African Journal of Rural Development, Vol. 3 (3): July-September 2018: pp.869-882. ISSN 2415-2838 This article is licensed under a Creative Commons license, Attribution 4.0 International (CC BY 4.0)

Biochemical and morphological roles of allelopathic crops in integrated weed management: A review

J.T. RUGARE.,1,2 P.J. PIETERSE.1 and S. MABASA.2 1Department of Agronomy, Stellenbosch University, South Africa, Private Bag X1 Matieland, 7602, South Africa 2Department of Crop Science, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe Corresponding author: [email protected]

ABSTRACT Poor weed control accounts for 40-60% of yield losses in arable crops. The widespread and injudicious use of synthetic herbicides currently obtaining in many farming communities leads to environmental damage and a surge in the development of herbicide resistant weed biotypes. These negative effects of improper herbicide use have created the need for efficacious, environmentally friendly and economically feasible alternatives or complements to herbicides. One such approach is the exploitation of allelopathy in ecological and integrated weed management. Several strategies of exploiting allelopathy in weed management have been widely investigated. In this review, the use of crop allelopathy, allelopathic residues, allelopathic water extracts and allelopathic cover crops is discussed. The study revealed that improved crop productivity where allelopathic /extracts are used is attributed to effective and selective suppression of weeds in weed-crop mixtures through competition, allelopathy and toxicant-induced hormesis that occurs when the extracts/mulches are applied or released at concentrations lethal to weeds but stimulatory to crops.

Key words: Allelopathy, , hormesis, weed management

RÉSUMÉ Un contrôle inapproprié des mauvaises herbes représente 40 à 60% des pertes de rendement sur les cultures arables. L’utilisation répandue et peu judicieuse des herbicides synthétiques actuellement utilisés dans de nombreuses communautés agricoles conduit à des dommages environnementaux et à la montée en puissance du développement de biotypes d’adventices résistantes aux herbicides. Ces effets négatifs de l’utilisation inappropriée d’herbicides ont créé le besoin d’alternatives ou de compléments efficaces, respectueux de l’environnement et économiquement réalisables aux herbicides. L’une de ces approches est l’exploitation de l’allélopathie dans la gestion écologique et intégrée des adventices. Plusieurs stratégies d’exploitation de l’allélopathie dans la gestion des adventices ont été largement étudiées. Dans cette revue, l’utilisation de l’allélopathie des cultures, des résidus allélopathiques, des extraits d’eau allélopathique et des cultures de couverture allélopathiques a été discutée. L’étude a révélé que l’amélioration de la productivité des cultures lorsque des plantes / extraits allélopathiques sont utilisés, est attribuée à la suppression efficace et sélective des adventices dans des mélanges adventices-cultures par compétition, allélopathie et hormèse provoquée par des substances toxiques se produisant lorsque les extraits / paillis de plantes

Cite as: Rugare, J.T., Pieterse, P. J. and Mabasa, S. 2018. Biochemical Received: 02 March 2018 and morphological roles of allelopathic crops in integrated weed Accepted: 14 May 2018 management: A review. African Journal of Rural Development 3 (3): Published: 30 June 2018 869-882. Biochemical and morphological roles of allelopathic crops in integrated weed management: A review

sont appliqués ou libérés à concentrations létales pour les adventices mais stimulantes pour les cultures.

Mots clés: allélopathie, culture de couverture, hormèse, lutte contre les adventices

