ALLELOPATHY AND GRAIN CROP PRODUCTION

TOP ALLELOPATHY AND GRAIN CROP... Rice (Oryza sativa) Wheat (Triticum aestivum) Maize (Zea mays) Sorghum (Sorghum bicolor) Barley (Hordeum vulgare) LITERATURE CITED

Allelopathy is the direct influence of a chemical released from one living plant on the development and growth of another. Many researchers have speculated that allelopathy might prove useful in controlling weeds and increasing grain yields. Selection for genotypes with enhanced allelopathic potentials has been carried out in several field crops, and evidence has accumulated that crop cultivars differ significantly in their ability to inhibit the growth of certain weed species. Although traditional breeding methods have not been successful in producing highly allelopathic grain crops with good yields, genetic engineering has the potential for overcoming this impasse (Duke et al., 2001). Conceivably, genetic engineers could enhance the production of allelochemicals already present in a crop or impart the ability to produce new compounds. With either strategy, there are potential problems that must be overcome with regard to tissue-specific promoters, autotoxicity, metabolic imbalances, and proper movement of the allelopathic compound to the rhizosphere. Thus, there is a considerable amount of research to do in all the subdisciplines surrounding this problem. This article highlights some recent developments in this field.

	Rice (Oryza sativa)

TOP ALLELOPATHY AND GRAIN CROP... Rice (Oryza sativa) Wheat (Triticum aestivum) Maize (Zea mays) Sorghum (Sorghum bicolor) Barley (Hordeum vulgare) LITERATURE CITED

Allelopathic rice germplasm has been identified in laboratory and greenhouse screenings. Rice cultivars of high allelopathic potential can suppress both mono- and dicot weed species. Field experiments revealed that allelopathy accounted for 34% of the overall competitive ability in rice. Recombinant inbred lines (RILs) have been developed for the identification of quantitative trait loci (QTL) controlling allelopathy (Olofsdotter et al., 2002a). The allelochemicals responsible for the growth inhibition of rice-associated weeds, however, have not yet been identified for certain. Several putative allelochemicals are found in extracts of rice leaf and straw, decomposing straw, and in rice soils. Although phenolic acids are often mentioned as putative allelochemicals, this idea has recently been criticized (Olofsdotter et al., 2002b). Phenolic acids concentrations are normally greater in submerged than in aerobic soils. A dose-response study, however, showed that seedlings of rice cultivars adapted to submerged anaerobic soils did not have a higher level of tolerance against p-hydroxybenzoic acid than did seedlings of varieties adapted to aerobic upland soils. Moreover, the rates at which rice plants released phenolic acids into solution cultures was calculated to be insufficient for attaining phytotoxic levels in the soil. Phenolic acids might be just one component in a mixture of chemicals that, when present simultaneously, are allelopathic (Olofsdotter et al., 2002b). Kato-Noguchi et al. (2002) have presented evidence that momilactone B may play an important role in rice allelopathy.

	Wheat (Triticum aestivum)

TOP ALLELOPATHY AND GRAIN CROP... Rice (Oryza sativa) Wheat (Triticum aestivum) Maize (Zea mays) Sorghum (Sorghum bicolor) Barley (Hordeum vulgare) LITERATURE CITED

Both wheat residue and wheat seedling allelopathy are being studied for their possible utilization in weed management. Wheat varieties differ greatly in their allelopathic potential against weeds. Wu et al. (2000) evaluated wheat seedling allelopathy against annual ryegrass (Lolium rigidum) in a collection of 453 wheat accessions originating from 50 countries. The effectiveness of different accessions in their ability to inhibit root growth of ryegrass ranged from 10% to 91%. Wheat allelopathic activity was normally distributed within the collection, indicating the involvement of multiple genes conferring the allelopathic trait. Thus, there is considerable genetic variation of allelopathic activity in wheat germplasm, and it may be possible to breed for cultivars with enhanced allelopathy for weed suppression. Several categories of allelochemicals for wheat allelopathy have been identified, including phenolic acids, hydroxamic acids, and short-chain fatty acids (Wu et al., 2001a). Wu et al. (2001b) have implicated 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one as another important allelochemical in wheat. Research is under way to identify genetic markers associated with wheat allelopathy. Wheat allelopathic activity is genetically controlled, and a multigenic model has been proposed (Wu et al., 2001a).

Allelopathy is also of agricultural importance because of the phenomenon of autotoxicity -a type of intraspecific allelopathy, where a plant species inhibits the growth of its own kind through the release of toxic chemicals into the environment (Singh et al., 1999). This phenomenon causes "soil sickness," particularly in semi-arid climates, where the soil "cleansing" affects of rainfall are less frequent. 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 (Wu et al., 2001a).

