Porterweed (Stachytarpheta spp.) attracts numerous butterfly species and other pollinators and is often planted in pollinator gardens (Gilman, 2014). It is a low-maintenance plant with high drought tolerance and is commonly used in the southern United States in cold hardiness zones 9b through 11 (Lamborn, 2017). Porterweed produces flowers from summer through fall before dying back during winter. The Stachytarpheta genus is vast, with 133 species identified in Australia (Munir, 1992) and 79 species classified in Brazil (Atkins, 2005). Seed is readily available to the public and can be found from many nurseries and online sellers. In the wild, seed is spread through the wind, vehicles, and machinery (The State of Queensland, 2016).
In Florida, the most sold porterweed species are jamaican porterweed (Stachytarpheta jamaicensis), nettleleaf porterweed (Stachytarpheta cayennensis), coral porterweed (Stachytarpheta mutabilis), purple porterweed (Stachytarpheta frantzii), and dwarf red porterweed (Stachytarpheta sanguinea) (http://plantANT.com). Jamaican porterweed is native to dunes, shell middens, pine rocklands, and disturbed sites of central and southern Florida (Wunderlin and Hansen, 2011); whereas nettleleaf porterweed was introduced to the United States from Central and South America and has escaped cultivation (USDA, NRCS, 2020). While it has not yet altered native plant communities in Florida, nettleleaf porterweed is listed as a Category II invasive plant by the Florida Exotic Plant Council (FLEPPC) due to its increased abundance or frequency (FLEPPC, 2019). The University of Florida Institute of Food and Agricultural Sciences’s (UF/IFAS) status assessment of nonnative plants in Florida’s natural areas is to “use with caution” (UF/IFAS Assessment, 2020). Hybridization potential between the native and invasive congeners is of concern. For example, in Florida, the introduced lantana (Lantana camara) has hybridized with the native pineland lantana (Lantana depressa), contaminating the gene pool of the native species (Hammer, 2004; Sanders, 1987). Proper identification and distinction between native and invasive species are crucial components of reducing the spread of invasive ornamentals (Steppe et al., 2019). Although plant experts could observe some differences between jamaican and nettleleaf porterweed (Wunderlin and Hansen, 2011), these two species share a lot of similarities in plant morphology. These similarities make it difficult to identify them reliably before making decisions for invasive plant management, especially for those who are not familiar with porterweed plants.
In prior work, Wilson et al. (2009) evaluated seed production and viability of eight porterweed selections in Florida and found three of them to be highly female sterile. Female sterile porterweed included the following: ‘Violacea’ porterweed (Stachytarpheta mutabilis), ‘Naples Lilac’ porterweed (S. cayennensis × S. mutabilis ‘Violacea’), and ‘Mario Pollsa’ porterweed (Stachytarpheta spp.). Through controlled crosses, the potential for nettleleaf porterweed to hybridize with jamaican porterweed was realized. However, there is a lack of information, such as ploidy level and pollen stainability on these porterweeds, which would be useful data in the breeding of noninvasive plants. Pollen staining has become a reliable method of determining pollen viability in hybridization studies (Czarnecki et al., 2014). Other porterweed studies have described pollen morphology and stainability for S. angustifolia, S. cayennensis, S. chamissonis, S. indica, and S. sericea (Adedeji, 2010; Atkins, 1991; Solanke et al., 2019). These studies revealed diverse morphologies and stainability of pollen grains.
Chromosome number and ploidy level are important plant characteristics; the latter is an important factor in determining plant fertility and hybridization potential. It was previously reported that the porterweed genus has varying numbers of chromosomes, from 2n = 18 to 2n =160, and varying levels of ploidy (Fedorov, 1974; Sanders, 2001). Flow cytometry has become a widely used tool to determine nuclear DNA content and ploidy level of plants due to high sample throughput (Doležel et al., 2007). Polyploids have been identified in porterweed using flow cytometry, but exact ploidy levels need to be determined or confirmed by chromosome counting (Wilson et al., 2009). To accurately interpret the ploidy level based on nuclear DNA content, somatic chromosomes must be determined to provide references for comparing nuclear DNA content (Doležel et al., 2007).
The purpose of this study was to characterize morphological and cytological features of five representative porterweed selections. The main objective was to count their chromosomes in root tip cells, determine nuclear DNA contents by flow cytometrical analysis of leaf tissues, and understand the relationship between ploidy level and nuclear DNA content in porterweed. A secondary objective was to assess pollen morphology and stainability among these porterweed selections. This information, along with complete leaf and flower morphological profiling, will improve identification, promote the use of native porterweed, and control the further spread of invasive porterweed.
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