PHYTOTOXIC EXUDATES FROM VELVETLEAF (ABUTILON THEOPHRASTI) GLANDULAR TRICHOMES

Nonvolatile exudates from velvetleaf glandular trichomes inhibited root and shoot growth of several weed and crop species in petri plate bioassays, but had no effect on seed germination per se. The exudate was efficiently collected by wiping both the stems and petioles with cotton swabs or by leaching with water, but was absent on the leaves ofvelvetleafplants. Cress (Lepidium sativum L.) was the most sensitive indicator species. Four types of trichomes appeared on the stem surface as revealed by scanning electron microscopy. Water soluble globules on the apices of 12to 15-celled glandular trichomes recurred and demonstrated their original potency within eight days after removal with cotton swabs. Both the quantity and phytotoxicity of the exudates from velvetleafplants cultured under varying environmental conditions were determined. While total exudate production was not affected at 16, 24, or 36 C, the exudates from plants cultured at 24 and 36 C were about twice as toxic as the exudate collected from plants grown at the lower temperature. Water stress decreased the amount of exudate collected, but the phytotoxic activity was increased by approximately the same factor. SECONDARY PLANT PRODUCTS are diverse in structure and distribution throughout the plant kingdom and have no obvious function in primary metabolism (Swain, 1977). Implicated in the defensive and protective mechanisms of plants, these secondary metabolites may have numerous ecological roles (Seigler, 1977; Swain, 1977; Lovett, 1982). They are released through volatilization, exudation, leaching or decomposition of plant residues. Some may act as allelochemicals which help plants defend against herbivores, insects, or neighboring plants. In addition to genetic differences, factors such as temperature, light quality and intensity, mineral deficiencies, age of plant, pathogens and predators, and water stress have been shown to affect the production of secondary compounds (Swain, 1977; Rice, 1984). The plasticity of secondary product formation in response to stress may aid in plant adaptation and survival. A variety of secondary compounds with biological activity are localized in or secreted from specialized plant structures called trichomes. Glandular trichome exudates from mite-resistant geraniums (Pelargonium hor' Received for publication 22 April 1986; revision accepted 5 September 1986. Journal Article No. 11930 of the Michigan Agricultural Experiment Station. 2Current address is Department of Agronomy, University of Wisconsin, Madison, WI 53706; Department of Horticulture and Landscape Architecture, University of Kentucky, Lexington, KY 40536; Pesticide Research Center, Michigan State University, East Lansing, MI 48824. We wish to thank Dr. Ingert Kuzych for assistance with the SEM studies. torum) contained two anacardic acid derivatives, o-pentadecenylsalicylic acid and o-heptadecenylsalicylic acid (Gerhold, Craig and Mumma, 1984). These compounds were moderately toxic when bioassayed with the twospotted spider mite, Tetranychus urticae. The aphid alarm pheromone, (E)-o-farnesene, was present in substantial quantities in air around wild potato (Solanum berhaultii) foliage (Gibson and Pickett, 1983). It was demonstrated that this chemical, which was secreted from leaf trichomes, induced rapid dispersal of settled aphid colonies. Green peach aphids (Myzus persicae) are killed by contact with secretions from the trichomes of several Nicotiana species (Thurston, Smith and Cooper, 1966). These secretions contain alkaloids, of which nicotine is the major constituent. Isoprenoids (mono-, sesqui-, and diterpenes) are also metabolites commonly found in the trichomes of flowering plants (Kelsey, Reynolds and Rodriguez, 1984). Some secondary compounds released from trichomes have also been implicated in allelopathy (Seigler, 1977; Swain, 1977; Lovett, 1982), the stimulatory or inhibitory effects that chemicals produced by one plant may exert on another (Rice, 1984). For example, the leaves of Artemisia absinthium bear glandular hairs which excrete ethereal oils and the alkaloid, absinthin (Funke, 1943). Plants in close proximity to A. absinthium have reduced vigor which has been attributed primarily to absinthin, which washes off the Artemisia plants onto the neighboring species. The glandular trichomes of Primula obonica secrete a phy-