Weed Management

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Bhowmik and Inderjit (2003) examined some considerable efforts in designing alternative weed management strategies due to increase in the number of herbicide-resistant weeds and environmental concerns in the use of synthetic herbicides. The conventional synthetic herbicides are becoming less and less effective against the resistant weed biotypes. These authors discussed the role of allelopathic cover crops/ crop residues, natural compounds, and allelopathic crop cultivars in natural weed management, and gave numerous examples of employing crop residues, cover crops, and allelopathic crop cultivars in weed management. They concluded that although we cannot eliminate the use of herbicides, their use can be reduced by exploiting allelopathy as an alternate weed management tool for crop production against weeds and other pests.

The use of allelopathy for controlling weeds could be either through directly utilizing natural allelopathic interactions, particularly of crop plants, or by using allelochemicals as natural herbicides. In the former case, a number of crop plants with allelopathic potential can be used as cover, smother, and green manure crops for managing weeds by making desired manipulations in the cultural practices and cropping patterns. These can be suitably rotated or intercropped with main crops to manage the target weeds (including parasitic ones) selectively. Even the crop mulch/ residues can also give desirable benefits. The allelochemicals present in the higher plants as well as in the microbes can be directly used for weed management along with the management of some herbicides (Singh et al., 2003b).

Singh et al. (2003b) also mentioned that the bioefficacy of allelochemicals can be enhanced by structural changes or the synthesis of chemical analogues based on them. Further, in order to enhance the potential of allelopathic crops, several improvements can be made with the use of biotechnology or genomics and proteomics. In this context either the production of allelochemicals can be enhanced or the transgenics with foreign genes encoding for a particular weed-suppressing allelochemical could be produced. These authors comment that in the former, both conventional breeding and molecular genetical techniques are useful. However, with conventional breeding being slow and difficult, more emphasis is laid on the use of modern techniques such as molecular markers and the selection aided by them. Although the progress in this regard is slow, nevertheless some promising results are coming and more are expected in future. In this sense, is important to point out that the potential use of transgenic plants and other genetically modified organisms (GMO's) with such or other proposal, cause a strong controversial with the principles of organic agriculture defined and established by the International Federation of Organic Agriculture Movements (IFOAM) founded with the aim to promote an agriculture that is ecologically, economically, and socially sustainable. IFOAM is opposed to genetic engineering in agriculture, in view of the unprecedented danger it represents for the entire biosphere and the particular economic and environmental risks it poses for organic producers (IFOAM, 2002)*.

Using a soil bioassay technique, Conkling et al. (2002) assessed seedling growth and incidence of disease of wild mustard (Brassica kaber) and sweet corn (Zea mays) in soil from field plots that received either of two treatments: incorporated red clover (Trifolium pratense) residue plus application of compost ('amended soil'), or application of ammonium nitrate fertilizer ('unamended soil'). Soils were analyzed for percent moisture, dissolved organic carbon, conductivity, phenolics, and nutrient content. A trend toward greater incidence of Pythium spp. infection of wild mustard seedlings grown in amended soil was observed during the first 40 days after

* IFOAM, 2002: International Federation of Organic Agriculture Movements (IFOAM). Position on Genetic Engineering and Genetically Modified Organisms. http://www.ifoam.org/pospap/geposition 0205.html 2002.

incorporation (DAI) of red clover and compost, with significant differences (a= 0.05) at two out of four sampling dates in 1997, and four out of four sampling dates in 1998. Incidence of Pythium infection was 10-70% greater in the amended soil treatment during that period. Asymptomatic wild mustard seedlings grown in amended soil were also on average 2.5 cm shorter (a= 0.05) at 5 DAI than those grown in unamended soil in one year out of two. Concentration of phenolic compounds in soil solution was correlated with decreased shoot and root growth (r = 0.50, 0.28, respectively) and increased incidence of disease (r = 0.48) in wild mustard seedlings in one year out of two. Dissolved organic carbon concentration was correlated with increased disease in wild mustard seedlings in both years (r = 0.51, 0.33, respectively). Growth of corn seedlings did not differ between the two soil treatments, suggesting that red clover green manure and compost may selectively reduce density and competitive ability of wild mustard in the field. Bioassay results corresponded well with emergence and shoot weight results from a related field study, indicating that this technique may be useful for screening potential soil treatments prior to field studies.