INTRODUCTION allelochemicals may also be released as the plant Allelopathy is a sub-discipline of chemical tissues usually applied under field conditions ecology which involves the production as mulches decompose to produce phytoactive of biochemical compounds by plants that secondary compounds by chemical or microbial influence the growth, survival, development, metabolization, a phenomenon called functional and reproduction of other plant organisms allelopathy (Zimdahl, 2007). Several allelopathic (Cheng and Cheng, 2015). This biological interactions of agricultural importance have phenomenon involves both detrimental and been reported including crop to weed, weed stimulatory effects due to secondary metabolites to crop, crop to crop, plant to insect and plant called allelochemicals (Rice 1984, Abbas et al., to pathogen (Zohaib et al., 2016, Abbas et al., 2017b). Literature abounds with information 2017b). There are several plants that produce on the inhibitory activity of allelochemicals allelochemicals that negatively influence on crops, weeds, pests and diseases (Farooq the growth of the same plant, a phenomenon et al., 2011). However, the stimulatory effect called autotoxicty (Singh et al., 1999). of allelochemicals at low concentrations, a phenomenon known as “hormesis” has been Allelopathy amongst plant species also plays reported by several researchers (Belz et al., an important role in plant succession through 2005). The term “hormesis” was first used by several mechanisms involving inhibition of Southam and Erlich in 1943 who reported the growth of neighbouring plants, stimulating that extracts of western red cedar heartwood weed germination (Abbas et al., 2017a) and (Juniperus virginiana L.) exhibited inhibitory protecting the donor plant from adverse biotic effects on fungi at high concentrations but conditions (Kong, 2010). The advancement of stimulated fungal growth at low concentrations allelopathic research has also been necessitated (Duke et al., 2006, Abbas et al., 2017b). by the rising demand for sustainable crop Conversely, some allelochemicals have been production systems like organic farming and found to be phytotoxic at low concentrations while conservation agriculture (CA) which require they are stimulatory at higher concentrations minimal or no use of synthetic pesticides (Ahmed et al., 2007). Therefore, the effect of the (Kruidhof et al., 2008). The utilisation of same allelochemicals may differ depending on allelopathic plant extracts and mulches for weed the concentration applied and recipient plants. control is one such strategy that has received worldwide attention. Moreover, the exploitation Allelopathy can be divided into two categories, of allelopathic properties of plants in weed “true allelopathy” and “functional allelopathy” management is considered a key strategy in (Zimdahl, 2013). True allelopathy involves the management of herbicide resistance which the direct release into the environment of is caused by excessive and injudicious use of allelochemicals without any chemical/microbial synthetic herbicides on the same target weeds metabolisation as leaf leachates, volatiles over a long period of time (Pieterse, 2010). Over or root exudates (Baratelli et al., 2012). the years researchers have developed several Allelochemicals can also be found in seeds and ways of exploiting the allelopathic properties (Farooq et al., 2011). On the other hand, of plants to achieve sustainable weed control

870 J.T. RUGARE et al. in arable fields. The most promising and plant (Zohaib et al., 2016). Liebman and Davis practically feasible strategies of exploiting (2000) reported that the allelopathic effects of allelopathy discussed in this review include crop residues are more pronounced on small (a) use of allelopathic residues as mulch from seeds than larger seeds. Since many crop cover crops in crop rotations, (b) application seeds are relatively bigger than weed seeds, of aqueous extracts as post emergence sprays, there are prospects for selective control of and (c) use of allelopathic varieties of main weeds using allelopathic mulches (Liebman crops commonly known as crop allelopathy. and Mohler, 2001). Moreover, the ability of crop seeds to tolerate allelopathic mulches Allelopathic crop residues. The use of can also be attributed to their ability to allelopathic crop residues is a promising metabolically deactivate the allelochemicals strategy of achieving cost effective, safe and and the presence of sufficient food reserves environmentally friendly weed suppression to support germination and successful in arable fields (Jabran et al., 2015). emergence (Bezuidenhout et al., 2012). Mtambanengwe et al. (2015) reported that surface mulching with retained cowpea Allelochemicals released by crop mulches (Vigna unguiculata L. [Walp]) residues may also influence plant growth indirectly suppressed weed emergence by between by altering soil characteristics and inhibiting 40% and 60% under CA in Zimbabwe. Weed soil micro fauna (Kobayashi, 2003; Zohaib suppression using allelopathic mulches et al., 2016). Consequently, accumulation can be achieved through crop rotation or of allelochemicals in the soil results in intercropping. Residues of allelopathic suppression of seed germination and plant plants may be left on the soil as mulch after growth, decrease in the volume of primary harvesting crops in reduced tillage systems roots and increased secondary roots, (Ashraf et al., 2017) or may be incorporated reduced uptake of water and nutrients and into the soil in conventional tillage systems subsequently chlorosis ultimately resulting in where they release putative allelochemicals the death of the plant (Narwal et al., 2005). during decomposition (Abbas et al., 2017b). For example, application of Malabar catmint However, mulching is only effective against [Anisomeles indica (L.) Kuntze] root residues weeds before or during germination and does as mulch in wheat (Triticum aestivum L.) at not provide effective weed control if done 2 t ha-1 reduced little canary grass (Phalaris after weed emergence (Ashraf et al., 2017). minor Retz) seedling emergence and growth Furthermore, allelochemicals are generally but increased wheat yield under natural field unstable in the soil and can only provide weed conditions (Batish et al., 2007). Abbas et al. suppression for a short duration after mulching. (2017a) reported that combining sorghum, Hence the exploitation of allelopathic mulches rice and maize mulches with reduced for weed control should be part of a bigger herbicide mixtures increased little canary integrated weed management programme grass seed mortality up to 98%, significantly (Nichols et al., 2015). Nevertheless, the use of reduced dry weed biomass and provided up allelopathic plant residues has been reported to 92% weed control efficiency. They further to be an effective way of suppressing weeds reported a reduction in seed viability and because the allelochemicals are released in the increased loss of little canary grass seedlings soil environment in close proximity to weed where allelopathic mulches were integrated seeds or the roots of weed seedlings and can with reduced herbicide mixtures. The therefore be readily absorbed by the receiver increase in seed decay and loss of viability

