A New Hybrid of Asimina (Annonaceae) Based on Morphological and Ecological Data
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CASTANEA 80(4): 262–272. DECEMBER Copyright 2015 Southern Appalachian Botanical Society A New Hybrid of Asimina (Annonaceae) Based on Morphological and Ecological Data Charles N. Horn Biology Program, Newberry College, 2100 College Street, Newberry, South Carolina 29108 ABSTRACT Two species of the genus Asimina (Annonaceae), A. triloba and A. parviflora, are well known to occur in South Carolina. Recent work investigating variation in leaf size, flower morphology, and habitat of Asimina in the state shows that a hybrid between the two species exists. The differences in the three taxa are best related to growth form, peduncle length, and outer petal length and width. This new hybrid, Asimina 3 piedmontana, is hereby described and distinguished from both parents. Key words: Asimina, paw paw, hybrid, parviflora, triloba, piedmontana INTRODUCTION The genus Asimina obovate to oblanceolate, 15–30 cm long; apex (paw paw), of the Annonaceae, was circum- acute to acuminate; base more or less gradually scribed by Kral (1997) to include eight species in attenuate to the short (0.5–1 cm) petiole.’’ Field subtropical and temperate North America. Since measurements of plants in South Carolina agree then, two species have been described, A. with that description, except that leaves were spatulata (Kral) D B. Ward (Ward 2001) and A. observed with lengths up to 45 cm. This species manasota DeLaney (DeLaney 2010), making a can also be recognized by the presence of floral total of 10 known species. Most species are peduncles over 1 cm at anthesis and flowers 2–5 restricted to Florida and Georgia; however, two cm in diameter (Kral 1997). The solitary flowers species are known over a large portion of include sepals 8–12 mm long and outer petals eastern North America, A. triloba (L.) Dunal 15–25 mm long (Kral 1960). and A. parviflora (Michx.) Dunal. Both of these Asimina parviflora (dwarf paw paw) has species are found in a variety of rich woodland been described by Kral (1997) as a miniature sites associated with stream and river valleys. version of the common paw paw with plants up Based on personal observation in the piedmont to 6 m tall; however, plants over 3 m tall are of South Carolina, A. triloba is commonly restricted to the karst country of Florida. Individuals grow on woodland slopes or ridges restricted to river flood plains and stream above streams and rivers where clonal growth bottoms, whereas A. parviflora is more com- has not been personally observed. Asimina monly found on well-drained slopes adjacent to parviflora leaves are ‘‘oblong-obovate to oblan- the bottoms. ceolate, 6–15 (À20) cm long; apex acute to Asimina triloba (common paw paw) grows as acuminate; base more or less gradually attenuate a shrub or tree and can reach a height of 14 m to the short (0.5–1 cm) petiole’’ (Kral 1960, p. (Kral 1997). Plants commonly grow clonally in 244). Measurements have revealed leaves up to populations that can cover several hectares of 27 cm long. Plants can be distinguished, as stream or river floodplains. However, Pomper et stated by Kral (1997), by a peduncle less than 1 al. (2009) demonstrated that a local population cm at anthesis and flowers 1.0–1.7 cm in of stems was not always genetically uniform. In diameter. The flowers include sepals that are his landmark revision of the genus, Kral (1960, p. 4–7 mm long and outer petals 10–13 mm long 242) described A. triloba leaves as ‘‘oblong- (Kral 1960). Received June 16, 2015; Accepted December 30, 2015. Geographically, A. triloba populations have DOI: 10.2179/15-067 been recorded from southern Ontario (Canada) 262 2015 HORN: A NEW HYBRID OF ASIMINA 263 to Georgia and west to eastern Nebraska and petals were found to be clearly narrower than eastern Texas (Kral 1997). Asimina parviflora those of A. triloba. Most striking was that the has a more southern distribution, occurring from outer petals were rolled outwards laterally along southern Virginia to northern Florida and west to their length on the hybrid (Figure 1c, 1d). This eastern Texas. The area of distributional overlap was in comparison to outer petals with the between the two species generally occurs within apical tip curved outward on both A. parviflora the piedmont of North Carolina and westward (Figure 1a, 1b) and A. triloba (Figure 1e, 1f). through South Carolina, Georgia, and Alabama. Vegetatively, the hybrid populations also have a Further overlap occurs within the coastal plain different growth form. The hybrids were ob- of Mississippi and Louisiana (Kral 1997). served to have a clonal growth pattern, with Previous work on the reproductive biology of most stems arching over and being no more than Asimina parviflora (Norman et al. 1992) and A. 1.5 m high. This clonal pattern is like that seen triloba (Willson and Schemske 1980) revealed with A. triloba populations. that both species are