Anaya et al. (1987) applied leaves of Alnus firmifolia, Berula erecta and Juncus sp., as green manures in corn fields with corn, bean, and squash grown using traditional techniques. The growth of weeds during the crop period was decreased by the presence of the green manures. At the same time, stimulation of bean root nodulation by Rhizobium was obtained with these particular green manure species, nodulation was also increased in plots with abundant weed growth. These results suggest that the presence of different secondary metabolites liberated by these green manures and by some living weeds in the field plots increase the ability of Rhizobium sp. to infect bean roots.

Weed control by rye, crimson clover, subterranean clover, and hairy vetch cover crops was evaluated in no-tillage corn during 1992 and 1993 at two North Carolina locations (Yenish, et al., 1996). Weed biomass reduction was similar with rye, crimson clover, and subterranean clover treatments, ranging between 19 and 95% less biomass than a conventional tillage treatment without cover. Weed biomass reduction using hairy vetch or no cover in a notillage system was similar averaging between 0 and 49%, but less than other covers approximately 45 and 90 d after planting. Weed biomass was eliminated or nearly eliminated in all cover systems with pre- plus postherbicide treatments. Weed species present varied greatly between years and locations, but were predominantly common lambsquarters, smooth pigweed, redroot pigweed, and broadleaf signalgrass. Corn grain yield was greatest using pre-herbicides or pre-plus post-herbicides, averaging between 16 to 100% greater than the nontreated control across all cover treatments depending on the year and location.

Studies were conducted by Burgos and Talbert (1996) at the Main Agricultural Experiment Station in Fayetteville and the Vegetable Substation in Kibler, Arkansas, in 1992 and 1993 on the same plots to evaluate weed suppression by winter cover crops alone or in combination with reduced herbicide rates in no-till sweet corn and to evaluate cover crop effects on growth and yield of sweet corn. Plots seeded to rye plus hairy vetch, rye, or wheat had at least 50% fewer early season weeds than hairy vetch alone or no cover crop. None of the cover crops reduced population of yellow nutsedge. Without herbicides, hairy vetch did not suppress weeds 8 wk after cover crop desiccation. Half rates of atrazine and metolachlor (1.1 + 1.1 kg al ha (-1)) reduced total weed density more effectively in no cover crop than in hairy vetch. Half rates of atrazine and metolachlor controlled redroot pigweed, Palmer amaranth, and goosegrass regardless of cover crop. Full rates of atrazine and metolachlor [2.2 + 2.2 kg al ha (-1)] were needed to control large crabgrass in hairy vetch. Control of yellow nutsedge in hairy vetch was marginal even with full herbicide rates- Yellow nutsedge population increased and control with herbicides declined the second year, particularly with half rates of atrazine and metolachlor. All cover crops except hairy vetch alone reduced emergence, height, and yield of sweet corn. sweet corn yields from half rates of atrazine and metolachlor equalled the full rates regardless of cover crops.

It is presently not known what effect wheat root residues have in regulating dicotyledonous (dicot) weed emergence in no-till management systems. Past research has focused almost entirely on the role of shoot residues, while the role of root residues in weed control has been essentially ignored. A field study was designed by Blum et al. (2002) to determine the respective effects of wheat shoot and root residues in regulating the emergence of three dicotyledonous weed species (morning-glory, pigweed and prickly sida). Glyphosate-desiccated wheat plots and fallow plots were surface seeded with morning-glory, pigweed and prickly sida during the spring of 1996 and 1997. Weed seedling emergence was determined for two months during each experimental period in plots with or without wheat shoot and/or root residues. The resulting data suggested that: a) the closer desiccation of the wheat cover crop occurred to the initial emergence of pigweed seedlings, the lower the emergence of that weed, b) the effects of wheat shoot and/or root residues on dicot weed seedling emergence vary considerably for the different weed species ranging from stimulation to inhibition and c) the role of root residues appear to be much more important to regulating weed emergence than that of surface shoot residues, Differences in soil moisture and temperature associated with the presence or absence of wheat residues could not be used to explain the observed treatment effects.

