Allelopathy is increasingly recognized in weed management strategies (11, 35). Research has explored weed suppression by cover crops (5, 22, 28, 31) and allelochemicals for herbicide development (25), as well as crop-mediated weed suppression (9, 21, 24). Studies on squash (Cucurbita spp.) (4, 17, 19, 20, 29) indicate that its weed suppression in traditional Mesoamerican polycultures (3, 6, 17) results from both light competition and allelopathy. As these factors coexist in the field (33), methods to isolate their effects have been proposed (13). Field and laboratory studies confirmed allelopathy as a key factor in weed suppression (14).

Title: Inhibitory Potential of Compounds Released from Squash (Cucurbita spp.) under Natural Conditions

Authors: P.T. Fujiyoshi¹, S.R. Gliessman^1,2,*, J.H. Langenheim¹

¹Department of Biology, University of California, Santa Cruz, CA 95064, USA
²Department of Environmental Studies, University of California, Santa Cruz, CA 95064, USA

Correspondence Author (*): S.R. Gliessman
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Squash (Cucurbita spp.) extracts were tested for phytotoxicity under natural conditions. Fog drip from leaves showed no inhibition of lettuce (Lactuca sativa L.) seed germination or radicle elongation. Water-soluble root exudates similarly did not affect pigweed (Amaranthus retroflexus) germination or seedling growth. Aqueous leachates from senescent leaves inhibited germination and radicle/hypocotyl elongation in corn (Zea mays) and lettuce, but only at concentrations exceeding typical field levels, with minimal impact on weeds. Hydrophobic root exudates, collected via resin bead adsorption, inhibited lettuce seed germination and radicle elongation.

allelochemical release, allelopathic stimulation, bioassay, Cucurbita, fog drip, root contact, root exudates, volatiles

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Allelopathy is increasingly recognized in weed management strategies (11, 35). Research has explored weed suppression by cover crops (5, 22, 28, 31) and allelochemicals for herbicide development (25), as well as crop-mediated weed suppression (9, 21, 24). Studies on squash (Cucurbita spp.) (4, 17, 19, 20, 29) indicate that its weed suppression in traditional Mesoamerican polycultures (3, 6, 17) results from both light competition and allelopathy. As these factors coexist in the field (33), methods to isolate their effects have been proposed (13). Field and laboratory studies confirmed allelopathy as a key factor in weed suppression (14).

5.1. Plant Extracts

Greenhouse-grown Cucurbita maxima Duch. ex Lam. ‘Blue Hubbard’, a variety known for weed suppression (14), was used. Field samples were collected from the University of California, Santa Cruz Farm, near Monterey Bay, central California, using varieties commercially available for over a century (37, 39).

5.2. Bioassays

For large extract volumes, bioassays followed McPherson et al. (26). Seeds of test species were soaked in extract or control solution for at least 1 hour and germinated in bioassay chambers. Statistical analyses used t-tests and ANOVA on SPSS 6.1.1 for Macintosh or William R. Rice’s STN program (7 March 1996). The Student-Newman-Keuls test was conducted on SPSS 6.1.1, and contingency tests used Rice’s STN-FREQ (6 March 1996).

6.1. Leaf Leachates

Leaf leachates from Cucurbita pepo ‘Small Sugar’ inhibited crop species more than weeds at lower concentrations (Table 1). At 2.5% concentration, radicle growth of corn and lettuce was reduced by 34% and 50%, respectively. Pigweed (Amaranthus retroflexus) germination and seedling elongation were inhibited only at 5% concentration. Malva parviflora germination was too low for conclusive results.

6.2. Fog Drip

Fog drip showed no inhibitory activity. Lettuce radicle length with fog drip (11.8 mm) was not significantly different from the control (12.0 mm). Although fog drip may concentrate in soil over time, the bioassay concentration was likely lower than field conditions.

We thank the Alfred Heller Endowed Chair and the W.K. Kellogg Foundation for funding, the Center for Agroecology and Sustainable Food Systems for providing materials and lab space, and Rob Kluson, Ana Luisa Anaya, Rob Franks, Swamp Wood, Ricardo Santos, Jerry Brownrigg, and Jonathan Krupp for their support.

Arrange references alphabetically, using the journal's style: full journal names in italics, volume numbers in bold, full page ranges, and a small dash between pages. Include recent references (preferably within the last 10 years). Examples:

1. Al-Saadawi, I.S., Rice, E.L., and Karns, T.K. (1983). Allelopathic effects of Polygonum aviculare L. III. Isolation, characterization, and biological activities of phytotoxins other than phenols. Journal of Chemical Ecology 9: 761-774.
2. Amador, M.F. and Gliessman, S.R. (1990). An ecological approach to reducing external inputs through the use of intercropping. In: Agroecology: Researching the Ecological Basis for Sustainable Agriculture (Ed., S.R. Gliessman), pp. 146–159. Springer-Verlag, New York, USA.
3. Anaya, A.L., Ortega, R.C., and Nava Rodriguez, V. (1992). Impact of allelopathy in the traditional management of agroecosystems in Mexico. In: Allelopathy: Basic and Applied Aspects (Eds., S.J.H. Rizvi and V. Rizvi), pp. 271-301. Chapman & Hall, London.
4. Anaya, A.L., Ramos, L., Cruz, R., Hernandez, J.G., and Nava, V. (1987). Perspectives on allelopathy in Mexican traditional agroecosystems: a case study in Tlaxcala. Journal of Chemical Ecology 13: 2083-2101.
5. Anaya, A.L., Sabourin, D.J., Hernandez-Bautista, B.E., and Mendez, I. (1995). Allelopathic potential of Ipomoea tricolor (Convolvulaceae) in a greenhouse experiment. Journal of Chemical Ecology 21: 1085-1102.
6. Chacon, J.C. and Gliessman, S.R. (1982). Use of the "non-weed" concept in traditional tropical agroecosystems of south-eastern Mexico. Agro-ecosystems 8: 1-11.
7. Connick, W.J., Jr., Bradow, J.M., Legendre, M.G., Vail, S.L., and Menges, R.M. (1987). Identification of volatile allelochemicals from Amaranthus palmeri S. Wats. Journal of Chemical Ecology 13: 463-472.
8. Dalton, B.R., Blum, U., and Weed, S.B. (1989). Differential sorption of exogenously applied ferulic, p-coumaric, p-hydroxybenzoic, and vanillic acids in soil. Soil Science Society of America Journal 53: 757-762.
9. Dilday, R.H., Lin, J., and Yan, W. (1994). Identification of allelopathy in the USDA-ARS rice germplasm collection. Australian Journal of Experimental Agriculture 34: 907-910.

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