Alan R. Putnam

Use of phytotoxic plant residues for selective weed control

Abstract Both fall- and spring-planted grasses were evaluated for weed-suppressing ability after desiccation by freezing, glyphosate or paraquat. Populations of Portulaca oleracea L. and Digitaria ischaemum (Schreb.) Muhl. were reduced by 70% and 98%, respectively, by residues of sorghum ( Sorghum bicolor L.). Total weed biomass and weight of several indicator species were also consistently reduced with residues of barley ( Hordeum vulgare L.), oats ( Avena sativa L.), wheat ( Triticum aestivum L.) and rye ( Secale cereale L.), as well as the sorghums. In general, the larger-seeded vegetables, particularly legumes, grew normally or were sometimes stimulated by the cover-crop residues, whereas several species of smaller-seeded vegetables were severely injured. The use of Populus wood shavings as a control mulch allowed separation of physical and chemical aspects of the residues. Glasshouse experiments with two soils confirmed both weed-suppressing and crop-stimulating effects of sorghum residues. Water extracts of sorghum herbage were toxic to indicator species in sterile bioassays: this suggests that phytotoxins are directly released by the plant residues.

Role of benzoxazinones in allelopathy by rye (Secale cereale L.)

Two phytotoxic compounds [2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) and 2(3H)-benzoxazolinone (BOA)] were previously isolated and identified in 35-day-old greenhouse-grown rye shoot tissue. Both compounds were also detected by TLC in greenhouse-grown root and fieldgrown shoot tissue. The concentration of DIBOA varied in the tissues, with the greatest quantity detected in greenhouse-grown shoots. DIBOA and BOA were compared with β-phenyllactic acid (PLA) and β-hydroxybutyric acid (HBA) for activity on seed germination and seedling growth and were consistently more toxic than either compound. Dicot species tested, including lettuce (Lactuca sativa L.), tomato (Lycopersicon esculentum Mill.), and redroot pigweed (Amaranthus retroflexus L.), were 30% more sensitive than the monocots tested. Of the two benzoxazinone compounds, DIBOA was most toxic to seedling growth. DIBOA and BOA reduced chlorophyll production inChlamydomonas rheinhardtii Dangeard, by 50% at 7.5 × 10−5 M and 1.0 × 10−3 M, respectively. Both DIBOA and BOA inhibited emergence of barnyardgrass (Echinochloa crusgalli L. Beauv.), cress (Lepidium sativum L.), and lettuce when applied to soil, indicating their potential for allelopathic activity.

Exploitation of allelopathy for weed control in annual and perennial cropping systems

A variety of crops, cultivars, and accessions have been evaluated over the past six years for superior capability to suppress weed growth. The most successful of these approaches has been to grow cover crops of rye (Secale cereale), wheat (Triticum aestivum), sorghum (Sorghum bicolor), or barley (Hordeum vulgare) to a height of 40–50 cm, desiccate the crops by contact herbicides or freezing, and allow their residues to remain on the soil surface. Often, up to 95% control of important agroecosystem weed species was obtained for a 30- to 60-day period following desiccation of the cover crop. The plant residues on the soil surface exhibit numerous physical and chemical attributes that contribute to weed suppression. Physical aspects include shading and reduced soil temperatures which were similarly achieved using poplar (Populus) excelsior as a control mulch. Chemical aspects apparently include direct release of toxins, as well as production of phytotoxic microbial products. Numerous chemicals appear to work in concert or in an additive or synergistic manner to reduce weed germination and growth.

Biological suppression of weeds: evidence for allelopathy in accessions of cucumber.

Cucumber (Cucumis sativus L.) accessions from 41 nations were grown with two indicator species in a search for superior competitors. Of the plant introductions tested, one inhibited indicator plant growth by 87 percent and 25 inhibited growth by 50 percent or more. The toxicity of leachates from pots containing inhibitory cucumbers to indicator plants germinated in separate containers suggested allelopathy. Incorporation of an allelopathic character into a crop cultivar could provide the plant with a means of gaining a competitive advantage over certain weeds.

Rye residues contribute weed suppression in no-tillage cropping systems.

The use of allelopathic cover crops in reduced tillage cropping systems may provide an ecologically sound and environmentally safe management strategy for weed control. Growers often plant winter rye (Secale cereale L.) for increased soil organic matter and soil protection. Spring-planted living rye reduced weed biomass by 93% over plots without rye. Residues of fall-planted/spring-killed rye reduced total weed biomass over bare-ground controls. Rye residues also reduced total weed biomass by 63% when poplar excelsior was used as a control for the mulch effect, suggesting that allelopathy, in addition to the physical effects of the mulch, did contribute to weed control in these systems. In greenhouse studies, rye root leachates reduced tomato dry weight by 25–30%, which is additional evidence that rye is allelopathic to other plant species.