871 Biochemical and morphological roles of allelopathic crops in integrated weed management: A review could be partially attributed to the phytotoxic (Gossypium hirsutum L.) yields than the full activity of allelochemicals released during pendimethalin dose although weed suppression the decomposition of the mulches (Nichols et was relatively less (Iqbal et al., 2009). The al., 2015). This demonstrates the potential of increase in cotton yield reported by these authors allelopathic mulches integrated with reduced suggests that the sorghum-pendimethalin herbicide dosages to contribute to reduction mixture may have caused desirable hormesis on in weed seed bank size in the long run. this crop despite poor weed control. Similarly, post emergence application of aqueous extracts Allelopathic weed suppression has been of moringa (Moringa oleifera Lam.) and demonstrated in studies conducted under sorghum [Sorghum bicolor (L.) Moench] at controlled environments using artificially high 3% w v-1 increased maize yield by 52% and concentrations despite the fact that these natural 42%, respectively (Iqbal, 2014). This shows herbicides can usually be found only in very that effective weed suppression may not be the minute quantities in the soil (Algandaby and El- only allelopathic mechanism that researchers Darier, 2016). Furthermore, variable efficacy by should focus on when developing strategies to the same mulch could be due to variations in the enhance crop production through allelopathy. concentration of allelochemicals in the plant, This increase in crop yield at concentrations variations in the amount of mulch and variations of allelopathic extracts lethal to young weeds in soil activity depending on environmental but stimulatory to crops has been attributed to and edaphic factors. As a result, increased enhanced membrane stability and water relations weed emergence and seedling growth was among other mechanisms (Munir, 2011). reported from fields treated with allelopathic mulches which could be partially attributed to Furthermore, desirable hormesis in fields the hormetic effects of the allelochemicals at sprayed with allelopathic water extracts can also low concentrations (Belz et al., 2005, Abbas be attributed to enhancement of physiological et al., 2017b). This underscores the need processes such as photosynthesis, respiration, for site specific evaluation of allelopathic increased gaseous exchange and stabilisation weed suppression by different mulches. of the plant’s photosynthetic pigment system (Kaya et al., 2009). Cedergreen et al. (2009) Allelopathic aqueous extracts. Several reported that the hormetic effect of the herbicide researchers have demonstrated that exploitation glyphosate was only transferred into harvestable of allelopathic water extracts has great potential yield when the herbicide was applied to for effective and sustainable weed control in barley (Hordeum vulgare L.) at anthesis stage agriculture (Jamil et al., 2009, Jabran et al., with earlier applications not translating into 2010). The application of allelopathic water an increase in crop productivity. This lends extracts for weed control can boost crop growth credence to the assertion that there is need to directly or indirectly (Farooq et al., 2011; apply the allelochemicals repeatedly within Abbas et al., 2017a). First, an increase in yields the same life cycle of a crop in order to sustain by allelopathic extracts could be attributed the hormetic effect of phytotoxins which may to toxicant-induced hormesis on crops which disappear if a single application is done early occurs when phytotoxins are applied at crop- in the season (Abbas et al., 2017a). Therefore, tolerant, low dosages (Abbas et al., 2017b). For maximum benefits of allelopathic water example, a tank mixture of sorghum water extract extracts are likely to be realized when they are at 12 L ha-1 and pendimethalin at one-third of the applied repeatedly commencing early in the recommended full dose resulted in higher cotton life cycle of the crop when the weed seedlings

