Gerald R. Leather

Potentials for exploiting allelopathy to enhance crop production

Strategies for utilizing allelopathy as an aid in crop production include both avoidance and application protocols. There are immediate opportunities for management of weed and crop residues, tillage practices, and crop sequences to minimize crop losses from allelopathy and also to use allelopathic crops for weed control. Varieties of grain and forage sorghums (Sorghum Spp.), sunflower (Helianthus annuus L.), oats (Avena sativa L.), wheat (Triticum sativum L.),rye (Secale cereale L.), and others may provide weed control and in some instances crop stimulation from their residues. Our four-year field study with cultivated sunflower resulted in no differences in weed biomass between plots with and without herbicide (EPTC) applications. Strip cropping that included sorghum showed that in the subsequent year weed density and biomass were significantly lower in the previous-year sorghum than in soybean strips. Possibilities exist for modification of crop plant metabolism to alter production of allelochemicals. Allelochemical-environmental interactions must be considered in efforts to benefit from allelopathy. Under greenhouse conditions, joint application of low levels of atrazine, trifluralin, alachlor, or cinmethylin with a phenolic allelochemical showed that these two categories of inhibitors acted in concert to reduce plant growth. Allelochemicals may also be adapted as yield stimulants or environmentally sound herbicides, such as cinmethylin and methoxyphenone. Isolation of bialophos, tentoxin, and others shows that bacteria and fungi are good sources of biologically active compounds.

Use ofLemna minor L. as a bioassay in allelopathy.

Investigations in allelopathy often require the use of a bioassay for evaluating limited quantities of potentially active growth regulators. A bioassay procedure was developed usingL. minor grown in 1.5-ml aliquots of nutrient medium with and without allelochemicals in wells of 24-well tissue culture cluster dishes with loose-fitting lids. Tests using six replications per treatment with several flavonoid compounds and derivatives of coumarin, benzoic acid, and cinnamic acid demonstrated that the bioassay was capable of measuring inhibition at levels of compound ranging from 50 to 1000 μmol. Strongly inhibitory treatments were visible after 1 or 2 days. After 7 days of growth, frond number, growth rate, and dry weight were used to evaluate effects. The bioassay system is relatively simple, very sensitive, reproducible, and can be used for testing small amounts and dilute concentrations of unknowns which have been separated by chromatography.

Bioassay of naturally occurring allelochemicals for phytotoxicity

The bioassay has been one of the most widely used tests to demonstrate allelopathic activity. Often, claims that a particular plant species inhibits the growth of another are based entirely on the seed germination response to solvent extracts of the suspected allelopathic plant; few of these tests are of value in demonstrating allelopathy under natural conditions. The veracity of the bioassay for evaluating naturally occurring compounds for phytotoxicity depends upon the physiological and biochemical response capacity of the bioassay organism and the mechanism(s) of action of the allelochemicals. The possibility that more than one allelochemical, acting in concert at very low concentrations, may be responsible for an observed allelopathic effect makes it imperative that bioassays be extremely sensitive to chemical growth perturbation agents. Among the many measures of phytotoxicity of allelochemicals, the inhibition (or stimulation) of seed germination, radicle elongation, and/or seedling growth have been the parameters of choice for most investigations. Few of these assays have been selected with the view towards the possible mechanism of the allelopathic effect.

Weed control using allelopathic crop plants

The concept that some crop plants may be allelopathic to common weeds of agricultural lands is receiving greater attention as an alternative weed control strategy. Several crops showing promise are: grain and forage species such as barley (Hordeum), oat (A vena), fescue (Festuca), and sorghum (Sorghum), and the agronomic species of corn (Zea) and sunflower (Helianthus). Among the problems that hinder the conclusive demonstration of allelopathic effects of crop plants are the loss of that capacity through selection and the variability among cultivars. Recent studies to evaluate the allelopathic potential of crop plants have shown that several sunflower varieties inhibit the germination and growth of associated weeds and to a greater extent than found in several biotypes of native sunflower. Aqueous extracts of dried sunflower and rape tissue inhibited or stimulated germination and growth of weeds, and the response depended upon the source of extract, the extract concentration, and the weed species tested. The validity of bioassay results was tested in a 5-year field study with sunflower and oat grown in rotation. Weed density increased in all plots but the extent of increase was significantly less in plots of sunflower than in control plots. The use of crop plants with increased allelochemical production could limit the need for conventional herbicides to early season application with late season control provided by the crop.

Plant growth regulatory activities of artemisinin and its related compounds

Artemisinin, a sesquiterpene lactone produced in the leaves ofArtemisia annua, was evaluated for its phytotoxicity in mono- and dicotyledonous plants. Artemisinin inhibited seed germination, seedling growth, and root induction in all species tested. The concentration of artemisinin required for 50% inhibition ofLemna minor growth was 5 μM. Inhibitory plant responses appeared to require the endoperoxide moiety of this compound since similar chemicals without endoperoxide, deoxyartemisinin, arteannuic acid, and arteannuin B, were less phytotoxic. InL. minor, artemisinin and arteannuic acid caused the leakage of proteins into the growth medium, suggesting the site of activity was at the plant cell membrane.

Allelopathic influence ofSorghum bicolor on weeds during germination and early development of seedlings

The allelopathic interaction between sorghum [Sorghum bicolor (L.) Moench] and 10 species of grass and broadleaf weeds was investigated. Germination of weed seeds was slightly inhibited or stimulated, depending on species, when incubated in closed Petri dishes with germinating sorghum. Subsequent radicle and hypocotyl or coleoptile elongation of weeds was significantly inhibited by the germinating sorghum. For weeds interplanted with sorghum and grown under greenhouse conditions. The inhibitory effect on some weed species was still evident after 2 months of growth. Significant differences were found in the dry matter per weed plant grown in pots in proximity to sorghum vs. weeds grown in monoculture. Aqueous leachates from pots planted with sorghum alone or from a system in which sorghum roots protruded into water had strong allelopathic activity. These results indicate that water-soluble allelochemicals are produced by germinating sorghum seeds and that production of these substances continues during seedling growth.

Weed control using allelopathic sunflowers and herbicide

SummaryField studies were conducted to determine if season long weed control could be achieved by combining the use of an herbicide with the natural allelochemicals produced by cultivated sunflower (Helianthus annuus L.). The weed biomass was reduced equally in plots planted with sunflowers, whether or not the herbicide was applied in each of 4 years. Weed control diminished the second year in all plots that received the same treatments as had been applied the previous year. This diminished efficacy was attributed to reduced emergence of sunflower (13.5 to 45.2 percent) in second-year plots, as a result of autotoxicity from sunflower crop residues remaining after the first-year harvest.

Effects of two sesquiterpene lactones isolated fromArtemisia annua on physiology ofLemna minor.

The effects of artemisinin and arteannuic acid extracted fromArtemisia annua on the physiology ofLemna minor were evaluated. Changes in frond production, growth, dry weight, and chlorophyll content ofL. minor were determined. Photosynthesis and respiration were evaluated with a differential respirometer. Artemisinin (5 µM) inhibitedL. minor frond production and dry weight 82 and 83%, relative to methanol controls. Chlorophyll content was reduced 44% by artemisinin (2.5 µM). Arteannuic acid (10 µM) was less active, inhibiting frond production 61% and reducing chlorophyll content 66% at 5 µM. Artemisinin (1 µM) reducedL. minor photosynthesis 30% and 2.5 µM reduced respiration 39%. Arteannuic acid had no significant effect on photosynthesis or respiration at the levels tested.