Abstract

allelopathic potential of squash (Cucurbita pepo L. cv. Scarlette) on certain weed species was investigated. Fresh shoot extract reduced germination and growth of Amaranthus retroflexus, Chenopodium murale, Eruca sativa, Malva sylvestris, Portulaca oleracea and Solanum nigrum in Petri-dishes. A. retroflexus, E. sativa, M. sylvestris and S. nigrum were most susceptible. Only 1 ml extract in the medium significantly reduced germination and growth of all weeds tested and the effect increased with concentration. Foliage leachates significantly reduced germination and growth of all weed species, and roots affected more than shoots. Volatiles from squash shoots were also phytotoxic. Soil-incorporated squash residues prevented seed germination of P. oleracea and arrested growth of other weed species. Soil-applied extract was highly phytotoxic to all weeds except M. sylvestris.

Media Summary

Different experiments showed that squash (Cucurbita pepo L. cv. Scarlette) is of an allelopathic potential on certain common weed species in Jordan, and could be incorporated in weed management programs.

Key Words

Introduction

Certain cucurbits have been reported to yield allelochemicals (Anaya et al. 1987; Batish et al. 2001). Fruit extract of cucumber accession PI 169391 and foliage leachates of certain accessions was toxic to Punicum miliaceum (Lockerman and Putnam 1979; 1981). Leachates of Cucurbita moschata inhibited corn (Zea mays) and bean (Phaseolus vulgaris) (Anaya et al. 1987). Squash (Cucurbita pepo) Leaf extract inhibited growth of several vegetables (Liebman and Dyck 1993), radicle elongation of corn, cowpea and cabbage (Gliessman et al. 1981; Chacan and Gliessman 1982, Gliessman 1983) and was suggested for inclusion into intercropping system for weed control. Squash effect was due to shade and allelochemicals (Liebman and Dyck 1993). Citrullus vulgaris, Cucumis sativus and Cucumis melo exhibit an autotoxicity effect (Yu 2001).

The work was designed to detect any allelopathic effect of a widely grown squash cultivar in Jordan on certain common weeds in vegetable crops.

Methods

Allelopathic effects of squash (Cucurbita pepo L. cv. Scarlette) on Amaranthus retroflexus, Chenopodium murale, Eruca sativa, Malva sylvestris, Portulaca oleracea and Solanum nigrum was investigated. Squash plants were grown in the field for 2 months before harvested and their shoots were used in different experiments. Weed seeds were collected from cultivated fields, and stored in vials until being used. Crop seeds were brought from local companies. The following experiments were carried out:

Experiment 1. Effect of extract concentration

Fifty seeds of each weed species were placed on a filter paper in each of four Petri-dishes (11 cm in diameter) per treatment. Full strength extract at 0 (control), 1, 2, 3 and 4 ml were added per dish. The final volume was completed to 5 ml by adding distilled water. Petri-dishes were incubated in the dark at 25°C. Germination percentage (G%) was recorded at 2 and 14 days of incubation, the experiment was terminated then stem and root lengths of weed seedlings were determined.

Experiment 2. Effect of foliage leachates

Seeds were sown as above. Five ml of squash foliage leachates was added per Petri-dish. Distilled water was used for the control treatment. Petri-dishes then incubated as usual for two weeks, and similar data on weeds as in experiment 1 were recorded.

Experiment 3. Effect of volatile materials

Twenty seeds of each weed were placed in each of four (5.5 cm diameter) Petri-dish. Two ml distilled water was added per dish. Dishes were left uncovered and each was placed in a larger Petri-dish to which 10 ml of extract or distilled water was added. Large dishes were closed with a tight cover and incubated at 25°C for one week before harvested. Germination percentage and stem and root lengths of weeds were recorded.

Experiment 4. Allelopathic effects of squash dried residues

Squash shoot residues were mixed in the soil at a rate of 16 g kg^-1. Twenty seeds of each weed were sown per each of four plastic pots filled with the residue-soil mixture. Pots with no residue added were included, sown by similar number of seeds as a control. Pots were water-irrigated when needed. Six weeks after emergence, data on weed germination and growth of weeds were recorded.

