Ruellia L. is one of the largest genera in the Acanthaceae and consists of ≈250 species of perennial herbs, subshrubs, and shrubs with mostly tropical and subtropical distribution (Ezcurra, 1993). Five species of Ruellia are native to Florida, and three non-native species are listed as naturalized in the state: R. blechum L., R. ciliatiflora Hook, and R. simplex Wright (Wunderlin and Hansen, 2012). In past years, R. simplex (commonly known as mexican petunia, mexican bluebell, or Britton’s petunia) was previously listed as R. tweediana Griseb. (or R. brittoniana Leonard). This species was introduced to Florida sometime before 1940 (Hupp et al., 2009) and since then has become a very popular landscape plant in the southern United States as a result of its prolific flowering and low-maintenance requirements (Gilman, 1999). Meanwhile, Ruellia coerulea Morong was considered restricted to subtropical South America and was considered closely related to North American species R. malacosperma Greenm. and R. brittoniana (Ezcurra, 1993). More recently, taxonomists have grouped both R. tweediana and R. coerulea under the name R. simplex. This is the name of a species of Ruellia described from Cuba in 1870, and because it is the oldest name recorded, it has taxonomic priority and reduces the latter names to synonyms (Ezcurra and Daniel, 2007).
Chromosome numbers of 2n = 34 for 27 taxa of Ruellia have been reported (Grant, 1955) as well as a few rare counts of n = 16 and n = 18 (Federov, 1969). Furthermore, chromosome numbers in several Ruellia species from Central and North America were all found to be n = 17. Thus, this chromosome number appears to be widespread in this large and variable genus from various parts of the world (Daniel and Chuang, 1993; Daniel et al., 1984, 1990). Ruellia simplex (as R. tweediana) was also reported as 2n = 34 (Piovano and Bernadello, 1991).
Ruellia simplex is found in sunny areas on periodically inundated soils in the south of the United States, Mexico, the Antilles, western Bolivia, southwestern Brazil, Paraguay, Uruguay, and northeastern Argentina (Ezcurra and Daniel, 2007). Ruellia simplex has the ability to grow in a wide range of environmental conditions, from wetlands to almost xeric (Hupp et al., 2009). It produces on average 20.6 seeds per capsule with 98% to 100% germination rate under ideal conditions of 30 °C day and 20 °C night (Wilson and Mecca, 2003). Explosive dehiscence of the seed capsule results in seed dispersal distances from the parent plant of 2.5 to 3 m (Witzum and Schulgasser, 1995). Furthermore, Ruellia seeds turn mucilaginous and adhesive on contact with water and in this way can be dispersed by animals (Ezcurra, 1993). Consequently, R. simplex has naturalized in disturbed uplands and wetlands of seven southern U.S. states (Florida, Georgia, South Carolina, Alabama, Mississippi, Louisiana, and Texas) plus the Virgin Islands, Puerto Rico, and Hawaii (Kartesz, 2012; U.S. Department of Agriculture, Natural Resources Conservation Service, 2012).
In Florida, R. simplex has formed naturalized populations in 28 counties throughout the state (Wunderlin and Hansen, 2012) and is listed in 18 areas reserved for conservation in South Florida (Gann et al., 2008). Since 2001, the Florida Exotic Pest Plant Council (FLEPPC) has considered Mexican petunia as a Category I invasive plant, described as “plants that are altering native plant communities by displacing native species, changing community structures or ecological functions, or hybridizing with natives” (FLEPPC, 2011). The Institute for Food and Agricultural Science (IFAS) Assessment of the Status of Non-Native Plants in Florida's Natural Areas states that Mexican petunia is invasive and not recommended for use in the central and south parts of Florida. It is also not recommended in northern Florida unless its specified use is approved (IFAS Invasive Plant Working Group, 2011).
Invasive species damage natural areas, alter ecosystem processes, displace native species, hybridize with natives, and/or support other potentially damaging plants, animals, and pathogens (Randall and Marinelli, 1996). Destruction of native ecosystems by invasive plants is an issue of worldwide concern with consensus that early eradication is critical. In the United States, an estimated 5000 plant species have escaped cultivation and are now established in natural ecosystems. The cost associated with invasive plant damage and control is estimated at nearly $35 billion a year and plant invasions are increasing at a rate of 10% annually (Pimentel et al., 2005). Conventional and new biotechnologies such as use of wide hybridization, selection and breeding for double flowers, development of triploid plants, and use of induced mutations can be used to develop new, non-invasive cultivars (Ranney, 2004). New approaches include using genetic engineering to develop “supersterile” cultivars that are both male- and female-sterile (Li et al., 2004).
Currently there are tall (‘Purple Showers’, ‘Chi Chi’, and ‘Snow White’) and dwarf (‘Katie’ and ‘Southern Star’ series) cultivars available of R. simplex in purple, pink, and white flower colors. These cultivars are propagated clonally to maintain their identity with the exception of the ‘Southern Star’ plants, which are propagated by seed (PanAmerican Seed Co., Chicago IL). All known cultivars are fertile, set fruit and produce viable seed, and are potentially invasive with the exception of ‘Purple Showers’, which does not set fruit by open pollination (Wilson and Mecca, 2003). A survey conducted in 2002 including 946 active nurseries in Florida indicated that 15.9% of them grew or sold R. simplex, and the total reported annual sales for this plant was estimated at ≈$12 million (Wirth et al., 2004). Sales of ‘Purple Showers’ in Florida were ranked third for herbaceous perennials after pentas and lantana (Ornamental Outlook, 2009).
New, sterile Ruellia cultivars with novel flower colors and growth habits have great potential for landscape use in Florida and other U.S. southern states as an alternative to the invasive wild R. simplex and fertile cultivars. Since 2007, we have developed at the University of Florida/IFAS in Gainesville the first Ruellia breeding program using a combination of polyploidization and hybridization techniques. The objective of this research is directed specifically toward creating fruitless cultivars for the landscape plant industry. Plants were evaluated in multilocation trials in Florida and selections were made for potential new cultivar releases.
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