The growth of four summer season crops, namely Cyamopsis tetragonoloba, Sorghum vulgare, Pennisetum americanum and Zea mays, in fields with or without residues of the preceding sunflower crop was poor. Crop density, weight of seed or grain and total yield were significantly lower in sunflower fields than in the control fields (i.e. those without previous sunflower crops). Growth in terms of plant height and biomass was drastically reduced after 60 days. The effect was more pronounced in the fields where sunflower residues were allowed to decompose than in those where residues were completely removed. The soil collected from sunflower fields (both with and without residues) was found to be rich in phenolics, which in a laboratory bioassay were found to be phytotoxic. The reduced growth and yield of crops can be attributed to the release of phytotoxic phenolics from decomposing sunflower residues (Batish, et al., 2002).

John and Narwal (2003) assessed that Leucaena leucocephala is the most productive and versatile multipurpose legume tree in tropical agriculture and has several uses, thus called 'miracle tree'. It is a popular choice for intercropping with annuals in hedgerow or alley cropping systems. Its allelopathic effects on oil cereals, pulses (peas and beans), oilseeds, vegetables, fodder crops, weeds, trees etc. are reviewed in this paper. The foliage and pods of Leucaena contain the toxic amino acid mimosine [beta-N-(3-hydroxy-4-pyridone)-alpha-aminopropionic acid] and many other phytotoxic compounds. The toxic effects of mimosine oil plants and physiology of its action also are discussed. The future areas identified for research in Leucaena are: (a) studies on its allelopathic compatibility with different crops to identify sustainable agroforestry systems (b) investigations to overcome its adverse allelopathic effects and mimosine toxicity and (c) possibility of using the allelopathic compounds in Leucaena as natural herbicides.

Marigold (Tagetes erecta) is another multipurpose crop with ceremonial, ornamental, medical and pharmaceutical uses, and reported antimicrobial properties. Gómez-Rodríguez et al. (2003) evaluated the effect of marigold intercropped with tomato (Lycopersicon esculentum) on Alternaria solani conidia germination in vitro, on conidial density and tomato leaf damage in vivo, as well as microclimatic changes, compared to tomato intercropped with pigweed (Amaranthus hypochondriacus) and monocropped tomato. They found that intercropping with marigold induced a significant (P < 0:05) reduction in tomato early blight caused by A. solani, by means of three different mechanisms. One was the allelopathic effect of marigold on A. solani conidia germination, as it was shown in vitro conditions; while pigweed did not have any of this inhibitory effect in conidia germination. The second way was by altering the microclimatic conditions around the canopy, particularly by reducing the number of hours per day with relative humidity 92%, thus diminishing conidial development. The third mechanism was to provide a physical barrier against conidia spreading. When intercroppped with tomato, pigweed plants worked also as a physical barrier and promoted reductions in the maximum relative humidity surrounding the canopy, but to a lesser extent than marigold.

The allelopathic properties of unburnt (UR) and burnt (BR) residues of Parthenium hysterophorus towards the growth of two winter crops-radish and chickpea were investigated (Singh et al., 2003c). The extracts prepared from both UR and BR were toxic to the seedling length and dry weight of the test crops, those from BR in particular. The difference was attributed to the highly alkaline nature of the extracts prepared from BR. Growth studies conducted in soil amended with UR and BR extracts and residues also revealed phytotoxic effects towards test crops, UR being more active than BR unlike crude extracts. These effects were attributed to the presence of phenolics rather than to any significant change in pH or conductivity.