Isolation and characterization of allelochemicals in rye herbage

Abstract Residues and aqueous extracts of rye (Secale cereale L. ‘Wheeler’) shoot herbage were previously shown to contain phytotoxic compounds. Sequential partitioning of aqueous extracts against a series of solvents of increasing polarity separated the most active compounds in the Et2O fraction based on a cress (Lepidium sativum L. ‘Curly’) root growth assay. Bioassays after TLC indicated two major zones of toxicity. Further separation of the Et2O extract revealed two new phytotoxic compounds in rye. The compounds were identified as 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) and a breakdown product 2(3H)-benzoxazolinone (BOA). In the cress bioassay, DIBOA and BOA inhibited root growth 50% or more at concentrations of ca 0.37 and 1.05 mM, respectively. Only DIBOA showed significant activity on barnyardgrass (Echinochloa crusgalli L. Beauv.). Neither compound inhibited seed germination at concentrations tested. DIBOA stimulated root and shoot growth of cress at the lowest concentration tested (0.09 mM).

Separating the competitive and allelopathic components of interference : Theoretical principles.

The terms “competition” and “allelopathy” should be used in ways consistent with some set of criteria, perhaps those proposed in this discussion. Proposed proof of competitive interference includes: (1) identification of the symptoms of interference; (2) demonstration that the presence of the agent is correlated with reduced utilization of resources by the suscept; (3) demonstration of which resource(s) depleted by the agent are limiting resources; and (4) simulation of that interference (in the absence of the agent) by reduction of the supply of resources to levels that occur during interference. Proposed proof of allelopathic interference includes: (1) identification of the symptoms of interference; (2) isolation, assay, characterization, and synthesis of the toxin; (3) simulation of the interference by supplying the toxin as it is supplied in nature; and (4) quantification of the release, movement, and uptake of the toxin. It would be desirable but not essential to show that the selectivity of the toxin to various species corresponds to the range of species affected by the allelopathic agent. We propose that fulfillment of the above criteria would constitute proof of competitive or allelopathic interference. According to these criteria, it is possible that neither competitive nor allelopathic interference has been unequivocally proven at the present time. These criteria are proposed as a basis for evaluation of experimental evidence and as an indicator of deficiencies in our technology.

2,2′-Oxo-1,1′-azobenzene - a microbially transformed allelochemical from 2,3-benzoxazolinone: I.

Abstract2,2′-Oxo-1,1 ′-azobenzene (AZOB), a compound with strong herbicidal activity, was isolated and characterized from a soil supplemented with 2,3-benzoxazolinone (BOA). A parallel experiment with 6-methoxy-2,3-benzoxazolinone (MBOA) yielded AZOB as well as its mono-(MAZOB) and dimethoxy-(DIMAZOB) derivatives. These compounds were produced only in the presence of soil microorganisms, via possible intermediates, I and II, which may dimerize or react with the parent molecule to form the final products. In the case of MBOA, it was shown that demethoxylation precedes the oxidation step. Although BOA and 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) were leached out of rye residues, there were no detectable amounts of the biotransformation products in the soil. When BOA was mixed with soil and rye residue, either under field conditions or in vitro, AZOB was detected. Levels of free BOA in the soil were greatly reduced by incubation with rye residue. AZOB was more toxic to curly cress (Lepidium sativum L.) and barnyardgrass (Echinochloa crusgatti L.) than either DIBOA or BOA.

Isolation and characterization of phytotoxic compounds from asparagus (Asparagus officinalis L.) roots

Potential allelochemicals from aqueous extracts of dried asparagus (Asparagus officinalis L.) roots were isolated and characterized. Active fractions separated by HPLC included ferulic, isoferulic, malic, citric, and fumaric acids. Soxhlet extraction of the residues also produced phytotoxic caffeic acid. Although none of these compounds, when applied singly, was active enough to account for the phytotoxicity of asparagus extracts, their combined effect might be additive or synergistic. An extract from lyophilized fresh root tissues contained a fraction that was one order of magnitude more toxic than any compound obtained from the dried roots. The most active component was isolated by TLC and characterized by [1H]NMR as methylenedioxycinnamic acid (MDCA). This compound provided severe inhibition of curly cress (Lepidium sativum L.) root and shoot growth at concentrations of 25 ppm or above.

2,2'-OXO-1,1'-azobenzene: Selective toxicity of rye (Secale cereale L.) allelochemicals to weed and crop species: II.

Three allelochemicals from rye or its breakdown products were evaluated for activity on garden cress (Lepidum sativum L.), barnyardgrass [Echinochloa crus-galli (L.) Beauv.], cucumber (Cucumis sativus L.), and snap bean (Phaseolus vulgaris L.). 2,4-Dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA), 2(3H)-benzoxazolinone (BOA), and 2,2′-oxo-1,1′-azobenzene (AZOB) were all applied singly at 50, 100, and 200 ppm and in two- and three-way combinations each at 50 and 100 ppm. AZOB at 100 and 200 ppm produced 38–49% more inhibition than DIBOA, while combinations of BOA/ DIBOA, which contained AZOB at 100 ppm had 54–90% more inhibition when compared to DIBOA/BOA combinations. All combinations were slightly antagonistic to barnyardgrass, while several combinations caused a synergistic response to garden cress germination and growth. Cucumbers and snap beans exhibited both types of responses, depending on the allelochemical combination and application rate. The plant-produced benzoxazinones were more inhibitory to crops than weeds. Therefore, improved herbicidal selectivity would be expected if there were rapid transformation of the benzoxazinones to the microbially produced AZOB.