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.
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