872 J.T. RUGARE et al. are still susceptible to phytotoxic damage. increase the operational costs of weed control. Furthermore, repeated application will require Secondly, improved crop productivity an increased supply of plant extracts which observed in crops sprayed with water extracts may not be feasible, especially in production of allelopathic plants can also be attributed systems with competing interests for the to effective weed control. For example, crop residues (Nyagumbo, 2008). Farooq sunflower (Helianthus annuus L.) water et al. (2011) reported that effective weed extracts reduced weed density by 46% and suppression paralleling herbicide control was increased plant height, leaf area, spike length achieved when extracts of different plants and grain yield of wheat by 14%, 26%, 17% were tank mixed compared to applications of and 76% over the control, respectively (Khan the sole extracts of individual plant tissues. et al., 2017). Utilisation of sole water extracts This underscores the need to explore the of plants offers prospects for effective weed effect of mixing extracts of different cover control in farming systems like organic crops in order to benefit from mixture effects farming where use of synthetic chemicals of different classes of allelochemicals. is prohibited (Santos et al., 2007, Jabran et al., 2010). For example, application of Combining plant aqueous extracts and phenolics obtained from jack [Canavalia reduced herbicide dosages. The other ensiformis (L.) DC] controlled lilac tassel promising strategy of exploiting allelopathy in [Emilia sonchifolia (L.) DC] and weed management is to combine allelopathic prickly fan petals (Sida spinosa L.) within water extracts and reduced dosages of 15 and 30 days after application, respectively synthetic herbicides (Duke, 2007). Several (Silva and Rezende, 2016). However, the researchers have reported satisfactory weed long time taken post application of sole control when reduced herbicide dosages were extracts could result in the crop suffering tank mixed with allelopathic water extracts. from early season weed pressure, especially For example, Khan et al. (2017) reported in crops with a long critical weed free period. a reduction in weed density of 50% when Parthenium weed (Parthenium hysterophorus Ashraf et al. (2017) reported that tank mixtures L.) extract was tank mixed with half of the of sorghum and sunflower reduced the weed recommended dosage of fenaxoprop-p-ethyl density in wheat by 65% and improved and bromoxynil plus MCPA resulting in higher wheat grain yield by 5.5%. The phytotoxic wheat yield than the control. In another study, activity demonstrated by both sunflower combining 10 L ha-1 of a mixture of sorghum, and sorghum could be attributed to the sunflower, sugar beet (Beta vulgaris L.) and presence of the allelochemicals sorgoleone, safflower (Carthamus tinctorius L.) water heliannnone and leptocarpin in the extracts of extracts and herbicides 2.4-D and tribenuron these crops (Bogatek et al., 2006). However, methyl resulted in a weed density reduction in most cases effective weed control was of 85-88% and an increase in wheat yield of obtained with repeated applications of 18% over the control (Miri and Armin, 2013). extracts in order to keep weed populations Elahi et al. (2011) reported that a 67% below the economic threshold. Repeated decrease in the dosage rate of isoproturon application of allelopathic water extracts when it was tank mixed with 12 L ha-1 each of required to achieve satisfactory weed control sorghum, sunflower and rice (Oryza sativa L.) is likely to be a deterrent to the adoption of water extracts reduced weed dry weight by this strategy since repetitive spraying may 92% and 93%, when the weeds were sprayed