	Maize (Zea mays)

TOP ALLELOPATHY AND GRAIN CROP... Rice (Oryza sativa) Wheat (Triticum aestivum) Maize (Zea mays) Sorghum (Sorghum bicolor) Barley (Hordeum vulgare) LITERATURE CITED

Allelopathy in maize has attracted less attention than allelopathy in rice or wheat. An important clue toward identifying the allelochemicals of maize is that the allelopathic potential of maize seedlings is enhanced by visible light. Kato-Noguchi (1999) found six substances with inhibitory activity in the acetone extract of germinating maize seedlings. One of these substances, identified as 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) was higher in light-grown maize than in dark grown. At concentrations greater than 0.03 mM, DIBOA inhibited the growth of roots and hypocotyls of lettuce (Lactuca sativa) seedlings. The concentrations of DIBOA in the light-grown maize seedlings and their root exudates were 43 and 0.38 µmol kg¹ fresh weight, respectively, and concentrations in the dark-grown seedlings and their root exudates were 19 and 0.17 µmol kg¹ fresh weight, respectively. The level of DIBOA in the dark-grown seedlings increased rapidly upon visible light irradiation. These results suggest that visible light may enhance allelopathic activity of germinating maize due to an increase in the level of DIBOA.

	Sorghum (Sorghum bicolor)

TOP ALLELOPATHY AND GRAIN CROP... Rice (Oryza sativa) Wheat (Triticum aestivum) Maize (Zea mays) Sorghum (Sorghum bicolor) Barley (Hordeum vulgare) LITERATURE CITED

Allelopathy by sorghum frequently harms wheat and peanuts (Arachis hypogea) when these crops are grown in rotation with sorghum. Much recent research has focused on cultivation techniques that may reduce allelopathic residues left by previous sorghum crops. Roth et al. (2000) found that prompt tillage of the mature sorghum stalks delayed development of the following wheat crop but did not affect grain yields, probably because allelopathic compounds degraded in the soil. No-till sorghum stover had little effect on stand establishment but frequently reduced grain yields of wheat, possibly because allelopathic compounds leached slowly. However, if erosion of soil is not a concern, allelopathy might be reduced by prompt tillage and other practices that promote rapid decomposition of sorghum stover. Sene et al. (2000) found that peanut seedling establishment was better between rows than on rows of previous sorghum crop. They proposed a geometrical sowing pattern for peanuts between the rows of the previous sorghum crop to escape the latter's "allelopathic heritage." They also examined the phenolic content of the row and interrow soils but did not find consistent data from year to year, suggesting possibly that phenolics are not the principal compounds responsible for sorghum allelopathy.

Although the introduction of sorghum into a crop rotation is often detrimental to crop yield, sorghum allelopathy, of course, does not only hinder the "good" plants in the fields: It also hinders many weeds. Paradoxically, if the weeds are inhibited by sorghum's allelochemical heritage more than the crops are, the crop plants can actually prosper because of the reduced competition by weeds. Thus, sorghum allelochemicals are actually used as a natural herbicide called sorgaab (a water extract of mature sorghum plants obtained after soaking in water for 24 h). Cheema and Khaliq (2000), for example, found that sorgaab applications reduced weeds by 35% to 49% and increased wheat yield by 10% to 21%.

	Barley (Hordeum vulgare)

TOP ALLELOPATHY AND GRAIN CROP... Rice (Oryza sativa) Wheat (Triticum aestivum) Maize (Zea mays) Sorghum (Sorghum bicolor) Barley (Hordeum vulgare) LITERATURE CITED

Autotoxicity has been found to be a problem in at least one cultivar of barley grown in Tunisia. Ben-Hammouda et al. (2001) reported that this same cultivar of barley was autotoxic to other cultivars of barley, though not to itself. Leaves were the most important source of allelopathic substances. This same cultivar of barley was also found to be phytotoxic to durum wheat (Triticum durum) and bread wheat (T. aestivum). Seedling growth bioassays demonstrated that the two wheat species responded differently to the allelopathic potential of barley with a greater sensitivity shown by the bread wheats. For both wheat species, radicle growth was more depressed than coleoptile growth, though stimulation of seedling growth was observed for durum wheat. Leaves and roots were the most phytotoxic barley plant parts for durum and bread wheats, respectively. Results suggested that the response by durum wheat and bread wheat varied depending on the source of allelochemicals (i.e. plant part) and the growth stage of the barley plant. Consequently, barley should be considered a depressive prior crop for both durum wheat and bread wheat in a field cropping sequence.