Allelopathy rice allelochemicals

Abstract All plants produce compounds that are phytotoxic to another plant species at some concentration. In some cases, these compounds function, at least in part, in plant/plant interactions, where a phytotoxin donor plant adversely affects a target plant, resulting in an advantage for the donor plant. This review discusses how such an allelochemical role of a phytotoxin can be proven and provides examples of some of the more studied phytochemicals that have been implicated in allelopathy. These include artemisinin, cineoles, P-triketones, catechin, sorgoleone, juglone and related quinones, rice allelochemicals, benzoxazinoids, common phenolic acids, l-DOPA, and m-tyrosine. Mechanisms of avoiding autotoxicity in the donor species are also discussed. 

Non-protein amino acids have previously been implicated in allelopathy. For example, mimosine has been associated with allelopathy of the legume tree Leucaena leucocephala [168]. l-DOPA, a compound structurally related to uj-tyrosine (Fig. 5), has been implicated in allelopathy of Mucuna pruriens ([158] and Section Rice Allelochemicals). Roots of pea (Pisum sativa) exude j3-(3-isoxazolin-5-on-2yl)-alanine which inhibits root growth on nonlegume plant species [169], although this non-protein amino acid is much less phytotoxic than m-tyrosine. 

One of the features of allelopathy, as pointed out by Rice (18). is that it is specific. Certain species, but not others, are affected by the allelochemics produced by a plant. This suggests that allelopathy by a weed must be positively demonstrated for each crop. References to specific reports of demonstrated allelopathy by a weed on a crop are given in Table 1. In some cases [crabgrass (Digitaria sanguinalis (L.) Scop.), spurge (Euphorbia spp.), etc.], no crop is listed in the allelopathy column. 

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). 

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). 

Mattice J, Lavy T, Skulman B, Dilday R (1998) Searching for allelochemicals in rice that control ducksalad. In Olofsdotter M (ed) Allelopathy in Rice, Proceedings of workshop on Allelopathy in Rice, IRRI, Manila, pp 81-98... 

Many compounds with phytoalexin activity are also implicated in allelopathy between plants (Rice, 1984). For example, isoflavonoids are important phytoalexins (Ingham, 1982 Paxton, 1981 Dakora and Phillips, 1996) and allelochemicals (Tamura et al. 1967 1969) from the Leguminosae. Parbery et al. (1984) found that the isoflavonoids biochanin A, formononetin and genistein increased in subterranean clover by 62%, 123% and 75% respectively following infection by pepper spot (Leptosphaerulina trifolii). In comparison, Tamura et al. (1967 1969) isolated a succession of isoflavonoids (including biochanin A, formononetin and genistein) from the shoots of red clover (Trifolium pratense) that inhibited its own germination by 50% at concentrations of 50 ppm. 

Mattice J., Lavy T., Skulman B., Dilday R. Searching for allelochemicals in rice that control duck salad. In Allelopathy in Rice. M. Olofsdotter, ed. International Rice Research Institute, Manila, Philippines, 1998 pp. 81-98. 

Rice E.L. Allelopathy An overview. Allelochemical Role in agriculture and forestry. American Chemical Society Symposium Series 330,1987 pp. 8-22. 

Rice EL (1983) Pest control with nature s chemicals allelochemics and pheromones in gardening and agriculture. University of Oklahoma Press, Norman, OK Rice EL (1984) Allelopathy. Academic Press, Orlando, FL... 

Wieland, I. et al. (1999) Detoxification of benzoxazolin-2(3//)-one in bigber plants. In Recent advances in allelopathy Yo. 1 (Macias, F.A. et al., eds). Servicio e Publicaciones-Univ. Cadiz, Spain, pp. 47 56 Rimando, AM. and Duke, S.O. (2003) Rice allelopathy. Inifice production Origin, history, and technology (Smith, C.S. and Dilday, RH. eds). Wiley, New York, pp. 221 244 Olofsdotter, M. et aL (2002) Why phenolic acids are unbkely primary allelochemicals in rice. J. Chem. Ecol. 28, 229 242... 

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