Seed dormancy is an evolutionary adaptation for increasing seedling survival by delaying germination until conditions are favorable for seedling establishment (Fenner and Thompson, 2005). Although seed dormancy is ecologically important, it imposes hardships during agronomic production and restoration. Classification systems define five classes of seed dormancy: physiological dormancy (PD), morphological dormancy, morphophysiological dormancy (MPD), physical dormancy (PY), and combinational dormancy (PY + PD) (Baskin and Baskin, 2004). Within the PD class, three levels can be defined as nondeep, intermediate, and deep, which can be partially elucidated by germination response to stimulatory chemicals (Baskin and Baskin, 2004). In many species, the depth of seed dormancy is controlled by the relationship between levels of gibberellic acid (GA) and abscisic acid (ABA) and is generally referred to as the hormone-balance theory (Wareing and Saunders, 1971). This theory proposes that as dormant seeds reach the optimum ratio of endogenous GA:ABA, germination can proceed. This is the foundation for the widely accepted practice of treating dormant seeds with GA3, a well-established dormancy-breaking chemical (Kirmizi et al., 2010), and GA4+7, which is almost 1000 times more effective than GA3 (Hilhorst, 2011). Germination stimulation by GA is limited to the nondeep and intermediate levels of PD (Baskin and Baskin, 2004). Another method to induce germination involves the use of fluridone, which decreases the levels of endogenous ABA during early stages of imbibition (Grappin et al., 2000). Ethylene, which is also a hormonal treatment, has been recognized to relieve dormancy in seeds. It is produced in trace amounts by almost all higher plants and is involved in the control of growth and development processes that range from germination to senescence (Abeles et al., 2012). Exogenous ethylene stimulates the germination of dormant and nondormant seeds, although in some cases seed germination is unaffected by this hormone (Keçpczyński and Keçpczyńska, 1997). Ethephon, an ethylene-releasing compound, has been found to break dormancy more efficiently compared with ethylene (a gas at room temperatures). Aside from hormonal treatments, reactive oxygen donors such as sodium nitroprusside (SNP) and hydrogen peroxide can enhance germination or relieve dormancy in seeds (Beligni and Lamattina, 2000; Bethke et al., 2011; Sarath and Mitchell, 2008). For species that are fire adapted, such as the native Hawaiian grass species tanglehead (Heteropogon contortus), simulated combustion products liquid smoke (a commercial food-grade flavoring) and cyanide can be effective germination promoters (Baldos, 2013).
Because seeds exhibiting intermediate or deep PD are not generally affected by stimulatory dormancy relief chemicals, it is necessary to understand optimum after-ripening storage conditions to maximize seed germination (Baskin and Baskin, 2004). Ultimate seed survival is directly related to the time the seed has been exposed to unfavorable conditions of temperature or humidity (Barton, 1961). Numerous studies have used sealed chambers to evaluate the factors of temperature and RH to achieve dormancy relief (Baldos et al., 2014; Santhoshkumar and Veena, 2012). When using sealed desiccation chambers, specific levels of RH can be established and maintained using saturated salt solutions (Winston and Bates, 1960; Young, 1967), but proportions of water added to the salt (forming the saturated salt solution) to desiccate a given volume of water (contained in fresh seeds) are unclear.
Torrid panicgrass is a native Hawaiian annual grass that has been identified as a re-vegetation candidate for roadsides and conservation plantings in Hawaii. The intent of this study was to evaluate seed dormancy relief in torrid panicgrass through 1) exposure to the exogenous chemicals cyanide, ethanol, GA3, GA4+7, liquid smoke, hydrogen peroxide, fluridone, ethephon, and sodium nitroprusside; and 2) storage conditions equilibrated to three levels of RH, stored at three temperatures over 10 months. The treatments of desiccation were chosen to resemble the natural ecological exposure of torrid panicgrass in the wild. Before the dormancy relief storage study, a calibration procedure was used to determine the components of the saturated salt lithium chloride (LiCl) solution that could absorb moisture released by seeds in a sealed vessel while lowering and maintaining the target RH level.
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