The genus Pavonia Cavanilles is usually found in tropical and subtropical areas and may be the largest genus in the Malvaceae species (Fryxell, 1999). The genus is mostly distributed in South America, Central America, the West Indies, and Mexico. The flowers of Pavonia are usually monadelphous, with a central staminal column and numerous anthers (Fryxell, 1999). Five petals are inserted at the base of the staminal column and form a gamosepalous calyx (Fryxell, 1999). The unique feature that distinguishes Pavonia from the other seven genera of the tribe of Malvavisceae is that it has two-times more styles and stigmas (10) in the flower than carpels (5) in the fruit (van Heel, 1978). The very young ovary has 10 carpels, but alternate carpels abort during their development. The fruits of Pavonia are composed of five mericarps, with each containing a single seed. In Pavonia sect. Pavonia and P. subsect. Lebretonia, the mericarps are indehiscent and variously ornamented (Fryxell, 1999). The different kinds of ornamentation involve a hard tissue that is protective of the enclosed seed, presumably creating dormant conditions for seeds to survive longer within the mericarp. Only a few are used as ornamental plants, such as P. hastata in Australia (Mitchell, 1982) and P. lasiopetala in central Texas (Miller, 1990; Nokes, 1986; USDA, 2010).
All three species of Pavonia are perennial shrubs with a similar size of ≈1.5 m tall and 1.5 m wide (Fryxell, 1999). Both P. hastata and P. missionum are native to South American countries such as Brazil, and P. hastata has been introduced in Georgia and Florida (Carter et al., 2009; Duncan, 1981; Fryxell, 1999; Wunderlin et al., 2008). P. lasiopetala is native to Texas and northern Mexico and inhabits dry and rocky soils (Hatch et al., 1990). All three species have different flower colors that are attractive to different people. P. hastata has a white flower with a deep red center, P. lasiopetala has a pink-rose flower, and P. missionum has an orange-red flower. Flowers of all three species only last 1 to 2 d, like most species in the Malvaceae species. Blooming periods for the three species are long, from summer to the first frost, which is ideal for ornamental use. Leaf shapes are different; they are hastate in P. hastata and cordate in P. lasiopetala and P. missionum. Pubescence is observed on all three species, with stellate hairs on P. hastata and P. lasiopetala and stellate and simple glandular pubescence on P. missionum (Fryxell, 1999). Unlike species in other families, Pavonia has an epicalyx external to the calyx (Esteves, 2000). All three species produce a hard mericarp with a reticulate-costate shape for P. hastata, smoothly rounded shape for P. lasiopetala, and reticulate-rugose shape for P. missionum (Fryxell, 1999).
Germination is one of the critical steps for plant studies and breeding. Seed dormancy is a mechanism that allows plants to regulate when and where to grow. One crucial function of delayed germination is allowing time for dispersal. Physical dormancy caused by the impermeability of seedcoats to water is common (Baskin and Baskin, 2001). This impermeable layer prevents the seed from taking up water or gases to prevent seeds from germinating until dormancy is broken. In nature, there are several ways to break physical dormancy, including high temperatures, widely fluctuating temperatures, fire, freezing/thawing, drying, and passage through the digestive tracts of animals (Baskin et al., 2000). Fifteen families of angiosperms show physical dormancy, including Anacardiaceae, Bombacaceae, Cannaceae, Cistaceae, Convolvulaceae, Cucurbitaceae, Fabaceae, Geraniaceae, Malvaceae, Musaceae, Nelumbonaceae, Rhamnaceae, Sapindaceae, Sterculiaceae, and Tiliaceae (Baskin and Baskin, 2001).
The fruit of Pavonia is a dry schizocarp that splits into one-seeded mericarps. The hard mericarp of Pavonia protects the seed and is also the cause of dormancy. Some species of Pavonia subgen. Malache have woody mericarp walls as well as indehiscent mericarps (Fryxell, 1999). The seeds of P. candida can be extracted from the mericarp only with the help of a hammer because the mericarp walls are 1 to 2 mm thick and bony (Fryxell, 1999). The physical or bacteriological degradation of such mericarp walls requires a significant amount of time.
