Allelopathy is defined as biochemical interactions between one plant or microorganism (alga, bacteria, or virus) and another plant through the production of chemical compounds - secondary metabolites (allelochemicals), which influence, direct or indirect, harmful or beneficial, plant growth and development (Rice 1984). Allelochemicals are present in almost all plants and in many tissues, like leaves, stems, flowers, fruits, seeds, roots, or pollen and may be released from plants into the environment by volatilization, leaching, root exudation, and decomposition of plant residues (Chou 1990).
Allelochemicals include terpenoids, phenolic compounds, phenylpropane derivatives, flavonoids, organic cyanides, long-chain fatty acids, and others. Potential allelopathic plants are most often connected with the total phenol content (Ben-Hammouda et al. 2001; Lee et al. 2004). For example, cereal residues release 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) and a breakdown product 2(3H)-benzoxazalinone (BOA) that both strongly inhibit the growth and germination especially of dicotyledonous plants (Tabaglio et al. 2008). Therefore BOA was suggested as a potential herbicide (Durtn-Serantes et al. 2002). Sorgoleone, benzo-quinone isolated from sorghum is another example of a strong allelopathic inhibitor.
Effects and presence of many other compounds in different crops was described by a lot of researchers, e.g., by Birkett et al. (2001).
The released chemicals are usually a mixture of many organic compounds and may exert toxicities in a synergistic manner. According to Hegde and Miller (1992), mixtures of five or more phenolic acids were more phytotoxic than their individual components except in the case of trans-cinnamic acid. Strong synergistic effects were observed among the identified allelochemics in vulpia (Vulpia spp.) residues (An et al. 1998). Sometimes small amount of an ingredient can enhance allelopathic effect dramatically. Shiming (2005) reported that Precoene I and Precoene II are the two major allelochemicals in Ageratum conyzoides L. but their mixture did not have the synergistic effect. On the other hand, bisabolene, caryophyllene, and fenchy-lacelate are not very important in allelopathy of this crop, but they caused enhancement of effects if they were mixed with Precoene II individually or all together.
The visible allelopathic effects include inhibition of germination; reduced growth; swelling or necrosis of roots; root curling; discoloration, lack of root hairs; increased number of seminal roots; reduced dry weight accumulation and decreased reproductive capacity (Rice 1984). The allelochemicals affect a large number of physiological functions and biochemical reactions: enzyme activities, cell division and ultrastructure, cell elongation, membrane permeability, and ion uptake. Some allelochemicals isolated from higher plants inhibit photosynthesis and respiration, e.g., juglone (a toxic compound present in black walnut (Juglans nigra L.), and increase oxidative stress (Terzi et al. 2003). Gniazdowska and Bugatek (2005) recorded individual physiological effects of different allelochemicals. The same compound can act as an allelochemical and in other case can share another role. Allelochemicals are probably directly transferred to target plants by cell-cell contact and the physical contact of roots with an allelochemical is more important than uptake of this compound (Inderjit 2003). Their toxicity is depended on concentration. Most of the isolated allelochemicals exhibit bioactivity at concentrations from 10-5 to 10-10 M (Macias et al. 2001). Lower concentrations can have stimulative effects. Plant survival in allelopathy stress depends on resistant mechanisms leading to detoxication.
Allelopathic potential of plants is affected by many factors. An increase of allelopathic effects was observed by the water deficit, high temperature, high irradi-ance, pathogen, insect and herbivore damages, or nutrients deficiency (Hura et al. 2006). In other case, the concentrations of the allelochemicals released from the allelopathic rice seedlings in soil increased dramatically (3-fold higher) when they were surrounded with barnyard grass (Echinochloa crus-galli (L.) Beauv.) (Kong et al. 2006). Some metabolites having allelopathic potential might be newly synthesized by UV irradiation (Kim et al. 2000).
On the other hand, allelopathic effect was negatively influenced by rainfall (Shiming 2005). The inhibition process was mitigated by shading and consequent moisture conservation (Muller 1966). Allelopathic activity can vary as well with photoperiod (Peng et al. 2004). Harder et al. (1998) found out that an increasing availability of nutrients decreased the concentration of allelopathic effective phenolic compounds in the plants of two winter wheat varieties. In soil, allelochemicals can be adsorbed by soil particles, decomposed by microorganisms and move with water.
Phenolic acids can be allelopathic but their presence in soil is ephemeral due to rapid degradation and/or sorption by soil particles (Inderjit 2004). Sorption of benzoic acid onto soil particles increased with concentration and it may explain the reason for the limited allelopathic effect of benzoic acid at concentrations often recorded in natural soil (Inderjit 2004). Microorganisms help to generate allelochemicals, but they may also modify toxic compounds into nontoxic compounds (Khanh et al. 2005). Allelochemicals are changed in composition and quantity during the residue decomposition. Allelopathy plays an important function in nutrient recycling (Rice 1984).
