Seed dormancy is an inherent adaptation in plants that delays the germination of newly dispersed seed (Tarasoff et al., 2007) allowing germination to be temporally staggered. In restoration and re-vegetation with native groundcovers, seed dormancy can both be an advantage and a disadvantage. Although seed dormancy can help in long-term proliferation and persistence of plantings, it can be a hindrance during planting and establishment. Direct seeding of dormant or partially dormant seed batches can delay establishment and canopy fill-in thereby making the site more prone to erosion and weed invasion.
In Hawaii, there is growing interest in the use of native species for restoration, roadside re-vegetation/erosion control, and urban landscaping. A number of native groundcovers have been identified and evaluated as potential re-vegetation species. However, there is a dearth of knowledge in seed production and germination biology of these plants, which hinders their use in large-scale re-vegetation projects. Tanglehead (Heteropogon contortus) is one of several native Hawaiian species used for re-vegetation that requires further study in these areas to facilitate its use in large-scale plantings.
Tanglehead is a drought-tolerant, perennial C4 bunchgrass that grows 0.4 to 1.0 m high (Wagner et al., 1999). It is a widely distributed grass species throughout the dry tropical and subtropical grasslands of the world (Carino, 1999; Carino and Daehler, 1999; Tothill, 1968). In Hawaii, it is found on all main islands, usually on dry rocky cliffs, ledges, or slopes near the ocean and from elevations ranging from sea level to 700 m (Wagner et al., 1999). As a result of its adaptability to low rainfall, low fertility soils (USDA-NRCS, 2007), and inherent cultural and ecological value (Daehler and Goergen, 2005), tanglehead is considered an important re-vegetation and restoration species in Hawaii.
A major constraint on the use of tanglehead in restoration and re-vegetation is its seed dormancy. Freshly harvested seeds do not germinate or have very low (less than 10%) germination. To relieve tanglehead seed dormancy, seeds can be scarified, treated with germination stimulants, or dry after-ripened. Scarification has been reported to relieve dormancy of tanglehead seeds. Studies conducted by the Kew Royal Botanic Gardens (2014) has shown high percent germination (77% to 100% germination) by chipping or partial removal of the covering structure. In contrast, scarification studies conducted by Baldos (2013) reported marginal to moderate dormancy relief. Removal of the husk (i.e., naked caryopsis) resulted in low and inconsistent germination (1.0% to 16.5% germination), whereas nicking to expose the embryo resulted in partial dormancy release (19.5% to 30.0% germination).
Aside from seed scarification, the application of germination stimulants such as gibberellic acid, aerosol smoke, food-grade liquid smoke, and smoke-infused water has been shown to relieve tanglehead seed dormancy. The application of a 1% solution of gibberellic acid can result in partial (Baldos et al., 2011) to complete dormancy release (Tothill, 1977). Application of cool aerosol smoke improved tanglehead seed germination by three to four times that of the untreated control (Campbell, 1995; Campbell et al., 1996). One-month-old seeds treated with a 1% v/v dilution of food-grade liquid smoke exhibited the highest percent germination (40%) in contrast with the control (0.5% germination) and the 1% gibberellic acid-treated seeds (20% germination) (Baldos et al., 2011). Water infused with smoke from burning tanglehead plant material resulted in 40% to 60% germination, whereas untreated seeds exhibited less than 5% germination (Baldos, 2013).
Dry storage (i.e., dry after-ripening) is a very effective means of breaking tanglehead seed dormancy. Seeds that have undergone a dry after-ripening period exhibit faster germination rates (Tothill, 1977) and high percent germination (80% to 90%) (Daehler and Goergen, 2005; Tothill, 1977). To break seed dormancy, tanglehead seeds must be kept in dry storage (at room temperature) for 6 to 12 months (Daehler and Goergen, 2005; Pater, 1993; Tothill, 1977; USDA-NRCS, 2007). Although dry after-ripening after drying is known to remove seed dormancy and subsequently improve germination, storage conditions to optimize this process for tanglehead seeds have yet to be determined.
Temperature and relative humidity are important storage conditions that affect the rate of seed dormancy loss as well as seed viability loss in a number of species (Baskin and Baskin, 1979; Bazin et al., 2011; Commander et al., 2009; Foley, 1994, 2008; Leopold et al., 1988; Probert, 2000; Steadman et al., 2003). Temperature is a well-known environmental factor that affects the rate of dry after-ripening (Iglesias-Fernandez et al., 2011). The application of heat during the dry after-ripening period typically accelerates dormancy loss, whereas dry storage at low temperatures tends to slow down or inhibit dormancy loss (Bannon et al., 1978; Cohn and Hughes, 1981; Reddy et al., 1985; Roberts, 1988; Steadman et al., 2003).
Relative humidity also plays a role in both the loss of seed dormancy and seed viability through seed moisture content. Together with temperature, seed moisture content influences dry after-ripening rates (Foley, 2008; Iglesias-Fernandez et al., 2011; Probert, 2000). Studies conducted on a number of species indicate that a specific range of seed moisture content is required for accelerated dry after-ripening at high temperatures (Baskin and Baskin, 1979; Bazin et al., 2011; Commander et al., 2009; Leopold et al., 1988; Steadman et al., 2003). Above or below this species-specific critical moisture threshold value, dormancy loss from dry after-ripening can be inhibited or delayed (Bewley and Black, 1994; Finch-Savage and Leubner-Metzger, 2006; Iglesias-Fernandez et al., 2011). Aside from seed dormancy loss, both temperature and seed moisture content can also impact the viability of dry-stored seeds. Storage at low temperatures and low seed moisture content generally maintains seed viability (Copeland and McDonald, 2001). In contrast, storage at high temperatures and high seed moisture content results in decreased seed viability as a result of accelerated aging (Copeland and McDonald, 2001). The objective of this study was to evaluate the effects of storage eRH, storage temperature, incubation period, and time of year (i.e., season) harvest on loss of seed dormancy and seed viability of tanglehead.
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