873 Biochemical and morphological roles of allelopathic crops in integrated weed management: A review at 40 and 70 days after sowing, respectively. sowing of the subsequent spring crop (Moore et al., 1994). They play an important role in Razzaq et al. (2012) reported higher wheat integrated weed management (Kruidhof et al., yields and a reduction in herbicide use of 70%, 2008). Weed suppressive ability of cover crops when sunflower and sorghum water extracts can be attributed to their fast emergence, rapid were combined with herbicides. In another canopy development and root growth (Rueda- study, a combination of sorghum and rice water Ayala et al., 2015). As a result, the level of extracts each at 15 L ha-1 mixed with 600 g a.i. weed suppression will depend on the cover ha-1 of pendimethalin gave a maximum weed crop species, weed species, environmental biomass reduction of 68 and 66% at 40 and 60 factors, amount and thickness of the mulch and days after sowing canola, respectively (Jabran et the management system used (Creamer, 1996, al., 2008). It was postulated that allelochemicals Liebman and Mohler, 2001). Bezuidenhout and herbicides may work additively or et al. (2012) reported the detrimental effects synergistically resulting in weed control of cover crops of oats (Avena sativa L.), rye paralleling full herbicide dosages (Farooq et (Secale cereale L.) and three annual ryegrass al., 2011). In addition, combining herbicides varieties on weed emergence and consequently and allelopathic water extracts has economic maize yield suggesting the need for screening of benefits. Jabran et al. (2008) reported a decrease cover crops and/or use of mulching rates that do in herbicide use of 50-67% and a concomitant not affect the growth of the main crop. increase in marginal returns of 1793-2059%. Apart from achieving satisfactory weed Cover crops may increase crop yields through control and reducing the cost of weed control, improved weed suppression and increased the application of low dosages of synthetic supply of nutrients that are released as their herbicides also reduces environmental pollution residues decompose thereby improving the vigor due to reduced application of herbicides in the and competitive ability of the main crop (Boddey agroecosystem. As a result, those herbicides et al., 2006). Leguminous cover crops also which are problematic in crop rotations due to improve crop vigor by supplementing nitrogen a long residual period may be safely used in through biological nitrogen fixation (Mhlanga seasons preceding planting of susceptible crops et al. 2015a). Although it has been demonstrated without fear of phytotoxic damage on non-target that cover crops can suppress weeds effectively plants (Muoni et al., 2014). under laboratory and greenhouse conditions, it still remains a challenge to separate the Allelopathic cover crops. Cover crops are allelopathic activity of the residues from other grown for several reasons, including prevention weed suppression mechanisms of mulches in of nitrogen leaching, improvement of soil field and pot trials (Rueda-Ayala et al., 2015). structure, soil enrichment by biological nitrogen The presence of live cover crop biomass or fixation and control of soil borne pests such residues on the soil surface may also reduce weed as nematodes (Kruidhof et al., 2008). By germination by interfering with phytochrome- definition, cover crops are leguminous or mediated germination of photoblastic weed non-leguminous plants that may be grown as seeds (Teasdale and Mohler, 1993). This rotational or relay intercrops during part or all suggests that the efficacy of cover crop mulches of the main crop season (Hartwig and Ammon, in suppressing weeds also depends on the weed 2002, Bezuidenhout et al., 2012). Alternatively, species present in the soil seed bank (Teasdale cover crops can be grown in late summer and Mohler, 1993). after harvest of the main cereal crop until the

874 J.T. RUGARE et al.

The presence of cover crop residues on the soil reduced herbicide dosages with allelopathic surface as mulch lowers soil temperature and cover crop mulches may provide effective results in reduced seed germination (Teasdale weed control and reduce the development of et al., 2007). However, care must be taken to herbicide resistance which occurs when weeds establish the optimum amount of mulch that are continuously exposed to herbicides with is required to optimize weed suppression the same mode of action (Jabran et al., 2010). without adversely affecting crop performance The cover crops currently being promoted in (Wicks et al., 1994). In addition, cover crop Southern Africa include jack bean [Canavalia biomass conserves soil moisture thereby ensiformis (L.) DC] and velvet bean [ affecting weed germination. In drought pruriens (L.) DC var utilis]. Morphological years, when water is in limited supply, the and biochemical characteristics of these two mulch may conserve moisture and supply promising cover crops are discussed below. weed seeds with water thereby promoting weed seed germination (Mashingaidze Jack bean [Canavalia ensiformis (L.) DC]. et al., 2012). Conversely, mulches may Jack bean, a belonging to the result in the retention of excess moisture family, is a climbing perennial commonly which creates waterlogging conditions and grown as an annual and is used as a fodder consequently reduce weed seed germination. crop and green manure cover crop (GMCC) This is due to seed-rot linked to excessive (Hauze and Tran, 2013). The tolerance of moisture experienced during seasons that jack bean to abiotic stresses such as low soil receive a lot of rainfall (Mtambanengwe et moisture and low fertility as well as its pest al., 2015). It is therefore difficult to extract suppression characteristics makes this cover useful generalizations since the moisture crop a promising source of natural pesticides conservation ability of cover crop biomass (Mendes and Rezende, 2014). In addition to to effectively suppress weeds is dependent improving soil fertility through biological on environmental conditions, edaphic factors nitrogen fixation jack bean is also a cover and weed species composition of the soil seed crop of choice in most parts of the world, bank (Mhlanga et al., 2015b). As a result, it because it is relatively free from pests and has is difficult to extrapolate the efficacy of cover nematicidal properties (Santos et al., 2007). crops in suppressing weeds based on results Intercropping jack bean with maize reduced obtained from research done elsewhere in the Pratylenchus zeae by 32% and reduced world. nematode disease severity by 26% (Arim et al., 2006). On the other hand, rotating maize Cover crop residues can also reduce weed seed with jack bean did not adversely affect maize bank size through increased seed predation productivity even when planted at the same because the mulches provide food and nesting time with maize but reduced total weed habitat for herbivorous micro and macro fauna density (Mhlanga et al., 2015a). Conversely, (Yang et al., 2013). Moreover, residues of lettuce (Lactuca sativa L.) and sorghum seed some cover crops may reduce the germination germination was reduced by 66% and 49%, and growth of weeds through allelopathy respectively when they were treated with (Farooq et al., 2011). Cover crops may also 3.3 % m v-1 aqueous extracts of jack bean suppress weeds through the production (Martínez Mera et al., 2016). These results of allelochemicals which suppress both demonstrate the need to establish the most germination and early seedling development effective way of using jack bean to achieve of certain weed species. In addition integrating effective weed control without compromising

