Crop Protection 21 (2002) 347–350 Short communication Allelopathic effect of two volatile monoterpenes against bill goat weed (Ageratum conyzoides L.) Harminder P. Singh*, Daizy R. Batish, Ravinder K. Kohli Department of Botany, Punjab University, Chandigarh 160 014, India Received 5 March 2001; received in revised form 10 July 2001; accepted 16 July 2001 Abstract A study was conducted to assess the allelopathic effect of two volatile monoterpenes viz. cineole and citronellol on Ageratum conyzoides with a view to explore the possibility of their exploitation for future weed management. Both the monoterpenes severely affected the germination, speed of germination, seedling growth, chlorophyll content and respiratory activity. After two weeks of exposure, the weed plants wilted. Out of the two monoterpenes, cineole was more toxic in causing injury to the weed. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cineole; Citronellol; Herbicidal activity; Weed control; Bioassay 1. Introduction Essential oils constitute an important group of plant products that provide a characteristic odour to the aromatic plants. These comprise of a mixture of volatile low-molecular weight monoterpenes, isoprenoids and sesquiterpenes and exhibit a great diversity of com- pounds more than any other group of natural com- pounds (Harborne, 1997; Vokou, 1999). The biological activity of the essential oils is dependent upon the synergistic or additive effects of constituent types present in different concentrations. Essential oils present in aromatic plants cause a number of positive or negative interactions (Vokou, 1999). Noteworthy among these are allelopathic interactions, for example in Salvia leucophylla (Muller, 1965), Artemisia sp. (Ahmad and Misra, 1994), Pinus sp. (Singh et al., 1999), and Eucalyptus spp. (Kohli and Singh, 1991). Essential oils and their constituents play a prominent role in food and perfume industries and more recently, their role as plant protection agents is being explored. This includes their insecticidal, pesticidal, and herbicidal properties for the management of harmful diseases, insects, pests and weeds (Isman, 1999, 2000; Kohli et al., 1998; Romagni et al., 2000). Such a wide array of biological activity attributed to the possession of certain characteristic properties such as: (a) lipophilicity and low vapour pressure resulting in their non-persistence in the soil and no leaching to the ground water, (b) no mammalian toxicity, (c) novel modes of action, and (d) providing prototypes for synthesis of available pesticides (Dayan et al., 1999a; Isman, 2000). As regards the management of noxious weeds with essential oils, only a few studies are available. Kohli et al. (1998) demonstrated that essential oils of Eucalyptus sp. have a potential to manage the ragweed parthenium (Parthenium hysterophorus). Volatile monoterpene ana- logs 1,4- and 1,8-cineole are reported to exhibit phytotoxicity against two weedsFEchinochloa crus-galli and Cassia obtusifolia (Romagni et al., 2000). Keeping in mind their potential for weed management, a study was conducted to explore the phytotoxic effect of two volatile monoterpenesFcineole and citronellol against bill goat weed (Ageratum conyzoides), a noxious weed of northern parts of India. 2. Materials and methods 2.1. General Uniform healthy seeds of bill goat weed (Ageratum conyzoides L.) were collected locally from wild growing *Corresponding author. Tel.: +91-172-534005; fax: +91-172- 541409. E-mail address: hpsingh 01@yahoo.com (H.P. Singh). 0261-2194/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0261-2194(01)00096-5