Abstract
The phytotoxicity of aqueous foliar extracts and ground dried residues of sunn hemp (Crotalaria juncea L.), cowpea [Vigna unguiculata (L.) Walp. cv. Iron Clay], and velvetbean [Mucuna deeringiana (Bort) Merr.] to crop and weed germination and growth was evaluated to compare the allelopathic potential of the cover crops. By 14 days after treatment (DAT), goosegrass [Eleusine indica (L.) Gaertn.] germination with 5% aqueous extracts of all cover crops (w/v fresh weight basis) was similar and greater than 75% of control. However, with the 10% extracts, goosegrass germination was lowest with cowpea extract, intermediate with velvetbean extract, and highest with sunn hemp extract. Livid amaranth (Amaranthus lividus L.) germination declined to ≈50% with cowpea and sunn hemp extracts and even lower to 22% with velvetbean extract. The suppression of livid amaranth germination was greater with the 10% extracts than the 5% extracts. Bell pepper (Capsicum annuum L.) germination was unaffected by velvetbean extract, inhibited more by the 5% cowpea extract than the 10% extract, and was also sensitive to the 10% sunn hemp extract. All cover crop extracts resulted in an initial delay in tomato (Lycopersicon esculentum Mill.) germination, but by 14 DAT, inhibition of germination was apparent only with cowpea extract. The phytotoxicity of ground dried residues of the three cover crops on germination, plant height, and dry weight of goosegrass, smooth amaranth (A. hybridus L.), bell pepper, and tomato was evaluated in greenhouse studies. Goosegrass germination was inhibited in a similar manner by residues of the three cover crops to 80% or less of control. Smooth amaranth germination, plant height, and dry biomass were more sensitive to sunn hemp residues than to cowpea and velvetbean residues. Bell pepper germination, plant height, and dry weight were greater than 90% of control except for dry weight with cowpea residue, which was only 78% of control. The greatest effect of cover crop residue on tomato occurred with dry weight, because dry weights with cowpea and sunn hemp were only 76% and 69% of control, respectively, and lower than with velvetbean. There was more evidence of cover crop phytotoxicity with the weed species than with the crop species and cowpea extracts and residue affected all species more consistently than those of sunn hemp and velvetbean.
In agricultural production systems, a variety of environmental conditions may be manipulated to sustain beneficial production. Nutrients and soil organic matter should be replenished; erosion prevented; beneficial physical properties of the soil maintained or improved; and pests, pathogens, and weeds managed or suppressed. Cover crops can contribute to the improvement of future agricultural production by providing some or all of these benefits and are particularly valuable for low-input and sustainable agrosystems (Bowman et al., 1998). In addition to protecting land from erosion, increasing soil organic matter and cation exchange capacity, improving moisture-holding characteristics, suppressing pests, pathogens, and weeds, reducing soil compaction, and serving as food, fiber, and forage, many legume species also have the potential to fix large amounts of nitrogen (Cherr et al., 2006; Creamer and Baldwin, 2000; Dabney et al., 2001).
Cover crops can suppress weeds in cropping systems by competing with weeds for available resources and by promoting conditions that are unfavorable for weed germination and establishment (Teasdale, 1998). The latter mechanism includes allelopathy, which is the inhibitory or stimulatory effect of a plant on another species as a result of the release of chemicals into the environment (Putnam and Tang, 1986). Allelopathy of crop species is currently underused for weed management in agricultural systems. However, its role can be expanded through the use of allelopathic cultivars of cash crops (Wu et al., 1999) and by preceding cash crops with cover crops that exude allelochemicals or produce residues that decompose to release allelochemicals that are phytotoxic to weeds (Batish et al., 2006).
Recently, there has been increased interest in using leguminous cover crops in sustainable and organic cropping systems in Florida (Abdul-Baki et al., 2005; Collins, 2004; Scholberg et al., 2006). Species such as cowpea, sunn hemp, and velvetbean can be used during summer fallow periods to suppress weeds through resource competition (Collins, 2004); however, it is likely that weed suppression by these cover crops may also be in part the result of allelopathy.