protogynous with an Observations of herbarium specimens at the overlap of at least one day between stigma University of South Carolina (USCH) revealed receptivity and pollen production. Norman et al. many specimens without flowers or fruits, (1992) then postulated that, at least for A. leading to question whether leaf features could parviflora, pollination and fruit production be used to separate the two species from the could either be the result of self-compatibility putative hybrid on a consistent basis. Further or out-crossing. Despite the two potential study was needed to clarify the status of the mechanisms of pollination, fruit set is typically hybrid and to determine specific morphological less than 10% in nature for both species (Willson and ecological features associated with interme- and Schemske 1980, Norman et al. 1992). diate plants. Thus, the purpose of this investiga- During the growing season of 2006, unusually tion was to determine if vegetative and flowering short-stemmed clonal populations of Asimina, characters would allow for taxonomic separa- typically no more than 2 m tall, were noticed by tion of the three taxa. Kate Goodrich, at that time a graduate student at MATERIALS AND METHODS Vegeta- the University of South Carolina. Further field tive and flowering material was observed and work by the author revealed additional popula- collected during the growing seasons of 2007–10 tions in the piedmont of South Carolina that fit within the piedmont of South Carolina. At least the above morphological description. It was three populations of each taxon were sampled suggested that the populations might represent for leaf and floral material. Voucher specimens hybrid material between the two well-known of both vegetative and flowering plants were species of the region. A review of literature collected and deposited in the herbarium at revealed that hybridization in the genus Asimina Newberry College (NBYC) and are listed in the is common, especially among the species of Appendix. Because plants flower before leaves Florida (Zimmerman 1941, Kral 1960, DeLaney are fully developed, multiple voucher specimens 2010). Kral (1960), in his treatment of the genus, were collected in most cases. noted that hybridization between Asimina tri- Initial identification and placement of popula- loba and A. parviflora is likely, noting herbarium tions into taxa during field trips was based on a specimens with intermediate flower sizes. He combination of two features, habitat and flower cited a collection from Brunswick County, North morphology. Small plants (about 1 m tall) on Carolina as an example. Later, Kral (1997, p. 16) upper slopes that did not appear clonal and had stated that ‘‘putative hybrids between the two flowers with apically excurved petals less than have been observed in northern Alabama.’’ The 13 mm long were considered A. parviflora. potential hybrid nature of the populations in Clonal populations with individuals over 3 m South Carolina, especially at Lynch’s Woods tall growing on floodplains, and having bell- Park (Newberry County) was recognized by shaped flowers with apically excurved petals Horn (2008). more than 15 mm long were considered A. Field work in April of 2007 and 2008 revealed triloba. Putative hybrid individuals (hereafter putative hybrid populations with distinctly inter- called hybrid) were identified from low-growing mediate-sized flowers. Especially noted were the (less than 2 m) clonal populations having petals intermediate length of sepals and petals. The that projected at right angles from the floral axis. 264 CASTANEA VOL.80 Figure 1. Photographs of Asimina species. Hand of author present for scale. (a–b) Asimina parviflora; both photographed at Lynch’s Woods, Newberry County, South Carolina. (c–d) Asimina 3 piedmontana; (c) photographed at Johnson Creek (Union County, South Carolina), (d) photographed at Broad River (Union County, South Carolina). (e–f) Asimina triloba; (e) photographed at Lynch’s Woods, (f) photographed at Pagett’s Creek (Union County, South Carolina). Leaf measurements were made on plants in used, and on each stem the largest 2–3 leaves the field during June 2009. At each population were measured after realizing that the first- site, 50 leaves were measured for total leaf produced leaves each year are typically smaller length, leaf length to the widest point, and leaf and the terminal leaves were the last produced width. Only branches with multiple leaves were and potentially still developing. Damaged leaves 2015 HORN: A NEW HYBRID OF ASIMINA 265 were not measured if any of the three measure- length population averages ranged from 29.6– ments was compromised. 33.0 cm, length to the widest point ranged from Floral material was collected during April 16.0–20.8 cm, and width ranged from 9.7–12.9 cm 2008 and field preserved in 90% ethanol for later (Figure 3a, 3b, 3c). Populations of hybrids measurement in the laboratory. Only mature, generally had intermediate leaf size.