The production and active release of allelochemicals from donor plants depends not only upon external environmental conditions but also upon the relative developmental stages of crops, plant tissues, and genetic disposition (Batish et al. 2001; Peng et al. 2004). Zuo et al. (2007) described the highest allelopathic potential in the tillering stage and the weakest in the seed filling stage of wheat. From plant tissues, leaves are usually the richest in the allelochemical content (Kalinova 2008). Significant varietal differences in allelopathic potential were established among winter wheat accessions (Zuo et al. 2007) and other crops (see Chapter: Varieties with strong allelopathic potential). The effect of allelochemicals can also depend on root absorption. According to Ferrarese et al. (2000), soya bean roots absorbed fer-ulic acid at greater rates when the concentrations ranged from 0.05 mM to 1.0 mM and the absorbed amount of the given compound was concentration dependent.
Allelopathic effects on a receiver plant are also affected by environmental factors. Environmental stresses affect both donor and receiver plants. They increase allelochemical production in the donor plant; on the other hand, they cause an increase in the dosage effect on the receiver plant (Shiming 2005). However, there exist genetic differences in sensitivity among accessions or species, e.g., large-seeded and deeper-seeded species are less sensitive to the allelochemicals than small-seeded and lower-seeded species (Chase et al. 1991). Small-seed species have greater absorptive surface area of roots through which allelochemicals may enter and they have fewer reserves which support seedling respiration during stress periods (Westoby et al. 2002). Ability to detoxify allelochemicals might also contribute to differences among species (Liebman and Sundberg 2006).
In nature, allelopathy forms a complex with competition for resources and both processes are very difficultly separable from each other (Kim and Shin 2003). Competition is the process in which a plant upon the habitat reduces the level of a necessary factor (radiant energy, oxygen, carbon dioxide, mineral nutrients, and water) to the detriment of another plant sharing the same habitat (either simultaneously or sequentially). Competition occurs only if the reaction involves a reduction demonstrably deleterious to another individual (Staman et al. 2001). Juvenile plants are less competitive than mature plants, e.g., deep-rooted, established alfalfa plants are better competitors for nutrients, water, and light than young seedlings. Well-established weeds may also compete with crop seedlings (Gray 1998).
Weed management in organic agriculture use preventive methods such as an appropriate crop rotation, precise soiled preparation before sowing crops, narrow seed spacing, etc. (Labrada 2003). Many of them include ways in which allelopathy (often together with competition) could play an important role (Fig. 14.1).
Preventive methods e.g. clean seeds; clean adjacent area; clean machines allelopathy
soil management (tillage) stand organisation etc.
Cultural methods crop rotation; variety selection; use of crop residues (green manure, mulching); cover crops, intercropping...
soil management (tillage) stand organisation etc.
Physical methods e.g. burning;
Biological methods e.g. insect, fungi plant extracts, pellets
Fig. 14.1 Utilization of allelopathy in organic weed management. Weed management in organic agriculture includes four groups of different methods for weed reduction. Allelopathy plays an important role in some cultural and biological methods
Allelopathic interactions of plants can provide weed control by: (a) use of allelopathic crops as cover crops, mulches or green manure, (b) use of allelopathic plants in crop rotations, (c) crop mixtures and intercropping, (d) varieties with strong allelopathic potential, and (e) use of allelopathic crop water extracts and other agents.
However, both crops and weeds can have allelopathic effects. For example, quack grass (Elymus repens (L.) Gould) shoots and rhizomes reduce the emergence and growth of alfalfa, cause chlorotic and stunted growth of oats and barley and reduce root nodulation in numerous legumes (Weston and Putnam 1985). Allelopathic potential were established in such weed species as Russian knapweed (Acroptilon repens (L.) DC) (Alford et al. 2007), birdsfoot trefoil (Lotus corniculatus L.), devil's beggarticks (Bidens frondosa L.), mile-a-minute weed (Polygonum perfoliatum (L.) H. Gross), jimson weed (Datura stramonium L.) and Cephalonoplos segetum (Bunge) Kitam. (Kim et al. 1987), Artemisia asiatica (Pampan.) Nakai ex Kitam., Shepherd's purse (Capsella bursa-pastoris (L.) Medik.), common purslane (Portulaca oleracea L.) and white clover (Trifolium repens L.) and others (Chun et al. 1988).
Hence, allelopathy alone can not be a sufficient tool for weed control. Combinations of methods that prevent weed germination and control weeds have to be used (Rasmussen 2004).
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