Allelopathy in velvetbean has been attributed to L-3-[3,4-dihydroxyphenylalanine (L-DOPA)] (Fujii et al., 1991), which is exuded from leaves and roots (Fujii, 1999; Nishihara et al., 2005). It is estimated that velvetbean can contribute 200 to 300 kg·ha−1 of L-DOPA to the soil each year (Fujii et al., 1991). Aqueous extracts of velvetbean have been shown to inhibit radicle elongation in germinating seeds of Amaranthus hypochondriacus L., barnyardgrass [Echinochloa crusgalli (L.) P. Beauv.], tomato, and cabbage (Brassica oleracea var capitata L.) (Caamal-Maldonado et al., 2001; Gleissman, 1983). In addition, the growth of corn (Zea mays L.), beans (Phaseolus vulgaris L.), and cabbage also was inhibited by aqueous extracts of fresh velvetbean leaves (Gleissman, 1983). L-DOPA suppression of germination has been shown to be selective. Inhibition of radicle growth was most pronounced in Cerastium glomeratum Thuill, Lactuca sativa L., Linum usitatissimum L., and Spergula arvensis L., whereas species from the Poaceae and Fabaceae families were the least affected (Fujii et al., 1991).
Sunn hemp is a fast-growing, 1- to 3-m tall, leguminous cover crop that generates a large quantity of biomass and a considerable amount of nitrogen (Akanvou et al., 2001; Mansoer et al., 1997). A previously unknown, nonprotein amino acid isolated from the seeds of sunn hemp and other Crotalaria species (Pant and Fales, 1974) can be considered to be a candidate allelochemical of sunn hemp. It was identified as delta-hydroxynorleucine (5-hydroxy-2-aminohexanoic acid) (Pilbeam and Bell, 1979) and demonstrated to be phytotoxic to lettuce (Wilson and Bell, 1979). Extracts of sunn hemp seeds were also reported to have reduced growth in 12 weeds tested, generally causing greater reduction in growth with greater concentration (Cole, 1991). Aqueous extracts of sunn hemp leaves applied to wheat plants decreased root length but did not affect plant height, number of leaves, or shoot and root dry weights (Ohdan et al., 1995).
Cowpea has been shown to suppress weeds when used as a cover crop, intercrop, or organic mulch (Hutchinson and McGiffen, 2000; Ngouajio et al., 2003; Unamma et al., 1986). Aqueous cowpea extracts were recently reported to reduce radicle growth in crop and weed seeds (Hill et al., 2006). However, no putative allelochemicals have as yet been reported for cowpea. The present study was intended to complement cover crop/weed competition studies with cowpea, sunn hemp, and velvetbean (Collins, 2004). The objective was to compare the allelopathic potential of cowpea, sunn hemp, and velvetbean leaf extracts and dried ground residue on germination and growth of weed and crop species.
Materials and Methods
The cover crops included in the study were selected on the basis of their suppressiveness of root-knot nematode (McSorley, 1999; Vargas-Ayala et al., 2000) for use in a biologically based system for vegetable production (Abdul-Baki et al., 2005). Our intention was to additionally exploit their potential for weed suppression.
Aqueous extracts and seed germination.
Leaves were obtained from cowpea, sunn hemp, and velvetbean plants grown in a greenhouse for ≈2 weeks in 61 cm × 38 cm × 24-cm boxes (Buckhorn, Milford, Ohio) containing Speedling tobacco peat-lite mix potting soil (Speedling, Sun City, Fla.). Plants were fertilized at the beginning of the second week with Triple Ten liquid fertilizer (10N–10P–10K; Growth Products, Ltd., White Plains, N.Y.). Fully expanded leaves of each cover crop were harvested in the morning and petioles and rachises were removed. The leaves were cut into pieces and 25 g was blended with 150 mL of distilled water. The homogenate was filtered through six layers of cheesecloth and one layer of 25 μm nylon mesh (Spectrum Medical Industries, Los Angeles, Calif.). The filtrate was then made up to a 250-mL volume to yield a 10% w/v solution on a fresh weight basis. A 5% w/v solution was prepared for each species by dilution. The extracts were stored at 4 °C until use within 24 h.
The effect of the extracts on seed germination was evaluated using two weed and two crop species. A 9-cm circle of Whatman #3 (Whatman, Inc., Florham Park, N.J.) filter paper was placed in the lid of each 9-cm petri dish and moistened with 5 mL of extract. Twenty-five seeds each of bell pepper cv. Sunbeam, tomato cv. Crusader, livid amaranth, and goosegrass were placed on the filter paper. The base of the dish was used to close the dishes, which were then sealed with Parafilm M (Alcan Packaging, Neenah, Wis.) laboratory sealing film to reduce moisture loss while promoting gas exchange. Controls were prepared using 5 mL of distilled water instead of extract. Dishes were held in darkness at room temperature (≈25 °C). The dishes were checked after 3, 7, 10, and 14 d, at which time germinated seeds were counted and removed. Germination was defined as emergence of the radicle or hypocotyl to a length equal to the longest dimension of the seed. The experiment was repeated using 4 mL of extract per petri dish. The extracts were used immediately upon extraction with no refrigeration. Cumulative germination was expressed as a percent of control and analysis of variance (ANOVA) was performed using the general linear models procedure (Littell et al., 2002) to determine significance at the P = 0.05 level. Comparison of the cover crop extracts was done using the least significant difference (lsd) method and the ANOVA F test was used to compare the effects of the 5% and 10% extracts.
