Corcuera et al., 1992; Givovich et al., 1994). Leszczynski et al. (1995) further reported that resistant winter wheat varieties contained higher concentrations of allelochemicals (such as phenolic compounds, hydroxamic acids and indole alkaloids), than susceptible ones. Breeding for a high level of Hx in plants has been suggested (Niemeyer & Perez, 1995; Niemeyer & Jerez, 1997). Clearly, there is potential to use allelopathy for integrated pest management. This is an area for further research. The detrimental effects of wheat allelopathy Wheat autotoxicity and the growth of wheat Poor seedling establishment and reduced grain yield of the following wheat crop have often been found
when previous wheat straw was mulched (Tang & Waiss, 1978; Hicks et al., 1989; Narwal et al., 1997). Alam (1990) reported that the germination and growth of wheat were significantly decreased by aqueous straw extract of the same variety. Research has shown that wheat autotoxicity varies with varieties (Guenzi et al., 1967; Kimber, 1967). For example, cv. Gabo exhibited a stronger inhibitory effect than cv. Insignia. Straw extracts of cv. Gabo and cv. Insignia inhibited root growth of cv. Gabo wheat seedlings by 27% and 4%, and shoot growth by 43% and 12%, respectively. Allelopathic substances from living wheat plants and straws may accumulate when wheat is cropped continuously. The inclusion of wheat in a farming system, either with monocropping or in rotation with other crops, has the tendency to cause higher autotoxic effects in comparison with those cropping systems without wheat (Young et al., 1989), indicating the accumulation of phytotoxins (Cast et al., 1990). Wheat allelopathy and the growth of other crops Hozumi et al. (1974) claimed that aqueous extracts from wheat residues suppressed growth of rice (Oryza sativa L.), barley and rye. Wheat residues were also allelopathic to the growth of soybean (Herrin et al., 1986). Narwal et al. (1997) reported that aqueous extracts of wheat straw adversely affected the germination and seedling growth of several forage crops, such as corn, sorghum, pearl millet [Pennisetum glaucum (L.) R. Br. emend. Stuntz], clusterbean [Cyamopsis tetragonoloba (L.) Taub.], and cowpea [Vigna unguiculata (L.) Walp.]. The negative effects of wheat residue allelopathy on the growth of other crops varied with wheat varieties. In greenhouse trials, the allelopathic potential of 20 wheat varieties has been shown to differentially influence the growth of soybean cv. Dare grown on soils containing 2% wheat straw residues (Collins & Caviness, 1978). Among the wheat varieties cv. Blueboy had the least allelopathic effect. Susceptibilities to wheat residue allelopathy varied with crops. Putnam et al. (1983) reported the growth of cabbage (Brassica oleracea L. var. capitata), corn , cucumber, lettuce (Lactuca sativa L.), pea, and snapbean responded differentially to wheat residues, and hypothesised that larger seeded crops are more tolerant than the smaller seeded species. Dias (1991) found that wheat, oats, and subterranean clover (Trifolium subterraneum L.) differed in their responses to decomposing wheat straw and associated soil, with mainly stimulation of subterranean clover and the inhibition of cereals. Different crop varieties also respond differentially
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to wheat residue allelopathy. Hicks et al. (1989) screened 11 varieties of cotton [Gossypium hirsutum (L.) Merr.] for the ability to tolerate the inhibitory effects of wheat straws in laboratory bioassays and in greenhouse studies. Tolerant variety, cv. Paymaster 404, and intolerant cv. Acala A246, were identified and used in field experiments. Cotton emergence was reduced by an average of 9% for cv. Paymaster 404 and 21% for cv. Acala A246. In both greenhouse and field trials, soybean varieties were also found to differ significantly in their tolerance to the allelopathic effects of wheat residues, with cv. Davis and cv. Centennial being the most tolerant and cv. Forrest the least (Caviness et al. 1986; Herrin et al., 1986). Managing the detrimental effects of wheat residues The deleterious effects of wheat residues are determined by a number of biotic and abiotic factors, including straw genotype, quantity and extent of decomposition, soil type, soil cultivation, and climatic conditions. Research