Torres et al. (2008) conducted a germination study of Pavonia cymbalaria (Pavonia subgen. Pavonia sect. Lebretonia subsect. Hastifoliae, the same as P. hastata and P. missionum). Their four treatments including removal of mericarp walls, mechanical scarification, nude seeds, and a control. The results indicated that germination only occurred on naked seeds, which implies the existence of a natural dormancy because of the thick mericarp walls. Mechanical scarification via sandpaper was not enough to induce germination. Nokes (2001) mentioned that the germination of P. lasiopetala seeds is delayed or staggered for several months. More uniform germination may be achieved by an overnight hot water soak or acid scarification. All the information indicates that species of Pavonia have physical dormancy, and pretreatments are needed for uniform germination. Few germination studies of Pavonia have been conducted; therefore, there is no standard protocol. However, there are some germination studies involving plants in the Malvaceae species, which would be good references to follow. Sulfuric acid scarification is routinely used to break seed physical dormancy and promote rapid and uniform germination (Sakhanokho, 2009). Sakhanokho (2009) conducted germination studies of Hibiscus (Malvaceae), and both H. acetosella and H. dasycalyx seed germination rates increased significantly after sulfuric acid scarification. Wang et al. (2012) also found that a 15-min sulfuric acid scarification treatment increased the seed germination rate and germination energy of Hibiscus hamabo.
Most dormant seeds have a palisade layer of lignified cells in the seedcoat (Baskin and Baskin, 2001). To understand how sulfuric acid aids seed germination, one approach is the use of scanning electron microscopy (SEM). SEM can achieve resolution of less than 1 nm; therefore, even small changes on seedcoat structures can be observed. Ruter and Ingram (1991) conducted a germination study of the hard-seeded legume Sophora secundiflora. The SEM results showed that acid scarification treatment removed the seed cuticle. By using SEM, researchers can achieve a better understanding of the seed structure and morphology of Pavonia species.
Pavonia hastata was previously known to only be naturalized in Charlton County, GA, and in Citrus and Levy counties in Florida (Jones and Coile, 1988); however, it has now spread to Camden County, GA (Carter et al., 2009), and Harris County, TX (Brown et al., 2007). Small and Rydberg (1913) in their Flora of the Southeastern United States, noted the presence of P. hastata on sandy soils in Georgia. Dr. John Ruter observed both P. hastata and P. missionum reseeding in Tifton, GA (zone 8b) (personal communication, 9 Feb. 2018). Flowers of all three species only last 1 d, but numerous flowers are produced during the growing season. Because each flower can provide five seeds, a high number of seeds can be produced in one season. Plants with reduced fertility often bloom longer during the flowering season (Acquaah, 2009). Breeding for sterile plants can reduce maintenance and labor costs of gardeners, landscapers, and nurseries.
Mutagenesis is a way to obtain new cultivars for sterility and compactness. Mutation breeding generates random variation, resulting in mutant plants with unique morphological traits (Ibrahim et al., 2018; Loewe and Hill, 2010; Schum, 2003). Plant breeding requires genetic variations for segregation and recombination. There are two main types of mutation: physical and chemical. Among the physical mutagens, X-rays and gamma rays are the two most commonly used for plant breeding. During the past 40 years, gamma irradiation was predominant compared with X-rays for mutation induction because of its wide availability and versatility of use. According to the Joint FAO/IAEA Division of the Nuclear Techniques in Agriculture, there are more than 1700 cultivars propagated by seed in 154 plant species developed from direct or indirect mutants (Maluszynski et al., 1995). Mutation breeding can improve quality traits for crops such as plant height, maturity, seed shattering, and disease resistance. According to the FAO/IAEA database, there have been 465 mutants released that are propagated vegetatively, most of which are ornamental plants with a few fruit trees. Among the ornamental plants, Chrysanthemum (187), Alstroemeria (35), Dahlia (34), Streptocarpus (30), Begonia (25), Dianthus (18), and Rhododendron (15) have been studied the most in mutation breeding (Maluszynski et al., 1992). Mutation breeding can be a faster method that is suitable because ornamental plant traits are often visible. Because many mutations are recessive, it is essential to keep the treated material for testing until at least the M2 generation to see if irradiation could enhance the segregation of novel traits.
Research by Li et al. (2010) indicated that irradiation could promote P. hastata seed germination at doses of less than 200 Gy. The putative radiation dosage suitable for mutation breeding was more than 250 Gy for P. hastata. High-dose irradiation can be used to develop desirable morphological traits or induce sterility. Preliminary research by Dr. John Ruter showed a curvilinear response of seed germination to radiation dosages between 0 and 800 Gy (Fig. 1) (unpublished data). Seed germination peaked at ≈350 to 400 Gy, increasing from 36% germination for the control seed to up to 55% germination. Radiation treatments have been shown to stimulate plant growth and development at low dosages (Sax, 1963).
Because seeds germinated at 800 Gy, additional seeds of P. hastata were treated with 1000 Gy, 1500 Gy, and 2000 Gy. Germinated seedlings from all three treatments were transplanted to the University of Georgia Durham Horticulture Farm (UGA Hort Farm) at Watkinsville, GA, and M2 seeds were collected for further evaluation. To determine the influence caused by gamma irradiation and the ideal rate for breeding, germination studies of three species of Pavonia were conducted in 2018 and 2019.
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