875 Biochemical and morphological roles of allelopathic crops in integrated weed management: A review crop productivity. The differences observed period of time (Bayorbor et al., 2006). A plant in the effects of jack bean extracts/residues density of 88 888 plants ha-1 produces more than could probably be due to differences in seed 5 t of biomass (Mhlanga et al., 2016). size of the target species which was reported to impact the influence of allelochemicals on seed Velvet bean [Mucuna pruriens (L.) DC]. Velvet germination (Bezuidenhout et al., 2012). bean, a legume belonging to the Fabaceae family, is commonly grown in Zimbabwe and In another study, Mhlanga et al. (2016) reported other Sub-Saharan African countries either as that the effect of jack bean mulches on weed a cover crop or a green manure (Appiah et al., growth varies with weed species. Previous 2015). It can tolerate drought, plant parasitic studies reported the inhibitory activity of jack nematodes and diseases thereby greatly bean residues and water extracts on reducing the use of synthetic chemicals in crus-galli (L.), P. Beauv, Mimosa pudica L., arable fields (Fujii, 2003; Lawson et al., Cassia spp, Ipomea plebia R.Br and Commelina 2007). It is also cultivated as a fodder crop bengalensis L., suggesting the presence of due to its ability to produce a lot of biomass allelochemicals with a wide spectrum of activity with high digestibility (Soares et al., 2014; in the tissues of this cover crop (Dinardo et al., Appiah et al., 2015). Velvet bean has been 1998; Santos et al., 2007; Mendes and Rezende, reported to contain levodopa, a direct precursor 2014; Poonpaiboonpipattana, 2015). L- of the neurotransmitter that has Canavinine, a non-protein amino acid contained profound aphrodisiac properties (Eucharia and in jack bean inhibited the elongation of the Edward, 2010) and is also used to treat the second leaf sheath of rice seedlings (Nakajima et neurodegenerative disorder, Parkinson disease al., 2001). Several other putative allelochemicals (Pulikkalpura et al., 2015). In cropping systems, have been isolated from jack bean including it is either planted as an intercrop or a rotational phenolics such as ferulic acid (Silva et al., 2016), crop in CA (Mhlanga et al., 2015b). It has been atropine, chlorogenic acid, genistein, naringenin reported that velvet bean can biologically fix and Kaempferol (Santos et al. 2007). Santos more than 100 kg N ha-1 (Lawson et al., 2007) et al. (2010) isolated phytotoxic polyamines rendering it a cover crop of choice in maize-cover from the roots of jack bean. Silva and Rezende crop rotations. Whitbread et al. (2004) reported (2016) reported that post emergence application a yield improvement of 64% when maize was of phenolics extracted from jack bean leaves grown following velvet bean demonstrating its caused chlorosis and necrotic brown spots on ability to improve crop productivity through the leaves of soya bean within 24 hours after increased N supply. Moreover, velvet bean spraying, suggesting that allelochemicals found exhibited significant weed suppression in the in jack bean interfere with the photosynthetic range of 79-90% (Lawson et al., 2006). However, pigment systems of susceptible plants. Jack timing of planting velvet bean in intercrops is bean seed extracts exhibited selective phytotoxic important as it influences whether the crops activity on broadleaved weeds in both transgenic will benefit from improved weed suppression and conventional soya bean at 25 and 50 g L-1 or suffer from interspecies competition. Lawson (Mendes and Rezende, 2014). However, the et al. (2006) reported that maximum benefits selective control of weeds demonstrated in pot in terms of weed suppression and maize yield experiments need to be confirmed under field improvement are obtained when velvet bean is conditions. Jack bean can also smother weeds planted at four weeks after planting maize and due to its ability to establish a lot of ground weeded once after five weeks from the date of cover (more than 80% in 60 days) within a short planting (Lawson et al., 2007). The ability of