has shown that these adverse impacts on agricultural production can be largely ameliorated by an efficient management strategy of wheat residues. Careful crop rotation allows the maximum utility of allelopathy with a minimum accumulation of allelochemicals (Putnam & Duke, 1978). Rotations with tolerant crop species or tolerant varieties within a crop species will minimise the allelopathic effects of wheat straws (Putnam et al., 1983; Caviness et al., 1986; Dias, 1991; Herrin et al., 1986; Hicks et al., 1989). Selection of non-allelopathic wheat varieties or tolerant wheat varieties as the next crop is an alternative option. If a specific wheat genotype has natural allelopathic activity, the choice of a tolerant wheat variety can be made. Alternatively, if the choice of a tolerant wheat variety is not feasible, then the first planting of a wheat variety with less allelopathic activity would cause fewer phytotoxic problems for the next wheat crop if the stubble is retained. Breeding for crop varieties with less allelopathic activity was emphasised in former USSR (Grodzinsky, 1987). The amount of aboveground residues that are incorporated into the seedbed during planting is critical to the allelopathic effects of wheat straws. The association between straw quantity and allelopathic activity is well demonstrated (Collins & Caviness, 1978; Hicks et al., 1989; Purvis & Jones, 1990). The negative effect of wheat straw can therefore be overcome by reducing the quantity of straw retained, increasing the sowing rates, and sowing tolerant varieties (Hicks et al., 1989; Purvis & Jones, 1990). The degree of straw decomposition prior to the next crop may affect the allelopathic responses of
the next crop. Allelochemicals leached from the residues into the soil will likely encounter soil adsorption, leaching, microbial transformation or breakdown processes and subsequently lose their allelopathic nature. The loss of allelopathic activity after a period of straw decomposition has been reported (Guenzi et al., 1967; Kimber, 1967). The allelopathic potential from straw would be greatest when the incorporation of residues occurs shortly before seed planting (Collins & Caviness, 1978; Purvis, 1990). An adequate time gap between straw decomposition and crop sowing seems to be a prerequisite for the healthy growth of the next crop. Supplementation of nitrogen fertiliser is a practical solution where N uptake is inhibited by the allelochemicals leached out from the straw and nitrogen immobilisation occurs. The influence of allelochemicals on nutrient uptake and nutrient cycling in the soil has been well documented (Rice, 1984). The supplement of N is effective in ameliorating the depressed growth and yield reduction resulting from mulched wheat straw (Hozumi et al., 1974; Hairston et al., 1987). The applied N may play a dual role in increasing productivity, because it compensates the short-term depletion of N by immobilisation, and it also enhances the activity of soil microbes to break down potential toxins (Rice, 1984). Ammonium ions may also help to detoxify some phytotoxins (Chou & Chiou, 1979). Pretreatment of crop seeds with allelochemicals could be a means of enhancing their resistance to wheat allelopathy. Cowsik & Jayachandra (1979) reported that wheat seeds were hardened by soaking for 4 h in caffeic acid (1 ppm), ferulic acid (5 ppm) and vanillic acid (1 ppm). This phenolic acid treatment also increased the leaf chlorophyll content and productivity, and retarded senescence. Pretreatment with an allelochemical adsorbent such as polyvinylpyrollidone also prevents the attack of allelochemicals (Macfarlane et al., 1982). Activated charcoal has been widely utilised to inactivate pesticides in soils and many other adsorbents could be used to minimise the detrimental effects of allelopathy (Putnam & Duke, 1978). Allelochemicals of wheat Advanced analytical techniques such as gas chromatography-mass spectrometry (GC/MS) and tandem spectrometry (GC/MS/MS) have been applied to identify and quantify wheat allelochemicals (Neves & Gaspar, 1990; Gaspar & Neves, 1993, 1995; Wu et al., 1999b). A number of phytotoxic substances suspected of causing allelopathic and autotoxic effects have been identified from wheat. There are three main categories identified, namely, phenolic acids, hydroxamic acids and short-chain fatty acids.
