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Panicum repens (Torpedo Grass) in Mississippi.

TAXON CHARACTERISTICS AND CIRCUMSCRIPTION

Panicum repens L. (Torpedo Grass) is a highly rhizomatous, perennial grass that forms dense mats (Fig. 1a) mostly through vegetative reproduction, via deep penetrating rhizomes (Fig. 1c). According to some researchers, this species never produces viable seed and the main mode of reproduction of the species is asexual by rhizomes (Wilcut et al. 1988). However, Martinez et al. (1992) found that fluctuating temperatures and salinity strongly encourage the germination of P. repens. They also found that varying depths of seed burial had no major effect on germination rates, which may be important for a species commonly found in coastal areas frequently disturbed by meteorological events. Tarver (1979) states that torpedo grass was planted by the USDA via seeds, so obviously at least some seeds of this species are viable (see below, polyploidy).

Leaves of P. repens are dark green to greenishgrey (sometimes almost glaucous), which often make colonies of the species very conspicuous growing among associate vegetation. The leaves typically are distichous and may be 3-25 cm long and 2-8 mm wide with a basally pilose adaxial surface and a glabrous to pubescent abaxial surface (Freckmann & Lelong 2003). The upper sheathing leaf bases usually are densely hirsute or pilose (Fig. 1b). Culms are erect to geniculate and range from 20-90 cm tall (Freckmann & Lelong 2003).

Inflorescences of P. repens are diffuse panicles from 3-24 cm long with spikelets ranging from 2.22.8 mm long (Freckmann & Lelong 2003) and often may be nearly glaucous in some specimens, or purplish in those specimens which have been referred to as P. gouinii E. Fourn. (Freckmann & Lelong 2003).

Panicum repens was formerly placed in the large (~ 450-500 spp.), and highly polyphyletic genus Panicum (sensu lato) and included in the subgenus Panicum (Zuloaga et al. 1993, Aliscioni et al 2003) of the subfamily Panicoideae in the family Poaceae. It currently is included within the section Dichotomiflora in the monophyletically circumscribed genus Panicum (sensu stricto), which contains around 100 species. Species in this genus are characterized by the C4 photosynthetic pathway and having a base chromosome number of x = 9 (Aliscioni et al. 2003). Panicum repens has been reported as diploid (Ashan et al. 1994) and also as forming a polyploid complex having tetraploid, hexaploid, and aneuploid cytotypes (Freckmann & Lelong 2003). Freckmann & Lelong (2003) report that another taxon, P. gouinii, is not recognizable on the basis of morphology and that intermediates are often found between the two species. They consider P. gouinii to be conspecific with P. repens. However, on the basis of the chloroplast gene ndhF, Aliscioni et al. (2003) found that the two taxa were molecularly divergent, although there was little support for this separation (e.g., Fig. 2). Some specimens from Mississippi have been identified as P. gouinii, but more work would be necessary to determine whether or not the taxon should actually be recognized as a good species.

ORIGIN AND DISTRIBUTION

Panicum repens is a cosmopolitan species found in many countries throughout the world (Freckmann & Lelong 2003). It has been reported as native to the Americas (Liu et al. 2006), although there is some debate over its place of origin. Freckmann and Lelong (2003) suggest that it may have been introduced into the Americas. Guglieri et al. (2004) consider the species native to the United States, Argentina, Belize, the Caribbean, and throughout Brazil, although they consider it as an invasive species of gardens and other cultivated areas. Schmitz et al. (1988) report that P. repens is a native of Europe. So obviously there is some confusion over the true origin of the species. Waterhouse (1994) reports the species from North Africa and the Mediterranean region and Hoyer et al. (1996) report it from Australia. It is a common weed in parts of Asia as well (Hossain et al. 2001, Liu et al. 2006).

Panicum repens can withstand relatively dry, sandy soils or "mucky" soils under flooded conditions (Sutton 1996). It is quite common in the southeastern United States being found from North Carolina to Texas (USDA, NRCS 2009) in a variety of habitat types but most frequently in coastal areas (Freckmann & Lelong 2003, Majure pers. observ.). Torpedo grass was planted throughout the southern states in the 1920s by the United States Department of Agriculture for use as a forage grass (Schmitz et al. 1988). It also is found in California, Utah (Freckmann & Lelong 2003) and Hawaii according to The Plants Database (USDA, NRCS 2009).