Experiment 5. Allelopathic effect of soil-applied extracts

Twenty seeds of each weed species were sown per pot. Squash extract at 75 ml was added per pot. For the control, 75 ml of water was added. The experiment was terminated at 5 weeks after weed emergence. Data on germination and growth of weeds were obtained.

Experiment 6. Allelopathic effects of squash root exudates

One squash plant was grown until flowering in 10-cm diameter pots before removed. The soil then was loosened and sown with 20 seeds of each weed per pot. Pots filled with fresh soil were sown and considered as a control. Hoagland nutrient solution (Hewitt 1966) was added at 100 ml per pot 3-4 days after emergence and continued until harvest. At 6 weeks after emergence, weeds were harvested and data on germination and growth were taken.

Experiment 7. Allelopathic effects of decayed squash residues

Dried squash shoots were mixed with the soil at a rate of 16g kg^-1. The mixture was placed in 10-cm diameter pots, and regularly irrigated for 40 days to allow residue natural decay, the soil was then loosened and each pot was sown with 20 seeds of a single weed species. Pots, with no residues added and sown with seeds of the same weed were included as a control. Pots were irrigated as needed. The experiment continued for five weeks, then terminated and data on germination and growth of weeds were recorded.

Statistics

All experiments were laid out in a randomized complete block design with four replicates. Data were statistically analysed by ANOVA, and treatments means were compared using LSD at P = 0.05.

Results

Experiments 1. Effect of shoot extract

Weeds stem and root lengths were inhibited with extract compared with the control. The inhibitory effect increased with extract concentration (Table 1). A. retroflexus, E. sativa, M. sylvestris, and S. nigrum were most susceptible. P. oleracea germination was least affected. Roots were inhibited more than shoots.

Table 1. Effect of aqueous extract concentration of squash on different weeds grown under laboratory conditions.

Treatments	A. retroflexus			C. murale			E. sativa
	G%	Stem length (mm)	Root length (mm)	G%	Stem length (mm)	Root length (mm)	G%	Stem length (mm)	Root length (mm)
Control	65	19	18	69	26	24	93	28	26
1	61	22	16	51	22	12	63	22	12
2	59	25	13	53	25	11	38	12	4
3	53	17	10	53	26	6	25	6	3
4	41	10	4	53	17	6	21	3	1
LSD (P = 0.05)	4	1	1	5	2	1	5	1	1
Treatments	M. sylvestris			P. oleracea			S. nigrum
	G%	Stem length (mm)	Root length (mm)	G%	Stem length (mm)	Root length (mm)	G%	Stem length (mm)	Root length (mm)
Control	80	58	25	90	31	29	65	32	18
1	65	30	15	81	32	23	65	44	7
2	48	15	8	81	25	13	60	27	6
3	32	10	5	82	17	7	41	12	3
4	25	9	2	77	7	5	38	9	2
LSD (P = 0.05)	5	1	1	4	1	1	5	2	1

Experiment 2. Effect of foliage leachates

Low concentration of leachates reduced germination of all weeds, and mostly inhibited their growth (Figure 1a). Weeds were different in their responses. M. sylvestris and P. oleracea root lengths were most affected, while C. murale and S. nigrum were least sensitive.

Experiment 3. Effect of volatile materials

Germination of M. sylvestris, P. oleracea and S. nigrum was the least affected (Figure 1b). Volatiles affected growth of all weed species with C. murale and S. nigrum were most affected.

Figure (1 a & b). Effect of foliage leachates (a) and volatiles (b) of squash shoots on germination and growth

of different weed species grown in Petri-dishes under laboratory conditions at 25^oC.

Experiment 4. Allelopathic effects of squash dried residues

Soil-incorporated residues reduced germination of A. retroflexus and M. sylvestris (Table 2). Growth of all weeds was reduced. C. murale, M. sylvestris and S. nigrum appeared the least affected.

Table 2. Effect of squash residues on germination (G%) and growth of different weeds grown in the glasshouse.

