Plants produce seeds to ensure the greatest establishment and survival success for future generations (Stoehr and El-Kassaby, 2011). One innate mechanism to increase seedling success is seed dormancy. The critical function of dormancy is to prevent germination when conditions are suitable but when the probability of survival and growth of the seedling is low (Fenner and Thompson, 2005). Generally accepted 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).
Uhaloa [Waltheria indica L. (Malvaceae)] is a pantropical shrub species which occurs in diverse populations in the Americas, Mexico, and Brazil (Wagner et al., 1990). Uhaloa in Hawaii is widely classified as a native plant, postulating that the small seeds may have attached to birds that distributed the species to the archipelago (Wester, 1992). Hawaiian uhaloa has been reported in regions receiving an annual rainfall around 500 mm with distribution ranging from sea level to 1220 m, commonly occurring in altered sites where soil disturbance has occurred (Long and Lakela, 1971; St John, 1979). Uhaloa form is generally described with a single strong stem that frequently branches near the ground (Howard, 1988). In Hawaii, segregation of upright and prostrate growth forms have been observed in locally sourced seedling populations. Axillary inflorescences are usually dense glomeruli that contain fragrant, yellow to orange flowers. Each 2-mm capsule holds one small, black, obovoid seed (Howard, 1974).
A single report on seed germination of uhaloa indicated 13% germination on freshly harvested seed after 16 weeks in a moist germination setting (Sánchez and Uranga, 1993). According to Baskin (2003), water-impermeable seeds (physical dormancy) commonly occur in the Malvaceae family, uhaloa is a member of this plant family (Baskin, 2003). In a study conducted by Boyd, it was determined that in Fremontodendron decumbens (also a member of the Malvaceae) 97.8% of seeds were dormant due to an impermeable seedcoat. Breaking of the seedcoat, mechanically or by heat allowed for germination 18–26 times greater than with the nontreated seeds (Boyd and Serafini, 1992). Physical dormancy has been well documented in the native Hawaiian plant Dodonaea viscosa, having similarities to the water-impermeable seedcoat of uhaloa and various physical treatments were tested to break dormancy in seeds of this species (Baskin et al., 2004). One treatment involved mechanically scarifying seeds, after 2 weeks of incubation; seeds that were mechanically scarified had germinated to 96% to 100% in light (Baskin et al., 2004). Nonscarified seeds, on the other hand, germinated to only 0% and 1% in light and darkness, respectively (Baskin et al., 2004). Other methods use to scarified seeds involve exposure to dry heat at a range of 80 to 160 °C, emersion in boiling water and exposing seeds to low RH conditions (Baskin et al., 2004).
Baskin suggests that storage can be used as a dormancy-breaking treatment, but it can also play a fundamental role in optimizing seed longevity. To maximize the usefulness of seeds, it is necessary to understand how possible dormancy relief conditions and long-term storage potential can be achieved through specified storage parameters. Ultimately, seed survival through storage is directly related to the time the seed has been exposed to unfavorable conditions of temperature or humidity (Barton, 1961).
The impact of storage conditions on nonscarified PY dormant Cassia angustifolia seed was determined for four levels of RH (5.5%, 11%, 33%, and 75%), established and maintained using saturated salt solutions in airtight desiccators, and three storage temperatures (5 °C, 20 °C, and ambient). Seed viability for C. angustifolia was optimized at storage temperatures of 5 and 20 °C when maintained at RH levels of 5.5% and 11%, increased levels of RH reduced seed viability over storage time (Santhoshkumar and Veena, 2012). Baldos et al. (2014) characterized storage and after ripening parameters of the native Hawaiian grass Heteropogon contortus. Optimal dormancy relieving conditions for H. contortus required a 28- to 30-d equilibration period in a 12% RH desiccation chamber to bring seed moisture levels to 6% followed by a 30 °C storage temperature for 9 to 12 months (Baldos et al., 2014). Although this study was conducted on a grass species with a permeable seed testa, the factors of RH and temperature over time can be applicable for PY dormant seeds which have been scarified before storage.
The assessment of water imbibition can be used to elucidate physical dormancy in seeds (Rolston, 1978). The objectives of this study were to 1) evaluate physical dormancy breaking mechanisms of uhaloa seeds by evaluating seed germination in response to manual and mechanical scarification and exposure to dry heat and hot water emersion and 2) determine the impact of storage humidity and duration at 5 °C on viability and germination response of nonscarified and scarified uhaloa seeds.
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