Phenolic acids Phenolic acids have been identified as allelopathic agents in wheat with p-hydroxybenzoic, vanillic, pcoumaric, syringic and ferulic acids being most frequently reported, and trans-ferulic and trans-pcoumaric acids being the dominant acids (Salomonsson et al., 1978; Lodhi et al., 1987; Wu et al., 1999b, 2000c,d, 2001b). Ferulic and p-coumaric acids inhibited the radicle growth of radish (Raphanus sativus L.) at concentrations of 10-4-10-3 M (Lodhi et al., 1987). Liebl & Worsham (1983) found that ferulic acid reduced germination and root length of pitted morning glory, tall morning glory, common ragweed (Ambrosia artemisiifolia L.) and prickly sida (Sida spinosa L.) at 5 ´ 10-3 M. Blum et al. (1991) reported that a mixture of phenolic acids similar to that obtained from the wheat soil under no tillage systems reduced radicle and hypocotyl length of crimson clover (Trifolium incarnatum L.). Individual phenolic acids also reduced radicle and hypocotyl length of crimson clover and ivyleaf morning glory (Blum et al., 1992). Recently, a GC/MS/MS technique was employed to investigate the chemical basis of wheat allelopathy by analysing seven known phenolic acids, phydroxybenzoic, vanillic, cis-p-coumaric, syringic, cis-ferulic, trans-p-coumaric, and trans-ferulic acids, in a worldwide collection of 58 wheat accessions previously found with varied allelopathic potential (Wu et al., 2000d, 2001b). It was found that accessions differed significantly in the production of each phenolic acid. The concentrations of phenolic acids in shoots of 17-day-old seedlings of the 58 accessions ranged from 9.8 to 49.3, 12.9 to 68.8, 0.8 to 11.2, 1.9 to 61.5, 0.2 to 17.0, 11.4 to 117.7, and 3.2 to 149.3 mg kg-1 dry weight for p-hydroxybenzoic, vanillic, cis-p-coumaric, syringic, cis-ferulic, trans-pcoumaric, and trans-ferulic acids, respectively. In the roots, these acids ranged from 24.5 to 94.5, 19.9 to 91.7, 3.7 to 15.4, 2.2 to 38.6, 1.0 to 42.2, 19.3 to 183.6, and 11.7 to 187.6 mg kg-1, respectively. The concentration of total identified phenolic acids in the 58 accessions varied from 93.2 to 453.8 and 85.9 to 547.4 mg kg-1 dry matter in the shoots and roots, respectively (Wu et al., 2000d, 2001b). Research has shown that wheat seedlings exuded phenolic acids into a growth medium, with the amounts varying with particular compounds and wheat accessions (Wu, 1999; Wu et al., 2000c). Multiple regression analysis showed that wheat seedling allelopathy was significantly associated with all the compounds analysed in the shoots, roots, and root exudates. In comparison with weakly allelopathic accessions, strongly allelopathic ones produced significantly higher amounts of allelochemicals in the shoots and roots, and also exuded larger quantities of allelochemicals into the growth medium (Wu, 1999; Wu et al., 2000c,d, 2001b).
Hydroxamic acids Cyclic hydroxamic acids (Hx), a class of alkaloids, were identified as another category of biologically active agents in resistance to pests (Bohidar et al., 1986; Leszczynski et al., 1995) and diseases (Niemeyer, 1988), and in weed suppression (Perez, 1990; Blum et al., 1992). DIMBOA and its decomposition product MBOA (6methoxybenzoxazolin-2-one) inhibited root growth of wild oat (A. fatua L.) by 50% at concentrations of 0.7 and 0.5 mM, respectively. MBOA also inhibited seed germination of wild oat (Perez, 1990). Blum et al. (1992) found MBOA was more potent than its precursor (DIMBOA) and inhibited germination and radicle and hypocotyl length of crimson clover and ivyleaf morning glory. Wheat varieties differ in DIMBOA production. The concentration of DIMBOA ranged from 1.4 to 10.9 mmol kg-1 fr. wt in 52 Chilean wheat varieties (Copaja et al., 1991), and from 0.99 to 8.07 mmol kg-1 fr. wt in a worldwide collection of 47 wheat varieties (Nicol et al., 1992). Niemeyer (1988) screened 55 accessions of 17 wheat progenitors and found that the content of hydroxamic acids was highest in Triticum speltoides (16.0 mmol kg-1 fr wt) and lowest in Triticum tauschii (0.21 mmol kg-1 fr. wt). Fifty-eight wheat accessions were analysed for the differential production of DIMBOA from the shoots, roots, and root exudates of 17-day-old seedlings (Wu, 1999). DIMBOA content differed significantly in the shoots and in the roots between accessions. The variation of the DIMBOA concentration in the 58 accessions was similar for both shoots and roots, ranging from no detectable amount to 730 mg kg-1 dry matter. Forty-seven out of the 58 accessions did not exude detectable amounts of DIMBOA through their living roots into a