Inhibitory potential of compounds released from squash (Cucurbita spp.) under natural conditions

P. T. Fujiyoshi, S. R. Gliessman¹* and J. H. Langenheim
Department of Biology, University of California, Santa Cruz, CA 95064, USA
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ABSTRACT

The squash (Cucurbita spp.) extracts released under the natural conditions were tested for phytotoxicity. The fog drip collected from the leaves did not inhibit lettuce (Lactuca sativa L.) seed germination or radicle elongation. Similarly, water soluble root exudates also did not inhibit germination or seedling growth of pigweed (Amaranthus retroflexus). Aqueous leachates from senescent leaves inhibited the germination, radicle and hypocotyl elongation of corn (Zea mays) and lettuce. However, the concentrations used were greater than those likely found under field conditions and showed little or no activity toward the weed species. Hydrophobic root exudates collected by adsorption to resin beads were inhibitory to lettuce seed germination and radicle elongation.

Key words: Allelochemical release, allelopathic stimulation, bioassay, Cucurbita, fog drip, root contact, root exudates, volatiles.
Please arrange in Alphabetical Order.

*Correspondence author, ¹Department of Environmental Studies. Foot Note, Author please add at end of Page 1 of Ms?

INTRODUCTION

The use of allelopathy in weed management has received significant attention (11,35). In addition to numerous investigations into weed suppression by cover crops (5,22,28,31) andallelochemicals suitable for herbicide development (25), there is a body of literature onallelopathic suppression of weeds by crop plants themselves (9,21,24). The studies involving squash (Cucurbita spp.) (4,17,19,20,29) suggest that its effectiveness in weed suppression in traditional Mesoamerican polyculture (3,6,17) is due to a combination of competition for light and allelopathy. Since allelopathy and light competition operate simultaneously in the field (33), methods for separating them have been proposed (13). In field studies and laboratory experiments, we explored the contributions of each factor. Results from the field studies supported the suggestion that allelopathy was a contributing factor to weed suppression (14).

MATERIALS AND METHODS

I. Plant Extracts
Greenhouse-grown squash plants were Cucurbita maxima Duch. ex Lam. 'Blue Hubbard', a variety shown to suppress weeds (14). Field-collected material came from theUniversity of California, Santa Cruz Farm, near the Monterey Bay of central California. Varieties were chosen based on their commercial availability for over a century (37,39).

II. Bioassays When large volumes of plant extracts were available, a bioassay similar to McPhersonet al. (26) was conducted. Seeds of test species were soaked for at least 1.0 h in the extract or the control solution and germinated in bioassay chambers. Statistical analysis: T -tests and ANOVA were performed on SPSS 6.1.1 for Macintosh or William R. Rice's program STN dated 7 March 1996. Student-Newman-Keuls test was performed on SPSS 6.1.1 for Macintosh. The contingency test was done on Rice's STN-FREQ dated 6 March 1996.

RESULTS AND DISCUSSION

Leaf leachates
Cucurbita pepo 'Small Sugar' leaf leachates were more inhibitorytowards the crops than the weeds at the lower concentration (Table 1). Radicle growth of corn and lettuce was significantly inhibited by 2.5% leachate by 34 % and 50 %, respectively, Amaranthus retroflexus germination and seedling elongation were inhibited only by 5%leachate and Malva parviflora germination was too poor to draw any conclusions.

Fog drip
Fog drip had no activity Radicle length of lettuce treated with fog drip (11.8 mm) was not significantly different from the 'control (12.0 mm). The fog drip falls on soil or nearby plants and while it was not active in bioassay, it is possible that on successive nights the drip falls in the same place and could have concentrated during the day. Thus the concentration used in the bioassay could have been less than that in the soil near the plants.

ACKNOWLEDGEMENTS

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

REFERENCES

(Please add, Recent References of last 10-years [Arranged in Alphabetical Order] are preferred)
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