Weston and Duke (2003) focused a review on a variety of weed and crop species that establish some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. They remarked that recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. in contrast, allelopathic crops offer strong potential for the development of cultivars that are more highly weed suppressive in managed settings. Both environmental and genotypic effects impact allelochemical production and release over time. A new challenge that exists for future plant scientists is to generate additional information on allelochemical mechanisms of release, selectivity and persistence, mode of action, and genetic regulation. In this manner, it is possible to further protect plant biodiversity and enhance weed management strategies in a variety of ecosystems.

Ohno et al. (2000) based on previous studies that suggested phenolics from legume green manures may contribute to weed control through allelopathy, investigated if red clover (Trifolium pratense) residue amended field soils expressed phytotoxicity to a weed species, wild mustard (Sinapis arvensis). Field plots involving incorporation treatments of wheat (Triticum aestivum) stubble or wheat stubble plus 2530 kg ha(-1) red clover residue, were sampled at -12, 8, 21, 30, 41, 63, and 100 days after residue incorporation (DAI). Soil-water extracts (1:1, m:v) were analyzed for plant nutrients and phenolic content. Phytotoxicity of the extracts was measured using a laboratory wild mustard bioassay. There was a 20% reduction of radicle growth in the green manure treatment in comparison with the wheat stubble treatment, but only at the first sample date after residue incorporation (8 DAI). The radicle growth reduction had the highest correlation with the concentration of soluble phenolics in the soil, water extracts. Bioassays using aqueous extracts of the clover shoots and roots alone predicted a radicle growth reduction of 18% for the quantity of clover amendment rate used in the field plots. The close agreement of the predicted and observed root growth reduction at 8 DAI further supports clover residue as the source of the phytotoxicity.

The allelopathic influence of sweet potato cultivar 'Regal' on purple nutsedge was compared to the influence on yellow nutsedge under controlled conditions. Purple nutsedge shoot dry weight, total shoot length and tuber numbers were significantly lower than the controls (47, 36, and 19% inhibition, respectively). The influence on the same parameters for yellow nutsedge (35, 21, and 43% inhibition, respectively) was not significantly different from purple nutsedge. Sweet potato shoot dry weight was inhibited by purple and yellow nutsedge by 42% and 45%, respectively. The major allelopathic substance from 'Regal' root periderm tissue was isolated and tested in vitro on the two sedges. The I50's for shoot growth, root number, and root length were 118, 62, and 44 |g/ml, respectively, for yellow nutsedge. The I50's for root number and root length were 91 and 85 |g/ml, respectively, for purple nutsedge and the I50 for shoot growth could not be calculated (Peterson and Harrison, 1995). These allelopathic substances, the resin glycosides mixture extracted from the periderm tissue of storage roots from sweet potato, Ipomoea batatas, was bioassayed for effects on survival, development, and fecundity of the diamondback moth, Plutella xylostella. The resin glycoside was incorporated into an artificial diet and fed to P. xylostella larvae. First instars were placed individually into snap-top centrifuge vials containing artificial diet with one of six concentrations of resin glycoside material (0.00. 0.25, 0.50, 1.00, 1.50, and 2.00 |g/ml). Each replication consisted of 10 individuals per concentration, and the experiment was repeated 13 times. Vials were incubated at 25oC and a photoperiod of 14:10 (L:D) h in a growth chamber. After 6 d, surviving larvae were weighed and their sex determined, then returned to their vials. Later, surviving pupae were weighed and incubated at 25oC until moths emerged. Females were fed, mated with males from the laboratory colony, and allowed to lay eggs on aluminum foil strips. Lifetime fecundity (eggs/female) was measured. There were highly significant negative correlations between resin glycoside levels and survival and between glycoside levels and larval weight after 0 d of feeding. For larvae that lived at least 6 d, there was no additional mortality that could be attributed to the resin glycoside material. However, there was a significant positive correlation between glycoside dosages and developmental time of larvae (measured as days until pupation). Lifetime fecundity also was negatively affected at sublethal doses. Resin glycosides may contribute to the resistance in sweet potato breeding lines to soil insect pests (Jackson and Peterson, 2000). It is important to consider that the use of allelopathic crops or plant residues in agricultural management will inevitably affect other crop pests, for example insect populations.