growth medium, although substantial levels of DIMBOA were found in their shoot or root tissues. For the 11 accessions which exuded DIMBOA into the growth medium, amounts ranged from 8.6 to 79.1 mg litre-1 of water agar. These results demonstrated that the exudation of DIMBOA by living wheat roots was highly accession-dependent, indicating the presence of genetic factors governing the exudation process of DIMBOA. Short-chain fatty acids Short-chain fatty acids (aliphatic acids) have been claimed as a third category of compounds implicated in wheat allelopathy. The production of short-chain fatty acids as a result of anaerobic fermentation of the insoluble polysaccharides which represent the major constituents of wheat straw, can also adversely affect crop development in soils of low redox potential (Lynch, 1978; Tang & Waiss, 1978). Lynch et al. (1980) reported that the acetic acid concentration of
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freshly harvested straw measured before absorption of soil moisture was 384 mM. Hairston et al. (1987) found that leachate from wheat straw at the concentration of 20 g litre-1 contained 10 mg kg-1 of acetic acid. Lynch (1977) commented that acetic acid inhibited barley root extension at concentrations of about 5 mM at pH 3.5 and 15mM at pH 6.5. Tang & Waiss (1978) also found that extracts from fermented suspensions of wheat straw inhibited wheat seedling growth. The major allelopathic agents were identified as salts of acetic, propionic and butyric acids. Traces of isobutyric, pentanoic, and isopentanoic acids were also identified (Tang & Waiss, 1978; Lynch et al., 1980). Synergistic effects of acetic, propionic, butyric and valeric acids were reported (Wallace & Whitehand, 1980). Lynch et al. (1980) concluded that the presence of acetic acid might be a major cause of poor establishment and growth when seeds and seedling roots come into contact with wheat straw. Other allelopathic agents Allelochemicals in wheat belong to far more than the three categories of compounds mentioned above. As the research deepens, more biologically active compounds are identified. Neves & Gaspar (1990) identified 20 allelopathically active compounds from wheat straw extracts. Most of them are phenolic acids but naphthoic acid, azelaic acid and 1,2,3,5tetrabromobenzene are also reported. Steroidal constituents were also found (Gaspar & Neves, 1993). Gaspar & Neves (1995) further identified 62 compounds in two allelopathic fractions from wheat straw. The compounds were carboxylic acid methyl esters, phenolic acids and triterpenoids. The involvement of a complex of allelochemicals suggests that further research is necessary to understand fully the chemical basis for wheat allelopathic and autotoxic effects. Genetic control of allelopathic activity in wheat Study of the genetic control of the allelopathic trait is important for the manipulation of crop varieties with elevated allelopathic potential. Dilday et al. (1992) postulated that the use of herbicides could be reduced, perhaps dramatically, if the genes for allelopathic effects on important weeds can be transferred into commercial varieties. Investigations have been made to understand the genetic basis of crop allelopathy (Niemeyer & Jerez, 1997; Dilday et al., 1998; Bach Jensen et al., 1999; Wu et al., 2000b). In a study of allelopathic activity of a population of 400 F 2 rice plants on ducksalad [Heteranthera limosa (Sw.) Willd.], Dilday et al. (1998) found that rice allelopathic activity was normally distributed, suggesting that the rice
allelopathic trait was quantitatively inherited. Recently, the restriction fragment length polymorphism (RFLP) technique has been employed to investigate the genetic markers associated with rice allelopathic activity on barnyardgrass [Echinochloa crus-galli (L.) Beauv.] in a population of 144 recombinant inbred lines derived from a cross between IAC 165 (japonica upland variety, strongly allelopathic) and CO 39 (indica irrigated variety, weakly allelopathic). Six putative quantitative trait loci (QTLs) for allelopathy were located on chromosomes 2, 3, 4, 5, 6, and 9, using single marker analysis (Bach Jensen et al., 1999). Only one genetic study of wheat allelopathy has been published (Wu et al., 2000b). Wheat seedling allelopathy on L. rigidum was normally distributed in a world-wide collection of 453 wheat accessions, indicating that this weed-suppressing ability is quantitatively inherited. Among the 453 accessions screened, 30 accessions shared the parent Condor and 12 accessions shared the parent