This species has been collected in Mississippi for quite some time. Records for this species exist at the Mississippi State University Herbarium (MISSA) from Deer Island, one of the barrier islands off of the Mississippi coast, from as early as 1891 collected by A. B. Seymour and 1894 by S. M. Tracy. Lloyd and Tracy (1901) recorded this species as a main component of the sand plain formation of the insular flora of Mississippi and Louisiana. Penfound and O'Neill (1934) and Pessin and Burleigh (1941) recorded Panicum repens from the Mississippi gulf islands Cat and Horn, respectively, also as a main component of the island vegetation.

Until recently torpedo grass was considered to be mostly restricted to coastal areas within the state of Mississippi. Panicum repens previously was only known from five southern counties (Forrest, Hancock, Harrison, Jackson, and Lamar; Freckmann & Lelong 2003), including records from state herbaria (MISS, MISSA, MMNS, USMS). However, more recent collections of this species from Lauderdale and Newton counties have increased its distribution to east-central Mississippi. Panicum repens found in Lauderdale County apparently originated from sod brought in from south Alabama (C.T. Bryson pers. comm.). It also was collected from Pearl River, Perry, Greene, and George counties, where it had not been recorded in recent treatments (Appendix 1). Panicum repens has been observed or suspected, but not collected, in other counties (Jeff Davis Co., Simpson Co.; H. Sullivan, Mississippi Museum of Natural Science, pers. comm.; Covington Co., Jones Co., Lee Co., Madison Co., Stone Co., C.T. Bryson, USDA-ARS, Southern Weed Science Laboratory, pers. comm.; Clarke Co., Wayne Co., V. Maddox, Mississippi State University, pers. comm.; Hinds Co., Rankin Co., W. Wells, Mississippi State University, pers. comm.) (Fig. 3). Considering the current distribution in the state, this species should also be found in several of the southwestern and south-central Mississippi counties that border southeastern Louisiana counties where the species has been recorded (Freckmann & Lelong (2003).

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POLYPLOIDY AND INVASIVE POTENTIAL

It is widely accepted that polyploidy (genome duplication) in plants is a major evolutionary force (Tate et al. 2005, and refs. therein). Polyploidy has had an enormous influence on the evolution of grasses, and it is estimated that all grasses are polyploids (paleo or neopolyploids; Levy & Feldman 2002). Polyploidy via hybridization (allopolyploidy) is often associated with the incurrence of an increased adaptive ability in progeny (i.e., hybrid vigor). Stebbins (1940) related this to the inclusion of the combined physiological traits of the putative parental taxa within the subsequent allopolyploid. A great example of this is the relatively recent speciation of the Chloridoid grass Spartina anglica C.E. Hubb., an allopolyploid derivative of the two hexaploid parental taxa (reviewed in Ainouche et al. 2003), the introduced S. alterniflora Loiseleur from the eastern United States and the European native, S. maritima (Curtis) Fernald (Ayres & Strong 2001). This species has become highly invasive in coastal marshes that it inhabits, subsequently occupying a larger geographical area than its parental progenitors (Baumel et al. 2002, Ainouche et al. 2003).

Polyploidy via genome duplication within one species (autopolyploidy) has typically been considered of little importance evolutionarily, although recent work suggests that plant systematists may be overlooking many species which have arisen through this process (Judd et al. 2007, Soltis et al. 2007). Polyploids in general can be considered to have an advantage over their diploid progenitors for several reasons. Polyploidy can lead to asexual reproduction and self-compatibility, so newly formed polyploids may be able to maintain viable populations without having a large group of mates to choose from. Polyploidy via hybridization can lead to heterosis, or hybrid vigor as mentioned above. Also the result of having more than one copy of most genes within a polyploid, or gene redundancy, provides a buffer system against deleterious gene mutations (Comai 2005). Being a polyploid species in some instances, Panicum repens may be more adaptable than taxa with which it is in direct competition. In this manner polyploidy may confer a higher degree of "weediness" in torpedo grass than for other species. As well though, some aneuploid cytotypes of P. repens may be sterile, and so accounts of nearly complete asexual reproduction of the species and non-viable seeds may thus be explained. Hence, polyploidy is not necessarily always advantageous.

