Flora of North America species comparison

Species 16 (15 in the flora).

Amorpha is notable among papilionoid legumes for having a non-papilionaceous corolla consisting solely of a banner petal, although deviation from the normal papilionoid floral form is common throughout Amorpheae. Even though recognition of the genus has never been in question, due to its easily distinguished floral characters, delimitation of species within the genus has long caused consternation among taxonomists. The range and intergradation of morphological variation have resulted in a prodigious list of recognized species, varieties, and forms, most often associated with the A. fruticosa complex. Early taxonomic treatments failed to result in a satisfactory circumscription of the genus (C. K. Schneider 1907; P. A. Rydberg 1919–1920; E. J. Palmer 1931), but the insightful and thorough work of R. L. Wilbur (1975), closely followed by D. Isely (1998) and here, was a marked improvement.

The “amorphoid” clade of Amorpheae, to which Amorpha belongs, has been strongly supported as monophyletic in molecular analyses (M. McMahon and L. Hufford 2004), although most relationships among genera in the clade remain unclear. Analyses of chloroplast, ribosomal DNA, and low-copy nuclear gene sequence data indicated that Amorpha may not be monophyletic because either Errazurizia rotundata or Parryella filifolia, or both, are nested within it (McMahon and Hufford 2004, 2005; McMahon 2005). Some of these analyses have indicated that E. rotundata and P. filifolia are sister species (combined analyses of chloroplast trnK, matK, ITS/5.8S rDNA, nuclear CNGC4), which may or may not be nested in Amorpha; other analyses indicated that E. rotundata is nested among Amorpha species while P. filifolia may or may not be (trnK, matK analyzed alone, CNGC4 analyzed alone). These conflicting outcomes leave the relationship of these two species to each other and to Amorpha unclear. Preliminary analyses of additional nuclear gene data sets have also indicated that E. rotundata and P. filifolia are nested in Amorpha but have not clarified whether or not they are sister species (S. C. K. Straub and J. J. Doyle 2014). Morphological evidence also supports the close association of E. rotundata (originally described as P. rotundata Wooton), P. filifolia, and Amorpha. Parryella filifolia lacks a corolla, and the corolla of E. rotundata is either absent or consists of a single petal, which suggests a closer association in terms of floral evolution to Amorpha than to the other 5-petaled genera of Amorpheae, an observation supported by developmental studies (McMahon and Hufford 2005). W. F. Mahler (1965) hypothesized a close relationship of Amorpha, E. rotundata, and P. filifolia based on shared pollen characteristics.

The phylogenetic relationships among Amorpha species remain unclear due to lack of molecular variation in non-coding chloroplast, nuclear rDNA spacer regions, and nuclear gene intron sequence data. Preliminary results from phylogenetic analyses, including DNA sequence data from most species of the genus, indicate that A. californica is the earliest diverging species in the genus and that A. georgiana and A. nana are closely related; however, the relationships among the other species of the genus are thus far unresolved (S. C. K. Straub and J. J. Doyle 2014). The lack of molecular variation may be indicative of a rapid radiation in the genus or of continued gene flow and partial genetic homogenization through hybridization and introgression, possibly mediated by widespread species. Ongoing genetic work has suggested that polyploidy is more common in Amorpha than previously recognized. Amorpha fruticosa has long been known to be a tetraploid, and new information from nuclear gene DNA sequences and microsatellites now indicates that A. confusa, A. crenulata, and A. roemeriana may also be tetraploids, although this is yet to be confirmed by chromosome counts (Straub et al. 2009; Straub and Doyle). Clearly, further work is needed to resolve species relationships within Amorpha, to update species concepts within the genus, and to determine whether or not Amorpha is monophyletic as circumscribed or if Errazurizia rotundata and P. filifolia should be added to Amorpha.

Amorpha has been a group of interest for many years in the search for biologically active compounds. In the area of medicinal biochemistry, anti-tumor and anti-inflammatory compounds have been isolated from A. fruticosa (L. Li et al. 1993; J. Y. Cho et al. 2000). Antimicrobial agents have been identified in both A. fruticosa and A. nana (L. A. Mitscher et al. 1981, 1985). Amorpha fruticosa has also been investigated for its insecticidal and insect repellant properties (R. C. Roark 1947). The glands on the fruits of this species have been shown to contain compounds that poison numerous types of insects through ingestion or contact (for example, chinch bug, cotton aphid, pea aphid, spotted cucumber beetle, mosquito larvae) and also insect repellant properties (for example, striped cucumber beetle, flour beetles, dog fleas, and houseflies; C. H. Brett 1946, 1946b).

Native Americans of the Great Plains employed several of the more common Amorpha species for a variety of uses. Amorpha fruticosa was used for bedding, horse feed, and arrow shafts, and stems were arranged on the ground to create a clean place to put butchered meat (M. R. Gilmore 1919; P. A. Vestal and R. E. Schultes 1939; D. J. Rogers 1980). Amorpha canescens was used to treat stomach pain, intestinal worms, eczema, neuralgia, and rheumatism, and powdered leaves were applied to wounds (W. J. Hoffman 1891; Gilmore 1909, 1919; Hu. H. Smith 1928). The leaves were also mixed with buffalo fat and smoked or used to make tea (Gilmore 1919). The dried leaves of A. nana were used to treat catarrh (F. H. Elmore 1944).

A few Amorpha species are found regularly in cultivation, the most common being A. fruticosa and A. canescens, while A. californica, A. herbacea, and A. nana are less commonly part of the horticultural trade. Amorpha fruticosa has also been used in the United States and abroad for soil stabilization, erosion control, and in windbreaks, and has been investigated as a potential forage and biomass production crop (S. Karrenberg et al. 2003; L. R. DeHaan et al. 2006; K. Török et al. 2003). Cultivation of A. fruticosa has led to its escape and naturalization in many parts of Europe and Asia.

Amorpha apiculata Wiggins is known from Baja California, Mexico.

(Discussion copyrighted by Flora of North America; reprinted with permission.)