Pavon. Results showed that the two distinct types of accessions differed significantly in their allelopathic activity. Accessions with Condor-background were more allelopathic than those with Pavon-background. Near isogenic lines derived from Hartog (Pavonbackground) and Janz (Condor-background) were further screened in order to investigate the genetic control of wheat allelopathy with L. rigidum as a test species (Wu et al. 2000b). Of the two parents, Hartog was weakly allelopathic, while Janz was strongly allelopathic. The allelopathic activity of BC 2 Hartog lines (backcrossed to Hartog) was very weak and was similar to that of parent Hartog. Janz lines had strong allelopathic activity, similar to that of Janz. These results suggest that allelopathic activity is genetically controlled (Wu et al., 2000b). Further work is underway to identify the QTLs conferring wheat allelopathic activity. Niemeyer & Jerez (1997) investigated the chromosomal location of genes controlling the production of hydroxamic acids in wheat by studying the euploid of variety Chinese Spring (high in DIMBOA) with aneuploids of the same variety, and substitution lines of variety Chinese Spring with variety Cheyenne (low in DIMBOA). A multigenic model was proposed for the accumulation of hydroxamic acids in wheat. Chromosome 4A and 4B may contain genes for the transformation of 2,4dihydroxy-2H-1,4-benzoxazin-3-one (DIBOA) into DIMBOA and chromosome 5B for the transformation of methoxylated lactam into DIMBOA. In addition, there might be a gene in chromosome 4D inhibiting the accumulation of Hx. The genetic study of allelopathy is still in its infancy: it does however represent a promising new frontier for further research.
Conclusions and future research The development of crop varieties with allelopathic capability for weed suppression is receiving worldwide attention (Wu et al., 1999a). Research is needed in understanding the genetic control of wheat allelopathy prior to the development of allelopathic wheat varieties. Modern DNA technology makes it easier to locate the genes for controlling allelopathic activity and the production of allelochemicals. Based on previous work, genetic mapping populations between allelopathic and non-allelopathic accessions are being constructed in order to identify genetic markers associated with wheat allelopathy. However, genetic manipulation of allelochemical production of a crop should not result in adverse effects on humans, livestock, other non-target organisms or the environment (Einhellig & Leather, 1988). Potential gene flow to other varieties, crops or to weed species will also need to be monitored. The penalties associated with the use of allelochemical control also need to be identified and quantified (Wu et al., 1999a). Numerous compounds have been identified as responsible agents for wheat allelopathy. However, most of the results are derived by chemical identification of agents from within wheat tissues. Compounds present in plant tissues may possess allelopathic potential (residue allelopathy), but they may not be leached or exuded from the plant into the environment to exhibit seedling allelopathy under natural conditions (Wu et al., 2000c). To establish their involvement in wheat allelopathy, it is also important to demonstrate their direct release from the plant into the growth environment. More research is needed to isolate, identify and quantify allelochemicals from root exudates of living wheat plants, especially during the early seedling stage. In the investigation of the chemical basis for differential wheat seedling allelopathy on the growth of annual ryegrass, it was found that the identified phenolic acids and DIMBOA do not completely account for all the observed variety-specific allelopathic activity. Together, the eight allelochemicals in wheat root exudates only explained 51.0% of the variation in the growth inhibition of ryegrass (Wu, 1999). Other active but unknown allelochemicals exuded by living wheat roots must therefore also be involved in wheat allelopathy and should be investigated. Once a relatively comprehensive array of allelochemicals is determined, there will also be a need to investigate the combined effects of artificial allelochemical mixtures using similar concentrations to those present in root exudates, as synergistic effects have been reported (Einhellig, 1996). A comprehensive analysis of multiple allelopathic compounds in their chemically distinct groups is required to help unveil the principles of wheat allelopathy.
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