Panicum repens (and P. gouinii) is listed on the website, A Global Compendium of Weeds (2009), so is considered by some authors to be a weedy species. Tarver (1979) considered this species a serious weed. In fact, control of this species has been tested using different herbicides in turf management (Brecke et al. 2001) and in natural areas, such as Lake Okeechobee, FL, where P. repens has invaded thousands of hectares of lakeshore and adjacent areas (Hanlon & Langeland 2000). Hanlon and Langeland (2000) also state that the invasion by P. repens has had deleterious effect on the sports fish habitat by reducing inhabitable space and dissolved oxygen for those species. Gordon (1998) lists this species as a Category 1 (on the most invasive plant species list) in Florida. Schmitz et al. (1988) state that torpedo grass is especially problematic in flood control canals. It has also been reported as impeding radio-telemetry work for tracking fish in southern Mississippi (M. Dugo pers. comm.), where P. repens forms large mats extending into the water. So there are obvious economic repercussions to large invasions by torpedo grass. Cuda et al. (2007) consider economic losses to be very large. The estimated cost of control annually in Florida in 1990 was $2 million (Schardt & Schmitz 1991). On the contrary, torpedo grass was found by Cuda et al. (2007) to contain a large number of associate insects (e.g., 12 orders, 37 families, and 54 genera) at Lake Okeechobee, although some of these insects are considered pests (e.g., Blissus insularis Barber). Panicum repens is also known for stabilizing edges of water bodies, which could either alter natural hydrological features of an area (Smith et al. 1992) or help prevent erosion.

DISCUSSION

It would be interesting to know whether or not polyploid cytotypes of P. repens represent auto or allopolyploids. If allopolyploidy were found, then the putative parental taxa could be determined and the evolutionary origin of polyploid P. repens could be established. Considering hybridization could be a factor and the fact that in phylogenetic analyses the morphologically similar P. gouinii and P. repens are not resolved as sister taxa (Alisconi et al. 2003; e.g., Fig. 2), this may actually be evidence for a hybrid origin for either P. repens or P. gouinii. Testing this hypothesis would require a biparentally inherited marker, such as from the nuclear genome (see Fig. 2). In any case, a close inspection of life history traits and ploidy in P. repens over a wide geographical range could reveal correlations among cytotypes and self compatibility, vegetative reproduction, and morphological features.

Considering the invasive potential of torpedo grass (Bodle & Hanlon 2001) and its status as a worldwide weed (Holm et al. 1977), this species probably should be considered as a weed to watch in MS. C.T. Bryson (pers. comm.) also considers this species as one that may need to be tracked as a potential weed in the state. It apparently is being spread by contaminated sod and is frequently found in disturbed areas (e.g., road construction, building construction areas, etc.), so anthropogenic actions are most likely the main cause for the increasing distribution of the species within the interior of the state. However, considering that sexual reproduction has been documented in P. repens (Martinez et al. 1992), seeds could be spread by any of numerous ways (e.g., hydrochory, zoochory, etc.) further increasing the distribution of the species in Mississippi.

APPENDIX 1. New records of P. repens in Mississippi.

Panicum repens L. George Co. in front of Salem Volunteer Fire Department, off of Hwy. 57 S, S of Benndale; common in lawn and ditch; L.C. Majure 2598 w/ T. Majure (MISSA, MMNS); Greene Co. off of Hwy. 57 S along small creek near junction with Thelma Road, just south of McLain past railroad tracks; 31.08494 [degrees]N, 88.81259 [degrees]W; L.C. Majure 2597 w/ T. Majure (MISSA, MMNS); Lauderdale Co. Meridian, Bonita Lakes Recreation Area off of Hwy. 19 S of I-20; 32.35934[degrees]N, 88.65360[degrees]W; common along roadside and bank of Bonita Lake; L.C. Majure 1200 (MISSA, MMNS). Newton Co. off of Hwy. 80 W; ca. 2 km W of the town of Chunky; 32.32627 [degrees]N, 88.93763 [degrees]W abundant along roadside in ditch and adjacent areas; L.C. Majure 2474 (MISSA, MMNS). Perry Co. off of Hwy. 15 S, ca. 0.5 km north of the town of Richton; 31.36247[degrees]N, 88.93543 [degrees]W; abundant adjacent to small stream. L. C. Majure 2596 w/ T. Majure (MISSA, MMNS). Pearl River Co. Picayune, 0.1 mi. W jct. of Hwy I-59 and MS 43 (N jct.), N of Hwy MS 43 between I-59 and Cooper Street, open area on sandy to silty loam soil, 16 Aug 1994, C.T. Bryson 14249 (SWSL).

ACKNOWLEDGMENTS

I wish to thank M. Alford, C.T. Bryson, V. Maddox, H. Sullivan, and W. Wells for information regarding herbarium collections and observations of P. repens in MS.

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Lucas C. Majure

University of Florida, Florida Museum of Natural History, P.O. Box 117800

Gainesville, FL 32611

Corresponding Author: Lucas C. Majure lmajure@ufl.edu
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