Ground residue effects on germination and growth.
Cowpea, sunn hemp, and velvetbean seeds were treated with cowpea-type rhizobium inoculant (Nitragin, Milwaukee) and 16 seeds per planting box were planted in ten planting boxes (61 cm × 38 cm × 24 cm) filled with Fafard custom mix soil (Conrad Fafard, Agawam, Mass.). All cover crops were irrigated immediately after seeding and regularly thereafter at 3-d intervals. Plants were grown in the greenhouse for 8 weeks. Leaves were harvested and dried in a drying oven at 72 °C for 3 d. The dried tissue was ground in a Wiley mill (Thomas Scientific, Swedesboro, N.J.) and then stored in the freezer at −20 °C until used. The allelopathic potential of the ground residue was evaluated using methods modified from Bewick et al. (1994) and Shilling et al. (1992). Ground foliage was thoroughly mixed with potting soil (Fafard custom mix soil) in plastic bags to give 0%, 2.5%, 5%, and 10% by mass. To each plastic bag 150 mL of water was added to saturate the soil. Each 10-cm diameter pot was filled with 60 g of soil/residue mixture and packed lightly to ≈1 cm from the top of the pot. Controls contained only soil.
Assay species were the same as for the extract experiment except smooth amaranth was used instead of livid amaranth. Smooth amaranth was tested with and without 3% activated carbon (Callaway and Aschehoug, 2000). Each pot was sown with either 25 seeds of tomato, bell pepper, or goosegrass or with 50 smooth amaranth seeds. Crop seeds were planted at 1-cm depth and weed seeds at 0.25-cm depth. The pots were arranged on weigh boats and immediately subirrigated after sowing and regularly thereafter at 3-d intervals. Data were collected for germination at 6, 11, 14, 18, and 21 d after sowing and plant heights were measured at 11 and 21 d after sowing. Plants were harvested at 21 d after sowing, dried at 75 °C for 3 d, and weighed to obtain dry biomass.
Each cover crop residue experiment was conducted twice and analysis of variance was performed using the general linear models procedure (Littell et al., 2002) for all parameters to determine significance at the P = 0.05 level. Comparisons among cover crop means were performed using lsd and the nature of the response to residue rates was determined using orthogonal polynomials. In assessing the effect of activated carbon, as a result of the interaction between the cover crop and activated carbon treatments, the simple effects on plant height and dry biomass of smooth amaranth were assessed by comparison of least squares means using the PDIFF option of the general linear models procedure (Littell et al., 2002).
Results and Discussion
Aqueous extracts and seed germination.
At 3 d after treatment (DAT), goosegrass germination was highest with 5% sunn hemp extract (91% of control), intermediate with cowpea extract (75%), and lowest with velvetbean extract (65%) (Table 1). Increasing extract concentration of cowpea and velvetbean resulted in a considerable decrease in goosegrass germination to less than 23% of control, whereas germination with 10% sunn hemp extract was the same as with the 5% extract. By 7 DAT, all cover crops had statistically similar germination with the 5% extracts; however, goosegrass germination with the 10% extracts of cowpea and velvetbean was still significantly lower than with the 10% sunn hemp extract by 14 DAT.
Effect of aqueous cover crop leaf extracts on the germination of goosegrass, livid amaranth, bell pepper, and tomato over a 14-d period.
Livid amaranth germination was inhibited by extracts of all three cover crops to less than 53% of control (Table 1). Velvetbean extracts caused greater inhibition of germination than cowpea and sunn hemp. By 14 DAT, germination with velvetbean was only 22%, but more than twice as much germination occurred with the cowpea and sunn hemp. The effect of extract concentration was also significant (P = 0.004). Germination with 5% extracts averaged over cover crops resulted in 49% germination compared with 33% with 10% extracts (data not shown).
