Model Research Paper
Inhibitory potential of compounds released from squash (Cucurbita spp.) under natural
P. T. FUJIYOSHI, S. R. GLIESSMAN1* and J. H. LANGENHEIM
Department of Biology, University of California, Santa Cruz, CA 95064, USA
Phone (Office): +390881741632, FAX (Office): +390881741632, Phone (Home): +390881686273,
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(Received in revised form: -------------)
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.
*Correspondence author, 1Department of Environmental Studies.
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).
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
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 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.
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.
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