The total resin glycoside content in the periderm of 37 sweetpotato cultivars and breeding clones was measured by HPLC and varied greatly among the clones, the highest content was 10.02 % of the periderm dry weight and the lowest was 0.05 %. Insect damage ratings of the clones and their periderm resin glycoside content were negatively correlated and all clones with high resin glycoside content exhibited moderate or low injury from insects. Resin glycosides extracted from 'Regal' periderm and incorporated into potato dextrose agar medium were inhibitory to the growth of four fungal species of sweetpotato roots; however, these fungi exhibited variable response. These observations provide evidence that sweetpotato resin glycosides contribute to the insect and disease resistance in the roots of some sweetpotato-clones (Harrison et al., 2003).

Barazani and Friedman (2001) discussed the impact of allelopathic, nonpathogenic bacteria on plant growth in natural and agricultural ecosystems. In some natural ecosystems, evidence supports the view that in the vicinity of some allelopathically active perennials (e.g., Adenostoma fasciculatum, California), in addition to allelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annuals. In agricultural ecosystems allelopathic bacteria may evolve in areas where a single crop is grown successively, and the resulting yield decline cannot be restored by application of minerals. Transfer of soils from areas where crop suppression had been recorded into an unaffected area induced crop retardation without readily apparent symptoms of plant disease. Susceptibility of higher plants: to deleterious rhizobacteria is often manifested in sandy or so-called skeletal soils. The allelopathic effect may occur directly through the release of allelochemicals by a bacterium that affects susceptible plant(s) or indirectly through the suppression of an essential symbiont. The process is affected by nutritional and other environmental conditions; some may control bacterial density and the rate of production of allelochemicals. Allelopathic nonpathogenic bacteria include a wide range of genera and secrete a diverse group of plant growth-mediating allelochemicals. Although a limited number of plant growth-promoting bacterial allelochemicals have been identified, a considerable number of highly diversified growth-inhibiting allelochemicals have been isolated and characterized. Some species may produce more than one allelochemical; for example, three different phyotoxins, geldanamycin, nigericin, and hydanthocidin, were isolated from Streptomyces hygroscopicus. Efforts to introduce naturally produced allelochemicals as plant growth-regulating agents in agriculture have yielded two commercial herbicides, phosphinothricin, a product of Streptomyces viridochromogenes, and bialaphos from S. hygroscopicus. Both herbicides have the same mechanism of action. Many species of allelopathic bacteria that affect growth of higher plants are not plant specific, but some do exhibit specificity; for example, dicotyledonous plants were more susceptible to Pseudomonasputida than were monocotyledons. Differential susceptibility of higher plants to allelopathic bacteria was noted also in much lower taxonomical categories, at the subspecies level, in different cultivars of wheat, or of lettuce. Therefore, when test plants are employed to evaluate bacterial allelopathy, final evaluation must include those species that are assumed to be suppressed in nature. The release of allelochemicals from plant residues in plots of 'continuous crop cultivation' or from allelopathic living plants may induce the development of specific allelopathic bacteria.