Bell pepper seed germination was inhibited by extracts of cowpea and sunn hemp but not by velvetbean, which resulted in germination that was at least 92% of control (Table 1). Bell pepper seeds were more sensitive to cowpea extracts than sunn hemp extracts; at 7 DAT, germination with 5% cowpea extract was 28% of control compared with 81% with 5% sunn hemp extract. The effect was greater with the 10% cowpea extract, which completely inhibited bell pepper germination, whereas germination with 10% sunn hemp extract was 52% of control by 14 DAT.
All three cover crops caused an initial delay in tomato germination (Table 1) so that at 3 DAT, the best germination was 41% of control followed by 22% with the 5% sunn hemp extract and velvetbean extracts, respectively. Germination with both cowpea extracts and the 10% extracts of sunnhemp and velvetbean ranged from 0% to 6% of control. By 14 DAT, germination with sunnhemp and velvetbean was similar—96% and 99% of control, respectively, and higher than the 81% germination obtained with cowpea extract. Germination with 5% and 10% extracts averaged over cover crop was 97% and 87%, respectively, which were not significantly different (data not shown).
Ground residue effects on germination and growth.
Residues of all three cover crops were equally effective in reducing goosegrass germination to 80% or less of control by 3 weeks after planting (WAP) (Table 2). Goosegrass heights ranging from 92% of control with velvetbean residue to 106% with cowpea residue were statistically similar. However, cowpea residue stimulated dry weight of goosegrass to 123% of control, whereas sunn hemp and velvetbean residues suppressed goosegrass dry weight to 81% and 85% of control, respectively.
Effect of cover crop residue source on germination, plant height, and dry weight of goosegrass, smooth amaranth, bell pepper, and tomato.
Smooth amaranth germination and plant height were suppressed by residues of all three cover crops but most strongly by sunn hemp residue. By 3 WAP, germination of livid amaranth was lowest with sunn hemp residue (61%), intermediate with cowpea residue (78%), and highest with velvetbean residue (87%). Plant heights with cowpea and velvetbean residues were 86% and 75%, respectively, of control and statistically similar; however, stronger inhibition of smooth amaranth height with sunn hemp residue resulted in smooth amaranth plants that were only 52% as tall as the control. The poor germination and short stature of smooth amaranth plants with sunn hemp residue resulted in lowest dry weights: 20% of control, whereas velvetbean residue suppressed smooth amaranth dry weight to 79% of control and cowpea residue stimulated smooth amaranth dry weight to 114%.
Bell pepper germination was greater than 90% with all cover crop residues (Table 2). Bell pepper plants with cowpea and velvetbean residues were 4% and 16% taller, respectively, than control plants and 5% shorter with sunn hemp residue than control plants. The largest adverse effect was a 22% reduction in bell pepper dry weight with cowpea residue.
By 1 WAP, tomato germination with velvetbean residue was 91% of control and did not change over the next 2 weeks. In contrast, initial suppression of tomato germination to 85% of control at 1 WAP with sunn hemp residue and to 93% with cowpea residue did not persist. By 3 WAP, complete germination had occurred with cowpea residue and germination with sunn hemp was not statistically different from that with cowpea. Although tomato germination with cowpea and sunn hemp residues showed little or no adverse effect by 3 WAP, dry weights were significantly affected with decreases of 24% and 31% observed, respectively.
The species most responsive to the rate of cover crop residue was smooth amaranth (Table 3). A linear decrease in germination was observed with increasing proportions of cover crop residue. Plant height was affected in a similar manner. Bell pepper was the least responsive species with no effect of cover crop residue rate on germination, plant height, or dry weight. Declines in tomato and goosegrass germination with increasing cover crop residue rate were significant at 1 and 2 WAP, respectively. Tomato dry weight measured at 3 WAP decreased linearly from 100% with control plants to 61% of control in plants treated with 10% cover crop residue.
Effect of cover crop residue rate on germination, plant height, and dry weight of goosegrass, smooth amaranth, bell pepper, and tomato.
The effects of the cover crop residues on smooth amaranth germination and growth were compared with and without activated carbon to confirm that the observed responses were the result of allelochemical interference. Reversal of the effects of putative allelochemicals has been demonstrated previously using this technique (Callaway and Aschehoug, 2000; Shilling et al., 1992). Germination of smooth amaranth was improved with the addition of activated carbon to cover crop residue/soil mixtures (Table 4). Plant heights and dry weights also were greater with the cowpea and sunn hemp residue/activated carbon/soil mixtures. Soil factors such as organic matter that promote adsorption of allelochemicals can result in decreased phytotoxicity (Kobayashi, 2004). Many Florida soils are sandy and low in organic matter and clay and thus adsorption of allelochemicals is likely to be low. As a result, greater phytotoxicity may be expected than in the organic potting soil used in our residue study.