876 J.T. RUGARE et al. velvet bean to suppress weeds is attributed effective weed suppression by allelochemicals partly to its ability to produce a dense canopy released by these crops. Allelopathic crops over 60-90 days after planting in the range may also stimulate crop growth and suppress of 5-13 t ha-1 biomass depending on rainfall weeds when used at low concentrations (Buckles, 1995). lethal to weeds but stimulatory to crop It is also widely documented that velvet bean growth. Finally, tank mixing allelopathic can suppress weeds through allelopathy. water extracts and reduced herbicide dosage The allelopathic potential of velvet bean is rates is an economic and environmentally attributed to a non-protein amino acid called benign method of weeds control. L-3, 4-dihydroxyphenylalanine (L-DOPA) (Fujii,, 2003). Velvet bean plant tissues contain ACKNOWLEDGEMENT 4-7% of L-DOPA and can release an estimated Support for this research was made possible 100-450 kg ha-1 of this allelochemical into the through a capacity building competitive soil through leaf deposition and root exudation grant Training the Next Generation (Fujii, 1999, Nishihara et al., 2005). The of Scientists provided by Carnegie amount of L-DOPA that is released by velvet Cooperation of New York through the bean roots into the rhizosphere is sufficient to Regional Universities Forum for Capacity suppress growth of neighbouring plants. The Building in Agriculture (RUFORUM). phytotoxic activity of velvet bean extracts on several weeds has been previously reported STATEMENT OF NO-CONFLICT OF INTEREST by several authors (Caamal-Maldonado et The authors declare that there is no conflict of al., 2001, Adler and Chase, 2007, Runzika interest in this paper. et al., 2013). Discoloration and suppression of radicle growth observed where aqueous REFERENCES extracts of velvet bean were applied on Abbas, T., Nadeem, M.A., Tanveer, A. and susceptible plant species was attributed to Chauhan, B.S. 2017. Can hormesis of the formation of reactive quinones during plant-released phytotoxins be used to the metabolism of L-DOPA (Fujii, 2003, boost and sustain crop production? Crop Appiah et al., 2015). The phytotoxic activity Protection 93: 69-76. of L-DOPA was shown to be selective. Abbas, T., Nadeem, M.A., Tanveer, A., Ali, Plants in the Brassicaceae, Compositae, H.H. and Farooq, N. 2017b. Role of Cucurbitaceae and Hydrophyllaceae families allelopathic crop mulches and reduced are more susceptible to L-DOPA in terms doses of tank-mixed herbicides in of root growth compared to those in the managing herbicide-resistant Phalaris Poaceae, Gramineae and Fabaceae families minor in wheat. Crop Protection 110: 245- (Adler and Chase, 2007, Soares et al., 2014). 250. Adler, J.M. and Chase, A.C. 2007. CONCLUSION Comparison of the allelopathic potential of Weed management strategies including the leguminous summer cover crops: Cowpea, use of allelopathic crops can provide effective, sunnhemp and velvet bean. Hortscience cheap and environmentally friendly weed 42: 289-293. control. Improved crop productivity where Ahmed, R., Uddin, M.B., Khan, M.A.S.A., allelopathic crops are used is attributed to Mukul, S.A. and Hossain, M.K. 2007. effective weed control due to the smothering Allelopathic effects of Lantana camara on effect of live mulches or crop residues and germination and growth behavior of some

877 Biochemical and morphological roles of allelopathic crops in integrated weed management: A review

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