Striga hermonthica is an obligate root-parasitic flowering plant that severely threatens cereal production in sub-Saharan Africa. A potential biological control option for reduction of crop yield-loss within the season of application is the use of soil-borne antagonists of S. hermonthica seed. A study was made (Ahonsi et al., 2002) with the aim to select soil-borne fluorescent pseudomonad strains capable of suppressing germination of S. hermonthica seeds and consequently reducing parasitism and damage to maize. An in vitro screening procedure was developed and was used to evaluate 460 fluorescent pseudomonad isolates from naturally suppressive soils. This resulted in the identification of 15 Pseudomonas fluorescens/P. putida isolates that significantly inhibited germination of S. hermonthica seeds. In a pot experiment using steam-sterilized soil, there was a significant reduction in the number of S. hermonthica plants on maize grown from seeds that were inoculated with any of the 15 bacterial isolates. Inoculation of maize seed with six of these isolates resulted not only in a reduced number of S. hermonthica plants, but also in an increased maize shoot biomass compared with the check. When soils inoculated with these bacterial isolates were left dried for 5 weeks after maize harvest and then planted with a second maize crop, no reduction in S. hermonthica parasitism was observed. This suggested that the bacteria did not persist in the soil after the first crop of maize. These results suggest that saprophytic fluorescent pseudomonads have potential for biological control of S. hermonthica in maize and that periodic application of bacteria, perhaps through seed treatment, may be necessary for sustained control.

Chittapur et al. (2001) asserted that integrated weed management systems involving catch and trap crops are needed to reduce herbicide use in agriculture and to help to control parasitic weed growth. The effective catch crops viz., fodder millet (Panicum miliaceum), sorghum (Sorghum bicolor), corn (Zea mays), sudangrass (Sorghum sudanense) have been identified for the management of Striga asiatica, and the cowpea (Vigna catjang) for S. gesnerioides. Cotton (Gossypium spp.), soybean (Glycine max) and peanut (Arachis hypogaea) are important trap crops. Intercropping of soybean or peanut with sorghum effectively controls S. hermonthica. Flax

(Linum usitatissimum) is a useful trap crop for Orobanche ramosa, O. cernua, O. crenata, and O. aegyptica. In India, sunnhemp (Crotolaria juncea), blackgram (Phaseolus mungo), greengram (Phaseolus aureus) and sesame (Sesamum indicum) have shown good potential for Orobanche control. Rotation of trap crop reduces the population of Orobanche and 3 to 4 years long rotation of catch/trap crops provides its effective control. Sorghum/maize/paddy (Oryza sativa)-tobacco (Nicotiana tabacum) rotation reduces the infestation and weed biomass of Orobanche. Relay cropping of tobacco in capsicum (Capsicum annuum), onion (Allium cepa) and peanut also reduces the incidence of Orobanche.

Nagabhushana et al. (2001) remarked that no matter how one may define sustainable agriculture, use of soil-conserving cropping practices, less synthetic herbicide inputs and better weed control would be compatible components. previously, these components were considered incompatible, since it was widely believed that soil-conserving practices required increased pesticide use, including herbicides. However, it has been shown that environmental and ecological differences between the no-till and conventional tillage can enhance the control of certain weed species in no-till cropping systems. With proper choice and manipulation of cover crops and residues, it is often possible to reduce the herbicides use. Thus, in eliminating tillage, by utilizing the surface mulch and allelochemicals leached from a killed cover crop and using most effective herbicides when needed, weed management has become much more effective in no-till. In North Carolina, these authors have grown soybean (Glycine max), tobacco (Nicotiana tabaccum), corn (Zea mays), sorghum (Sorghum bicolor) and sunflower (Helianthus annuus) in killed heavy mulches of rye (Secale cereale) without herbicides, other than a non-selective one to kill the rye. Early-season control of broadleaf weeds such as sicklepod (Cassia obtusifolia), morningglory spp. (Ipomoea spp.), cocklebur (Xanthium strumarium), prickly sida (Sida spinosa), common purslane (Portulaca oleracea) and pigweed spp. (Amaranthus spp.) has been 80 to 95%. Rye is the most weed suppressing cover crop among several small grains and subterranean clover (Trifolium subterraneum) and crimson clover (Trifolium incarnatum) the most suppressive legumes. This approach will still enhance agricultural sustainability because: (a) productive top-soil will be conserved, (b) herbicide use (especially preemergence herbicides) can be reduced and (c) herbicides for cover crop kill and postemergence selective herbicides, even if used, have little potential for environmental contamination.

