Lantana camara, a member of Verbenaceae L., originated in the West Indies (Sanders, 2001) and was introduced and spread by European explorers to almost all the tropical colonies by 1900 (Howard, 1969). Plants of this species produce attractive flowers, attract numerous species of pollinators [including at least 24 species of butterflies (Goulson and Derwent, 2004; Schemske, 1976)], tolerate harsh environmental conditions (droughts, salts, etc.), and have low maintenance requirements. These attributes make L. camara a popular plant for landscape use (Arnold, 2002; Mugnai et al., 1999; Starman and Lombardini, 2006). Lantana camara is an important floricultural/nursery crop in many parts of the world, especially in the southern United States. However, L. camara has been a major invasive plant species, reportedly being invasive in more than 30 countries (Morton, 1994). It is especially problematic in tropical and subtropical areas around the world where the plant is only limited by cold winters (Sanders, 2006). Lantana camara has been cited as one of the 100 worst weeds in the world (Lowe et al., 2000). In the United States, escaped L. camara has been found in 14 contiguous southern states, from North Carolina to California. Its escape also has been observed in Hawaii, Puerto Rico, and the Virgin Islands (USDA NRCS, 2020). The Florida Exotic Pest Plant Council (FLEPPC) classified L. camara as a Category I invasive species (FLEPPC, 2020). Category I invasive plant species are those that have shown the ability to change the structure or ecology of an environment and/or to cross-pollinate native species.
The ability to produce and disperse seeds is one of the most critical aspects of a species’ survivability. The degree to which a plant is able to accomplish this goal is also one of the main factors determining the invasive potential of a species (Dozier, 1999). Seed production and seed germination have been the primary criteria in evaluating exotic species’ invasive potential (Trueblood et al., 2010; Wilson and Mecca, 2003). In L. camara, seed is borne inside a round, fleshy drupe (berry). Each drupe generally contains one seed and occasionally one additional seed (reviewed by Sharma et al., 2005). The fruit is initially green but turns purple then blue-black as the fruit ripens.
Lantana camara can flower and produce fruit all year round if adequate temperature, moisture, and light are available. Several previous studies examined the fruit production of naturalized L. camara plants or seed densities in the soil seed bank under naturalized plants. Significant intraspecific variation seems to exist. An Australian study showed that each lantana inflorescence could bear approximately eight fruit (Barrows, 1976), whereas in India, as many as 25 to 28 fruit were observed on individual peduncles (inflorescences) (Sharma et al., 2005). An even greater variation has been observed in the density of lantana seed in the soil seed bank. Reported lantana seed density in soil ranged from <5 to 2690 seeds per square meter (Sharma et al., 2005). However, little information is available in the literature regarding the fruit (and seed) production capacity and seed germination of commercial lantana cultivars that are used in the landscape.
Several researchers have attempted to understand the relationships between ploidy level and fruit or seed production in L. camara. Natarajan and Ahuja (1957) suggested that ploidy level would be an influencing factor in fruit production. In their study, diploid L. camara plants had “no seed” to “good” seed production, whereas triploids produced no seed; 30% of tetraploid plants did not produce seeds, while the rest of tetraploid plants had “none” to “good” seed production. Two subsequent studies by Raghavan and Arora (1960) and Khoshoo and Mahal (1967) indicated that triploid plants did produce “good” amounts or at least a few seeds. Spies (1984) collected seeds from all observed ploidy levels in South Africa and found a range of seed production capacity across diploid to pentaploid plants of 0 to 2485 (per plant). These studies indicated that tetraploid and diploid plants were the highest seed producers at 856 (4x) and 565 (2x) seed per plant, respectively. The triploid plants were expected to be sterile but still produced 342 seeds per plant. In these studies, few pentaploid and hexaploid plants were available, and one pentaploid produced 638 seeds on a single plant.
Lantana camara seeds can germinate at any time of the year with sufficient conditions (Gentle and Duggin, 1997). Studies from Australia and India indicated a range of seed germination: 12% in diploids, 28% in triploids, and 56% in tetraploids (Raghavan and Arora, 1960; Spies, 1984). An earlier study (Heit, 1946) investigating the best methods for L. camara seed germination determined the highest average rate of seed germination to be 53% after 40 d, with an individual accession reaching as high as 70% after 60 d. Nevertheless, in their study, only one individual was sampled for each of the three ploidy levels (diploid, triploid, and tetraploid) (Raghavan and Arora, 1960).
In a previous study, we identified five ploidy levels among 32 L. camara cultivars/breeding lines, determined their pollen stainability, and gained a better understanding of the relationship between ploidy level and pollen stainability or male fertility in L. camara (Czarnecki et al., 2014). In an earlier study (Czarnecki and Deng, 2009), we performed ploidy analysis of more than 1500 lantana progeny from self, open, and/or controlled pollinations and observed frequent production of unreduced female gametes (UFGs) in some lantana cultivars/breeding lines. The objectives of this study were to assess female fertility (fruit production and seed germination) in the 32 cultivars/breeding lines and to understand the relationship between female fertility and ploidy level and UFG production in L. camara.
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