Sunn hemp, a multipurpose species used for fiber, fodder, and biomass (Cook and White, 1996), is widely grown in tropical and subtropical agricultural systems for its usefulness as a cover crop and green manure. A cover crop of sunn hemp can provide sufficient dry matter to protect the soil from erosion and add substantial amounts of nitrogen in its residues (up to ≈126 kg·ha−1) for use by the subsequent crop (Mansoer et al., 1997). In addition to these benefits, sunn hemp competes effectively with weeds (Collins et al., 2007, 2008), making it especially useful for weed management in organic and sustainable production of row crops. A nonprotein amino acid delta-hydroxynorleucine was identified in sunn hemp seeds (Pant and Fales, 1974; Pilbeam and Bell, 1979) that was demonstrated to be phytotoxic, inhibiting hypocotyl and radicle growth during germination (Wilson and Bell, 1979). Nonprotein amino acids from other legumes also have been shown to be phytotoxic. These include l-3-(3,4-dihydroxyphenyl)alanine (l-DOPA) from velvet bean [Mucuna pruriens var. utilis (Nishihara et al., 2005)], mimosine and albizziine from Leucaena leucocephala and Albizia species, respectively (Williams and Hoagland, 2007), and m-tyrosine from Festuca species (Bertin et al., 2009). Cole (1991) found that extracts of sunn hemp seeds reduced the growth of several weeds. Aqueous extracts of leaves also showed inhibitory effects on the roots of wheat seedlings [Triticum aestivum (Ohdan et al., 1995)] and extracts from macerated sunn hemp leaves and leachate from whole sunn hemp leaves inhibited maize (Zea mays) germination at 15% and 30% w/v (Cruz-Silva et al., 2015).
Because of the potential to use sunn hemp’s allelopathic properties for weed suppression in horticultural crop production systems, we examined the phytotoxicity of its aqueous foliar extracts and ground, dried residues in an earlier study (Adler and Chase, 2007). Both sunn hemp extracts and ground, dried residues inhibited the germination and growth of livid amaranth (Amaranthus lividus), goosegrass (Eleusine indica), bell pepper (Capsicum annuum), and tomato (Solanum lycopersicum) germination with greater negative effects on livid amaranth than in other species (Adler and Chase, 2007). Skinner et al. (2012) reported that ground, dried sunn hemp residues inhibited germination of lettuce and smooth pigweed (Amaranthus hybridus), and aqueous leaf extracts reduced germination and seedling growth of various crops. Despite this progress, several aspects of allelopathy by sunn hemp are not well understood. Sunn hemp has considerable genetic diversity (Wang et al., 2006), but possible differences in allelopathic potential of different accessions and the nature and properties of the allelochemicals are not known. The objectives of the current study were to test the relative allelopathic potential of leaves, stems, and seeds of a commercially available sunn hemp cultivar and to evaluate the allelopathic potential of aqueous leaf eluates from 14 accessions from eight countries. In addition, we examined whether the phytotoxic compound—delta-hydroxynorleucine—was the major allelochemical in sunn hemp aqueous leaf eluate.
Abdul-Baki, A.A., Bryan, H.H., Zinati, G., Klassen, W., Codallo, M. & Heckert, N. 2001 Biomass yield and flower production in sunn hemp: Effects of cutting the main stem J. Veg. Crop Production 7 83 104
Adler, M.J. & Chase, C.A. 2007 Comparison of the allelopathic potential of leguminous summer cover crops: Cowpea, sunn hemp, and velvetbean HortScience 42 289 293
Bertin, C., Harmon, R., Akaogi, M., Weidenhamer, J.D. & Weston, L.A. 2009 Assessment of the phytotoxic potential of m-tyrosine in laboratory soil bioassays J. Chem. Ecol. 35 1288 1294
Cole, S.D. 1991 Allelopathic effects of Crotalaria juncea, MA thesis, Univ. South Dakota, Vermillion, SD.
Colegate, S.M., Gardner, D.R., Joy, R.J., Betz, J.M. & Panter, K.E. 2012 Dehydropyrrolizidine alkaloids, including monoesters with an unusual esterifying acid, from cultivated Crotalaria juncea (sunn hemp cv. ‘Tropic Sun’) J. Agr. Food Chem. 60 3541 3550
Collins, A.S., Chase, C.A., Stall, W.M. & Hutchinson, C.M. 2008 Optimum densities of three leguminous cover crops for suppression of smooth pigweed (Amaranthus hybridus) Weed Sci. 56 753 761
Collins, A.S., Chase, C.A., Stall, W.M. & Hutchinson, C.M. 2007 Competitiveness of three leguminous cover crops with yellow nutsedge (Cyperus esculentus) and smooth pigweed (Amaranthus hybdridus) Weed Sci. 55 613 618
Cook, C.G. & White, G.A. 1996 Crotalaria juncea: A potential multi-purpose fiber crop, p. 389–394. In: J. Janick (ed.). Progress in new crops. ASHS Press, Arlington, VA.
Cruz-Silva, C.T.A., Matiazzo, E.B., Pacheco, F.P. & Nóbrega, L.H.P. 2015 Allelopathy of Crotalaria juncea L. aqueous extracts on germination and initial development of maize IDESIA 33 1 27 32
Ferguson, J.J., Rathinasabapathi, B. & Gal, M. 2004 A method to screen weed-suppressing allelochemicals in Florida biomass Proc. Florida State Hort. Soc. 117 231 233
Mansoer, Z., Reeves, D.W. & Wood, C.W. 1997 Sustainability of sunn hemp as an alternative late-summer legume cover crop Soil Sci. Soc. Amer. J. 61 246 253
Nishihara, E., Parvez, M.M., Araya, H., Kawashima, S. & Fujii, Y. 2005 L-3-(3,4-Dihydroxyphenyl)alanine (L-DOPA), an allelochemical exuded from velvetbean (Mucuna pruriens) roots Plant Growth Regulat. 45 113 120
Ohdan, H., Diamon, H. & Mimoto, H. 1995 Evaluation of allelopathy in Crotalaria by using a seed pack growth pouch Jpn. J. Crop. Sci. 64 644 649
Pilbeam, D.J. & Bell, E.A. 1979 A reappraisal of the free amino acids in seeds of Crotalaria juncea (Leguminosae) Phytochemistry 18 320 321
Rathinasabapathi, B., Ferguson, J. & Gal, M. 2005 Evaluation of allelopathic potential of wood chips for weed suppression in horticultural production systems HortScience 40 711 713
Skinner, E.M., Diaz-Perez, J.C., Phatak, S.C., Shomberg, H.H. & Vencill, W. 2012 Allelopathic effects of sunnhemp (Crotalaria juncea L.) on germination of vegetables and weeds HortScience 47 138 142
Vranova, V., Rejsek, K., Skene, K.R. & Formanek, P. 2011 Non-protein amino acids: Plant, soil and ecosystem interactions Plant Soil 342 31 48
Wang, M.L., Mosjidis, J.A., Morris, R.E., Dean, T.M. & Pederson, G.A. 2006 Genetic diversity of Crotalaria germplasm assessed through phylogenetic analysis of EST-SSR markers Genome 49 707 715
Williams, R.D. & Hoagland, R.E. 2007 Phytotoxicity of mimosine and albizziine on seed germination and seedling growth of crops and weeds Allelopathy J. 19 423 430