Staman et al. (2001) stated that in order to demonstrate that allelopathic interactions are occurring, one must, among other things, demonstrate that putative phytotoxins move from plant residues on or in the soil, the source, through the bulk soil to the root surface, a sink, by way of the rhizosphere. These authors hypothesized that the incorporation of phytotoxic plant residues into the soil would result in a simultaneous inhibition of seedling growth and a stimulation of the rhizosphere bacterial community that could utilize the putative phytotoxins as a carbon source. If true and consistently expressed, such a relationship would provide a means of establishing the transfer of phytotoxins from residue in the soil to the rhizosphere of a sensitive species under field conditions, presently, direct evidence for such transfer is lacking. To test this hypothesis, cucumber seedlings were grown in soil containing various concentrations of wheat or sunflower tissue. Both tissue types contain phenolic acids, which have been implicated as allelopathic phytotoxins. The level of phytotoxicity of the plant tissues was determined by the inhibition of pigweed seedling emergence and cucumber seedling leaf area expansion. The stimulation of cucumber seedling rhizosphere bacterial communities was determined by the plate dilution frequency technique using a medium containing phenolic acids as the sole carbon source. When sunflower tissue was incorporated into autoclaved soil (to reduce the initial microbial populations), a simultaneous inhibition of cucumber seedling growth and stimulation of the community of phenolic acid utilizing rhizosphere bacteria occurred. Thus, it was possible to observe simultaneous inhibition of cucumber seedlings and stimulation of phenolic acid utilizing rhizosphere bacteria, and therefore provide indirect evidence of phenolic acid transfer from plant residues in the soil to the root surface. However, the simultaneous responses were not sufficiently consistent to be used as a field screening tool but were dependent upon the levels of phenolic acids and the bulk soil and rhizosphere microbial populations present in the soil. It is possible that this screening procedure may be useful for phytotoxins that are more unique than phenolic acids. Such an inverse relationship between phytotoxicity and the response of rhizosphere bacterial populations was also observed by Blum et al. (2000), and such interactions provide indirect evidence for the transfer of allelochemicals from the plant root to the rhizosphere.

In relation with resistance of weeds to herbicides, Duke et al. (2000) mentioned that new mechanisms of action for herbicides are highly desirable to fight evolution of resistance in weeds, to create or exploit unique market niches, and to cope with new regulatory legislation. Comparison of the known molecular target sites of synthetic herbicides and natural phytotoxins reveals that there is little redundancy. Comparatively little effort has been expended on determination of the sites of action of phytotoxins from natural sources, suggesting that intensive study of these molecules will reveal many more novel mechanisms of action. These authors gave some examples of natural products that inhibit unexploited steps in the amino acid, nucleic acid, and other biosynthetic pathways: AAL-toxin, hydantocidin, and various plant-derived terpenoids.

Natural products have not been utilized as extensively for weed management as they have been for insect and plant pathogen management, but there are several notable successes such as glufosinate and the natural product-derived triketone herbicides. The molecular target sites of these compounds are often unique. Strategies for the discovery of these materials and compounds are outlined by Duke et al. (2002a). Numerous examples of individual phytotoxins and crude preparations with weed management potential are provided by these authors. They described an example of research to find a natural product solution of a unique pest management problem (blue-green algae in aquaculture), and mentioned the two fundamental approaches to the use of natural products for weed management: i) as a herbicide or a lead for a synthetic herbicide and ii) use in allelopathic crops or cover crops (Duke et al., 2002b).