Treatments	A. retroflexus				C. murale				E. sativa
	G%	Plant height (mm)	Shoot dry weight (mg/pot)	Root dry weight (mg/pot)	G%	Plant height (mm)	Shoot dry weight (mg/pot)	Root dry weight (mg/pot)	G%	Plant height (mm)	Shoot dry weight (mg/pot)	Root dry weight (mg/pot)
Control	71	42.3	190	167	56	31.7	369	90	56	36.3	563	644
Squash	26	9.8	78	42	46	22.8	218	47	25	11.9	187	108
LSD (P = 0.05)	9	6.8	30	18	10	5.6	50	9	9	6.8	37	61
Treatments	M. sylvestris				P. oleracea				S. nigrum
	G%	Plant height (mm)	Shoot dry weight (mg/pot)	Root dry weight (mg/pot)	G%	Plant height (mm)	Shoot dry weight (mg/pot)	Root dry weight (mg/pot)	G%	Plant height (mm)	Shoot dry weight (mg/pot)	Root dry weight mg/pot)
Control	79	52.1	363	594	86	61.9	234	100	69	38.7	341	338
Squash	65	45.6	403	415	0	0	0	0	41	16.8	208	93
LSD (P = 0.05)	10	ns	ns	109	10	1.1	22	8	6	2.8	37	20

Experiment 5. Allelopathic effect of soil-applied extract

Squash shoot extract in the soil reduced germination and growth of all weeds tested (Fig. 2a). M. sylvestris and S. nigrum were the least affected but A. retroflexus was the most. Roots inhibited more than shoots except for M. sylvestris.

Experiment 6. Allelopathic effects of squash root exudates

Shoot and root dry weights of A. retroflexus, C. murale, and E. sativa were significantly reduced with squash root exudates compared with the controls (Figure 2b). Shoots of A. retroflexus were the most affected.

Experiment 7. Allelopathic effects of decayed residues

Germination of A. retroflexus was reduced, while root growth of A. retroflexus, C. murale, E. sativa and P. oleracea were the most affected (Figure 3).

a. Soil applied extract	b. Root exudates

Figure (2 a & b). Effect of soil applied extract (a) and root exudates (b) of squash on germination and growth of different weed species grown in pots under glasshouse conditions.

Figure (3). Effect of soil decayed squash residues on different weed species grown under glasshouse conditions.

Conclusions

1. Squash (Cucurbita pepo L. cv. Scarlette), is of an allelopathic potential on different weed species.

2. Squash residues or inclusion of squash crop in rotation system may aid in weed management strategies.

3. Further work on squash allelopathic effects in the field, genetic studies and isolation and identification of allelopathic agent(s) are important future research lines.

References

Anaya AL, Ramos L, Cruz R, Hernandez JG, Nava V (1987) Perspectives on allelopathy in Mexican traditional agroecosystems: A case study in Tlaxacala. Journal of Chemical Ecology 13, 2083-2101.

Batish DR, Singh HP, Kohli RK, Kaur S (2001) Crop allelopathy and its role in ecological agriculture. Journal of Crop Production 4, 121-161.

Chacan JC, Gliessman SR (1982) Use of the non-weed concept in traditional tropical agroecosystem of southern Mexico. Agroecosystem 8, 1-11.

Gliessman SR (1983) Allelopathic interactions in crop/weed mixtures: application for weed management. Journal of Chemical Ecology 9, 991-999.

Gliessman SR, Garcia RE, Amadar M (1981) The ecological basis for application of traditional agricultural technology in the management of tropical agroecosystems. Agroecosystem 7, 173-185.

Liebman M, Dyck E (1993) Crop rotation and intercropping strategies for weed management. Ecological Application 3, 92-122.

Lockerman RH, Putnam AR (1981). Growth inhibitors in cucumber plants and seeds. Journal of American Society of Horticulture Science 106, 418-422.

Lockerman RH, Putnam AR (1979) Evaluation of allelopathic cucumbers (Cucumis sativus) as an aid to weed control. Weed Science 27, 54-57.

Yu JQ (2001) Autotoxic potential of cucurbit crops: phenomenon, chemicals, mechanisms and means to overcome. Journal of Crop Production 4, 335-348.