The genus Phlox consists of ≈60 species native to North America (Steffey, 1987; Wherry, 1955). The “Phlox pilosa complex,” a taxonomic grouping considered to be a natural phyletic unit, consists of four species: P. pilosa, P. divaricata, P. amoena, and P. floridana (Levin, 1966; Wherry, 1955). The perennial species Phlox pilosa (also known as Prairie Phlox or Downy Phlox) consists of several subspecies (Levin, 1966). All species in the “Phlox pilosa complex” are self-incompatible, but there is a great deal of compatibility and potential for outcrossing within the complex (Levin, 1966).
Wildflowers are now widely used in residential and commercial landscaping, habitat restoration, and highway beautification projects (Milstein, 2005). Phlox pilosa is potentially useful in these situations. This species is found in all states from North Dakota to Texas and eastward into Florida and New York (Barkley, 1986). Germination of P. pilosa seeds is erratic (Specialty Perennials, 2006), and poor seedling emergence in its natural habitat has been reported (Christiansen, 1967). The inconsistent germination of many perennial species has long discouraged growers from starting plants from seed; however, several companies have recently begun to focus on improving germination of perennial species through seed treatments (Hamrick, 2005).
Although considerable research has been done on seed germination of the annual species P. drummondii (Carpenter et al., 1993a, 1993b, 1995), there is a paucity of scientific literature concerning germination in the perennial Phlox species. Several seed companies have published recommendations for germinating perennial Phlox seeds. For example, Prairie Moon Nursery (2004) suggests cold stratification at 33 to 38 °F (0 to 5 °C) for 2 months to break dormancy in P. pilosa seeds. Specialty Perennials (2006) recommends freezing P. pilosa seed for 2 weeks before sowing, then incubating seed trays in darkness at 70 °F (20 °C) for 3 weeks, followed by 35 °F (2 °C) for 3 weeks before moving trays back to 70 °F (20 °C). Similarly, Jelitto Staudensamen (2004) recommends prestratification of P. pilosa seeds at 18 to 22 °C in moist medium for 2 to 4 weeks, followed by cold stratification at –4 to 4 °C for 4 to 6 weeks, and a period (duration not specified) of moderate temperatures (5 to 12 °C) before exposure to warmer temperatures. Simplification of these procedures would be beneficial to commercial growers and seed producers. The Association of Official Seed Analysts (AOSA) makes recommendations for germinating P. drummondii but no other Phlox species (AOSA,1988).
Germination rate and uniformity of many wildflower species can be improved by cold stratification (Baskin and Baskin, 1998; Bratcher et al., 1993; Wartidiningsih et al., 1994). In their germination study of 91 Wisconsin prairie species, Greene and Curtis (1950) observed that germination of Phlox pilosa seeds increased from 2% to 10% after stratification at 40 °F (6 °C) for 3 months. The majority of prairie species tested by these researchers responded positively to cold stratification.
Release from dormancy by cold stratification is indicative of physiological dormancy, a condition in which the embryo lacks the ability to penetrate the seedcoat (Baskin and Baskin, 2005). Physiological dormancy is found in numerous plant families. Baskin and Baskin (2005) classify seeds in this category as having deep, intermediate, or nondeep physiological dormancy, depending on the temperature requirements for dormancy break, response to gibberellins, and the growth of embryos excised from seeds. The germination of some species with physiological dormancy is enhanced by a period of warm stratification (temperature not specified) before cold stratification (Baskin and Baskin, 2005).
In some species, cold stratification can replace a light requirement for germination (Bewley and Black, 1994). Recent studies have revealed that cold stratification has a direct effect on production of gibberellins (GAs) in seeds of Arabidopsis thaliana (Yamaguchi and Kamiya, 2000, 2002; Yamauchi et al., 2004). Exogenously applied GA3 overcomes seed dormancy in several species (Baskin and Baskin, 1998; Hartmann et al., 1997) and promotes germination in some species that normally require cold stratification, light, or after-ripening (Bewley and Black, 1994). GA promotes the production of enzymes such as endo-β-mannanase, which loosen cell walls in the endosperm, thereby reducing resistance to radicle emergence (Bewley, 1997; Groot and Karssen, 1987: Yamaguchi and Kamiya, 2002).
In addition to its role in plant nutrition, nitrate acts as a signal molecule in several processes in plant development and metabolism (Wang et al., 2003). A solution of 0.2% potassium nitrate has been found to enhance germination of Phlox drummondii (Heit, 1957); however, Springer and Tyrl (1989) observed no significant enhancement of germination in seeds of P. oklahomensis treated with 0.2% KNO3. Exogenously applied nitrate enhanced germination in Arabidopsis (Alboresi et al., 2005; Hilhorst and Karssen, 1988). Batak et al. (2002) found that exogenously applied nitrate reduced the light requirement for germination in seeds of Arabidopsis Landsberg erecta ecotype. The degree of dormancy in Arabidopsis seeds is correlated with nitrate nutrition of the mother plant and with the nitrate content of seeds themselves; the greater the available nitrate, the lower the dormancy (Alboresi et al., 2005). Nitrate accumulation in Arabidopsis seed is also correlated with a decreased requirement for GAs during germination (Alboresi et al., 2005).
Pathogenic fungi, bacteria, and viruses may be found on seedcoats or within seed tissues. Seedborne fungi may cause poor germination and impair seedling development (Halmer, 2000). Certain species of Alternaria can infect seed while it is still on the mother plant and can be responsible for decreased germination (Mycock and Berjak, 1995).
The present study was conducted to determine the effects of cold stratification with prestratification, GA3, potassium nitrate, light, and surface disinfestation on the germination of Phlox pilosa seeds.
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Gray, D. Thomas, T.H. 1982 Seed germination and seedling emergence as influenced by the position of development of the seed on, and chemical applications to, the parent plant 81 110 Khan A.A. Elsevier Biomedical Press New York
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Wang, R. Okamoto, M. Xing, X. Crawford, N.M. 2003 Microarray analysis of the nitrate response inPlant Physiol. Arabidopsisroots and shoots reveals over 1,000 rapidly responding genes and new linkages to glucose, trehalose-6-phosphate, iron, and sulfate metabolism 132 556 567
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