As it was mentioned, crops may be genetically engineered for weed management purposes by making them more resistant to herbicides or by improving their ability to interfere with competing weeds. Transgenes for bromoxynil, glyphosate, and glufosinate resistance are found in commercially available crops. Other herbicide resistance genes are in development. Glyphosate-resistant crops have had a profound effect on weed management practices in North America, reducing the cost of weed management, while improving flexibility and efficacy. in general, transgenic, herbicide-resistant crops have reduced the environmental impact of weed management because the herbicides with which they are used are generally more environmentally benign and have increased the adoption of reduced-tillage agriculture. crops could be given an advantage over weeds by making them more competitive or altering their capacity to produce phytotoxins (allelopathy). Strategies for producing allelopathic crops by biotechnology are relatively complex and usually involve multiple genes. one can choose to enhance production of allelochemicals already present in a crop or to impart the production of new compounds. The first strategy involves identification of the allelochemical(s), determination of their respective enzymes and the genes that encode them, and, the use of genetic engineering to enhance production of the compound(s). The latter strategy would alter existing biochemical pathways by inserting transgenes to produce new allelochemicals (Duke et al., 2002c).(See controversy between organic agriculture and biotechnology - Control of Weeds and Management of Agroecosystems, Pag. 18, first paragraph)

More sophisticated techniques will be used to search for alternative to herbicides in agroecosystems. The use of winter cover crops is beneficial to agriculture. Stanislaus and cheng (2002) tried to design a cover crop that self-destructs in response to an environmental cue, thereby eliminating the use of herbicides and tillage to remove the cover crop in late spring. Here, this novel concept is tested in a model system. The onset of summer brings with it elevated temperatures. using this as the environmental cue, a self-destruction cassette was designed and tested in tobacco. A heat-shock-responsive promoter was used to direct expression of the ribonuclease Barnase. Because Barnase is extremely toxic to cells, it was necessary to coexpress its inhibitor, Barstar, whose expression was under the control of the CaMV 35S promoter. The wild-type and two mutated Barnase genes, one missense and one translation attenuated, were tested. The results indicated that the translation-attenuated version of the Barnase gene was most effective in causing heat-shock-regulated plant death. Analysis of the T-2 progeny of a transgenic plant carrying this Barnase mutant showed that the Barnase gene expression was sixfold higher in heat-shock-treated plants compared with untreated plants. This level of Barnase gene expression was sufficient to kill transgenic plants.

Many advances in disciplines such as chemistry, biochemistry, plant breeding, genetics, engineering, and others have been applied in a positive manner to improve knowledge in weed science. The emerging field of genomics is likely to have a similar positive effect on our understanding of weeds and their management in various plant agriculture systems. Genomics involves the large-scale use of molecular techniques for identification and functional analysis of complete or nearly complete genomic complements of genes. commercial application of genomics has already occurred for improvement in certain crop input and output traits, including improved quality characteristics and herbicide and insect resistance. Additional commercial applications of genomics in weed science will be identification of genes involved in crop ability. Genes controlling early crop root emergence, rapid early-season leaf and root development for fast canopy closure, production of allelochemicals for natural weed control, identification of novel herbicide target sites, resistance mechanisms, and genes for protecting crops against specific herbicides can and will be identified. successful crop improvement in these areas using the tools of genomics will dramatically affect weed-crop interactions and improve crop yields while reducing weed problems. In relation to improved basic knowledge of weeds and the resulting ability to improve our weed management techniques, genomics will offer the weed science community many new and exciting research opportunities. Scientists will be able to determine the genetic composition of weed populations and how it changes over time in relation to agricultural practices, Identification of genes contributing to weediness, perennial growth habit, herbicide resistance, seed and vegetative structure dormancy, plant architecture and morphology, plant reproductive characters (outcrossing and hybridization, introgression), and allelopathy will be identified and utilized with high-throughput DNA sequencing and other genomics-based technologies. Using genomics to improve our understanding of weed biology by determining which genes function to affect the fitness, competitiveness, and adaptation of weeds in agricultural environments will allow the development of improved management strategies. Information is provided concerning the current state of molecular research in various areas of weed science and specific genomic research currently being conducted at Purdue University using transfer DNA (TDNA) activation tagging to generate large populations of mutated plants that can be screened for genes of importance to weed science (Weller et al., 2001).

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