Chemical Composition and Allelopathic Activity of Oil and Volatile Fractions of Conyza albida Willd. ex Sprengel from Greece

Journal of Essential Oil Research: JEOR, Sep/Oct 2004 by Tzakou, O, Gani, A, Economou, G, Yannitsaros, A

Abstract

The oil of the fresh aerial parts of Conyza albida and its fractions were analyzed by GC/MS. Fifty-five components were identified with cis-lachnophyllum ester (30.0%) being the main compound. Furthermore, the allelopathic potential of the oil and its fractions was evaluated using two sensitive indicators, Avena satina and Spirodella polyrhiza.

Key Word Index

Conyza albida, Compositae, essential oil composition, lachnophyllum ester, allelopathic activity, Avena sativa, Spirodella polyrhiza.

Introduction

Essential oils are used for medicinal drugs, controlling harmful insects, antimicrobial/antifungal activity and more often recently for allelochemicals among plants of agricultural importance (1-4). Several monoterpenes and sesquiterpenes have been implicated in allelopathy. Allelopathy, the direct or indirect effect of one plant on another through the production of chemical compounds that escape into the environment, occurs widely in natural plant communities. Plants have their own defense mechanisms and allelochemicals are, in fact, natural herbicides.

Conyza albida Willd. ex Sprengel (Compositae) is an annual-biennial herb of American origin (5-7). It is a widespread species found in many parts of the world; a difficult weed to control and a persistent invader spreading throughout Greece. C^sub 10^ acetylenic compounds are common in Conyza, Erigeron and related genera of the tribe Astereae (8). The composition of the oil of C. albida has not been reported previously.

Experimental

Plant material: Conyza albida young plants (rosette) were collected during winter 1999 from a natural population in the University of Athens Campus. Voucher specimens have been deposited in the Herbarium of the Laboratory of Pharmacognosy, University of Athens.

Chemical analysis: Hydrodistillation of fresh aerial parts for 3 h using a modified Clevenger-type apparatus afforded a yellowish oil with a characteristic odor in 0.25% yield. The obtained oil was dried over anhydrous sodium sulphate and kept at -4�C until it was analyzed.

Flash chromatography: The oil of C. albida (1.528 g) was fractionated by CC over a silica gel-60 column (2.5 x 30 cm) using pentane, pentane-diethyl ether (50:50) and diethyl ether.

GC/MS analysis: The GC/MS analysis was performed on a Hewlett Packard 5973-6890 system operating on EI mode [equipped with a split/splitless injector (200�C); 1/10 split ratio, HP 5MS 30 m x 0.25 mm, 0.25 �m film thickness capillary column]. The temperature program of the column was 60�0 -280�C at 3�C/min. The identification of the compounds was based on comparison of their retention indices (RI) and mass spectra with those obtained from authentic samples and/or the NIST/NBS, Wiley libraries and the literature (9).

Allelopathic evaluation: Bioassay experiments were used to determine the inhibitory potential of the obtained oil and its fractions on growth of Avena sativa L. (oat) radicle and Spirodella polyrhiza (L.) Schieiden (a species of duckweed) fresh weight. The oil and fractions were dispersed as an emulsion in water using Tween 20. Seven concentrations were used. Dilutions were made with distilled water. Avena sativa seeds were placed on filter paper in conical tapped vial (six seeds per vial) and were soaked with 3 mL of the tested water dilutions. The vials were wetted with distilled water and incubated in darkness at 25�C. Avena sativa radical growth was measured after seven days and used as an index to evaluate allelochemical activity. Seeds that did not germinate were considered to have a radicle length of 0 mm. The experimental design was a randomized block with four replicates for each treatment and control. Data typically were expressed as a percentage of radicle ellongation in control vials. The dose needed to inhibit A. sativa radicle growth to 50% of control radicle growth (hereafter called the I^sub 50^ value) was determined from dose-response bioassays (10).

The above dilutions from the oil and fractions were tested via another bioassay using as test plant S. polyrhiza and measuring the decrease of its fresh weight. Duckweeds have been used as indicators in several allelopathic studies as they grow easily and the bioassay is easy to conduct. They are high sensitive to chemical disturbance and response by chlorosis and drastic decrease of their weight (11). The damage was estimated by using visual assessment of fronds yellowing getting the color (dark-green, green, yellow, etc.) into different classes according to European Weed Research Society classification scale of phytotoxicity symptoms (12).

Results and Discussion

The composition of the oil and fractions can be seen in Table I. Fifty-five components were identified. In the oil, the sesquiterpenes and acetylenic compounds were found in almost equal level (39.1% and 37.0%, respectively). The monoterpenes represented 23.2% of the oil.

As it was expected fraction A was dominated by monoterpenes (71.8%), while fraction B was characterized by a high content of sesquiterpenes (65.9%). In fractions C, D and E contained mainly acetylenic compounds (97.7%, 88.4% and 53.5%, respectively).