Effect of activated carbon on smooth amaranth germination and growth.
Target species varied in their sensitivity to cover crop extracts and residues. The amaranth weeds were the most sensitive of the species tested because germination of livid amaranth was suppressed by aqueous extracts of all three cover crop species at both the high and low concentrations, and residues of all three cover crops inhibited germination and growth of smooth amaranth. This sensitivity of Amaranthus species to aqueous extracts of leguminous cover crops is supported by other studies. Velvetbean leaf extract inhibited germination of A. hypochondriacus and A. retroflexus was inhibited by leaf extracts of hairy vetch and cowpea and by seed extracts of sunn hemp (Caamal-Maldonado et al., 2001; Cole, 1991; Hill et al., 2006).
Velvetbean extract has also previously been shown to inhibit a grass weed, barnyardgrass (Caamal-Maldonado et al., 2001); so it was not unexpected that in our study, goosegrass was suppressed by velvetbean extracts. However, the insensitivity of goosegrass to sunn hemp extracts underscores that broad-spectrum weed control cannot be expected with allelochemicals. In fact, consistent with our findings, Hill et al. (2006) concluded that effects of aqueous extracts of hairy vetch and cowpea vary with species and extract concentration. Differences in the sensitivity of crop cultivars may also occur. This would explain why in our study, bell pepper was more sensitive to cowpea extract than tomato, whereas Hill et al. (2006) found tomato to be more sensitive. Alternatively, the differences observed may be because Hill et al. (2006) used whole plant extracts, whereas our extracts were foliar. The time at which seed germination is evaluated can also affect study results. Caamal-Maldonado et al. (2001) reported that velvetbean leaf extract strongly inhibited tomato germination at day 3. We also found that tomato germination was only 5% of control at day 3. However, by day 7, germination had exceeded 90%.
There is a tendency for smaller-sized seed to exhibit greater sensitivity to allelochemicals and this was again recently supported by the work of Liebman and Sundberg (2006) with red clover extracts. The crop species in our study, which had larger seeds than the weeds, were less sensitive to cover crop extracts and residues. The use of transplants may also impart some protection for crop plants in which allelopathy will be used as one of the weed suppression tools in a cropping system. Bell peppers and tomatoes for fresh market are typically transplanted in Florida. Caamal-Maldonado et al. (2001) showed that although tomato seed germination was inhibited by aqueous velvetbean leachate, biomass of tomato transplants was unaffected by soil-incorporated velvetbean leaf tissue. However, the sensitivity of transplants may also differ with species because broccoli transplants appeared to be inhibited by velvetbean residues but were unaffected by cowpea and soybean residues (Harrison et al., 2004).
Despite variability in target species sensitivity, allelopathic cover crops can also contribute to the management of nonsensitive weed species through alternative mechanisms (Hutchinson and McGiffen, 2000; Teasdale and Mohler, 1993). Mechanisms of weed suppression of cover crops and their residues include interference (Radosevich et al., 1997), which encompasses resource competition and allelopathy, physical impedance, modification of the soil microclimate to adversely affect weed germination, and providing favorable conditions for weed seed decay and predation (Teasdale, 1998). In organic and sustainable conventional cropping systems, cover crops can be used during the off-season to suppress weed growth (Collins, 2004; Creamer and Baldwin, 2000; Ngouajio et al., 2003). Weed suppression can also be obtained by intercropping cover crops as living mulches for part or all of the cash cropping season (Teasdale, 1998) and by retention of cover crop residue as organic mulch in a subsequent cash crop (Hutchinson and McGiffen, 2000).
Our findings complement the results from a previous study that indicate that although smooth amaranth is inherently more competitive than sunn hemp and velvetbean, smooth amaranth biomass declines with increasing planting densities of the cover crops (Collins, 2004). Therefore, during the off-season, these cover crops may contribute to weed suppression through rapid canopy closure, utilization of water and nutrients that would otherwise be used by weeds, modification of soil microclimate, and, in the case of velvetbean, exudation of allelochemicals. During the cropping season, retention of surface residue offers soil microclimate modification and physical impedance of weed germination and growth; and surface mulches and incorporated residues may contribute to continued weed suppression by allelopathic suppression of weed seed germination and seedling establishment. Future studies should focus on isolating and identifying the putative allelochemicals in shoots and roots of sunn hemp and cowpea and determining whether the allelochemicals are exuded during cover crop growth in addition to being released after cover crop incorporation into the soil.
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