Inhibitory effects of combinations of three phenolic acids on grain sorghum germination

The Joint Action of Sesquiterpene Lactones from Leaves as an Explanation for the Activity of Cynara cardunculus

The work described herein is a continuation of a previous study centered on the bioprospect of cardoon (Cynara cardunculus) leaf extracts through the isolation of secondary metabolites with phytotoxic activity. Chromatographic fractionations of the ethyl acetate extract and spectroscopic analysis showed that the majority of the components were sesquiterpene lactones. Of these compounds, aguerin B, grosheimin, and cynaropicrin were very active on etiolated wheat coleoptile, standard target species, and weed growth. The joint action of binary mixtures of these three active sesquiterpene lactones and one nonactive compound (11,13-dihydroxy-8-desoxygrosheimin) was studied. The activities of fixed-ratio mixtures were assessed on wheat coleoptile. The results can be interpreted with respect to a reference model by considering dose-response analyses and isobolograms with linear regression analyses. A total of 17 binary mixtures at different levels of inhibition (ED25, ED50, and ED75) were studied, and predominantly they responded additively (25). Deviations from additivity included seven synergistic responses and two antagonistic responses. The joint action of major sesquiterpene lactones isolated from C. cardunculus can explain the activities observed in extracts and fractions. The results reported here reiterate the utility of the wheat coleoptile bioassay as a quick tool to detect potential synergistic effects in binary mixtures.

Biologically active secondary metabolites of barley. II. Phytotoxicity of barley allelochemicals

The release of alkaloids by barley was quantified by HPLC. Hordenine was released from the roots of barley in a hydroponic system for up to 60 days. The amount reached a maximum, 2μg/plant/day, at 36 days, then declined. Effects on white mustard by hordenine and gramine included reduction of radicle length and apparent reduction in health and vigor of radicle tips. Transmission electron microscopic examination of white mustard radicle tips exposed to hordenine and gramine showed damage to cell walls, increase in both size and number of vacuoles, autophagy, and disorganization of organelles. The evidence of the morphological and primary effects of barley allelochemicals at the levels released by living plants indicates that the biologically active secondary metabolites of barley may lead to a significant role in selfdefense by the crop.

Synthesis and structure--activity relationships of substituted cinnamic acids and amide analogues: a new class of herbicides

In the present investigation, substituted cinnamic acids (3-hydroxy, 4-hydroxy, 2-nitro, 3-nitro, 4-nitro, 3-chloro, and 4-methoxy) and their amide analogues with four different types of substituted anilines have been synthesized. The synthesized compounds have been screened for their germination inhibition activity on radish (Raphanus sativus L. var. Japanese White) seeds at 50, 100, and 200 ppm concentrations, and the activity was compared with standard herbicide, metribuzin formulation (sencor). Significant activity was exhibited by all of the compounds. It was observed that with the increase in concentration of the test solution, the activity also increased. All of the compounds showed more than 70% inhibition at 100 ppm concentration except 4-hydroxy cinnamanilide. The compound, 2-chloro (4'-hydroxy) cinnamanilide was the best among the tested compounds, and it was found to be at par with the standard, metribuzin at all concentrations. Thus, it can be concluded that substituted cinnamic acids and their amide analogues may be developed as potential herbicides.

Phytotoxic effects of leukamenin E (an ent-kaurene diterpenoid) on root growth and root hair development in Lactuca sativa L. seedlings

Leukamenin E, an ent-kaurene diterpenoid isolated from Isodon racemosa (Hemsl) Hara, showed phytotoxic effects on root growth and root hair development of lettuce seedlings (Lactuca sativa L.). Lower concentrations (10 microM) of leukamenin E did not affect root growth, but at concentrations higher than 50 microM, the rate was inhibited. The influence of leukamenin E on root growth rate was closely correlated with alterations in the mitotic index. A low incidence of aberrant mitosis image was observed when lettuce roots were treated with higher concentrations (100 and 200 microM) of leukamenin E. This suggests that inhibition of root growth may be due to inhibition of cell division. All tested concentrations of the diterpenoid (10 microM or more) inhibited root hair development in a dose-dependent manner. At a concentration of 80 microM, leukamenin E completely blocked root hair initiation. Application of Ag(+)-an ethylene action inhibitor-to lettuce seedlings inhibited root hair elongation similar to the diterpenoid. Enhanced root hair length was stimulated by exogenous ethephon-an ethylene-releasing agent-and could be reversed by addition of leukamenin E. This suggests that leukamenin E may act as a potential ethylene action antagonist in the inhibition of lettuce root hair development. We conclude that leukamenin E may curb root hair development by interfering with ethylene action at concentrations above 10 microM and inhibits root growth via inhibition of cell division at concentrations above 50 microM.

Joint action of benzoxazinone derivatives and phenolic acids

The joint action of binary and ternary mixtures of benzoxazinone derivatives and phenolic acids was studied using the additive dose model (ADM) as reference model. The activity of fixed-ratio mixtures of phenolic acids [ferulic acid (FA), p-coumaric acid (CA), vanillic acid (VA), and p-hydroxybenzoic acid (HBA)] and benzoxazinone derivatives [2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), 6-methoxybenzoxazolin-2-one (MBOA), benzoxazolin-2-one (BOA), 2-aminophenol (AP), and N-(2-hydroxyphenyl)acetamide (HPAA)] on Lolium perenne and Myosotis arvensis root growth was assessed in Petri dishes. Root length was recorded 6 days after seeding, and EC(50) and EC(90) values were estimated using nonlinear regression analyses. The benzoxazinone derivatives were found to be more phytotoxic than the phenolic acids, particularly on M. arvensis. Binary mixtures of phenolic acids responded predominantly additively on both plant species. Deviations from additivity were species-specific with antagonistic responses on L. perenne and synergistic responses on M. arvensis. Similarly, binary mixtures of benzoxazinone derivatives also followed the ADM, although synergistic responses were observed for BOA + AP and BOA + HPAA. Binary and ternary mixtures of benzoxazinone derivatives and phenolic acids responded primarily antagonistically; however, a significant synergistic performance was observed with DIMBOA + FA and DIMBOA + VA on L. perenne. These results do not support the assumption that allelopathic effects of wheat can be attributed to synergistic effects of otherwise weakly active allelopathic compounds, and it is suggested that future research be directed toward identifying and studying the effects of other potential allelochemicals including the degradation products of the most abundant wheat allelochemicals.

Search for a standard phytotoxic bioassay for allelochemicals. Selection of standard target species

In the search for a standard bioassay of phytotoxicity for allelochemicals, 22 commercial varieties of eight plant species [four dicotyledons: lettuce (Compositae), carrot (Umbelliferae), cress (Cruciferae), tomato (Solanaceae); and four monocotyledons: onion (Liliaceae), barley, wheat, and corn (Gramineae)] proposed as models for the most common weed families have been tested at different pH and solution volumes per set conditions. Nine commercial varieties selected as standard target species (STS) were tested with standard commercial herbicides to ensure their sensitivity to phytotoxic compounds. Results are discussed to establish the proper growth requirements, and sensitivity of commercial seeds of STS and to find the most suitable commercial herbicides that allow comparison with an internal standard to validate the response of potential allelochemicals.