Buffalograss [Buchloë dactyloides (Nutt.) Engelm. Syn. Bouteloua dactyloides (Nutt.) Columbus], a dioecious, warm-season grass native to the U.S. plains region, is touted for characteristics like drought tolerance, winterhardiness, and requiring minimal fertility and pest control inputs compared with other turfgrass species (Pozarnsky, 1983; Wenger, 1940; Wenger, 1943). Ornamental native grasses are used for a variety of landscape purposes that include beautification, erosion control, landscape restoration, and turfgrass alternatives. However, researchers, agriculturalists, and others have discovered the challenges in reliable seed germination of warm-season grasses, many of which are also native to the Great Plains region (Baskin and Baskin, 1998).
A buffalograss seed, called a bur, consists of 1–5 caryopses tightly wrapped in a bur coat. When planted without any seed treatment, bur germination rates are <30%, making stand establishment from seed a difficult task. However, nontreated caryopses (removed in a deburring process) will germinate at high rates (>90%), but remain viable for only short periods of time, and significant losses can occur during the time-consuming deburring process. In addition, deburring adds to postharvest seed production costs and is not economically practical for the buffalograss seed producer’s industry. To improve buffalograss establishment and use, seeds are primed, a postharvest hydration seed treatment that is commonly used to increase seed germination (Hilhorst, 1990; Wenger, 1943). Industry professionals typically soak harvested burs in a 0.05 m KNO3 solution followed by a moist chill treatment at 5 °C for 5 weeks, a treatment adapted from Wenger (1941) (R. Fritz, personal communication). The KNO3 treatment adds about $0.50 per pound in seed costs for consumers, but these costs are offset by significant improvement in seed germination when compared with untreated burs. However, little is understood about the physiology of the bur–KNO3 interaction, how priming improves germination in buffalograss, and what factors contribute to seed dormancy in this species.
Seed dormancy occurs when complete germination is blocked in viable seeds grown under favorable conditions (Finch-Savage and Leubner-Metzger, 2006). A dormant seed cannot germinate over a period of time even though advantageous physical and environmental conditions may exist (Bewley, 1997; Hilhorst et al., 2010; Hoang et al., 2013). Over the course of time, seed dormancy has likely evolved to allow germination to occur and seedlings to establish when conditions are suitable.
Unlike economically important crops that lack dormancy from selection and breeding, most native warm-season forage grasses have not been heavily altered by the breeding process, and still possess characteristics like those of wild varieties (Adkins et al., 2002). Seed dormancy has been categorized into five classes: physiological (PD), morphological (MD), morphophysiological (MPD), physical (PY), and combinational (PY + PD). Some classes fall under a coat-imposed dormancy, whereas others result from a problem with the embryo, caused by genetics, immaturity, or a promoter imbalance due to environmental conditions (Adkins et al., 2002; Bewley and Black, 1985; Foley, 1999; Villiers and Wareing, 1964). Physiological dormancy, a seedcoat-imposed type of dormancy, is divided into deep and nondeep dormancy. The difference between deep and nondeep PD is determined by growth response of the excised embryo. If the excised embryo grows abnormally or not at all, the dormancy is categorized as deep; however, if the embryo grows normally, the dormancy is classified as nondeep (Baskin and Baskin, 2004; Finch-Savage and Leubner-Metzger, 2006). In nondeep PD, the seedcoat prevents water uptake, gas exchange, and/or impedes the embryos’ ability to expand (Adkins et al., 2002). In amenity grasses, it is common to find tissue enveloping the embryo, as in the bur of buffalograss, which could inhibit oxygen uptake and carbon dioxide release (Adkins et al., 2002), and thereby contribute to PD. To break PD, several months of warm or cold stratification of seeds or seed-containing structures are required. Nondeep PD, the type of dormancy observed in buffalograss (Finch-Savage and Leubner-Metzger, 2006), can be overcome by GA treatments, scarification, warm or cold stratification, postharvest ripening in cold storage, fire, heat, or smoke, in several species, although such treatments may not effectively overcome dormancy in all seeds. Salt marsh grass, Distichlis spicata, is similarly treated with nitrate to overcome hormonal control of seed dormancy (Amen et al., 1970). Also, for some species, initial promotion of germination rates induced by external treatments may decrease significantly under storage.
The plant hormones ABA and GA play significant and well-known roles in seed dormancy and germination processes (Finch-Savage and Leubner-Metzger, 2006; Hilhorst et al., 2010; Hoang et al., 2013; Rodriguez-Garcia, et al., 2009). Relatively high levels of ABA promote dormancy, and release from dormancy results in the decline of ABA levels and an increase in GA levels, suggesting the ABA/GA ratio influences seed germination (Bewley, 1997; Finch-Savage and Leubner-Metzger, 2006; Villiers and Wareing, 1964). In Arabidopsis, germination promoted by increasing GA content weakens the seed tissue around the radical (Ibarra et al., 2016; Kucera et al., 2005). However, there is no information currently available on plant hormone status in buffalograss burs or caryopses.
Several factors likely contribute to low germination rates in buffalograss. Ahring and Todd (1977) extracted uncharacterized oils from buffalograss burs. Soaking deburred caryopses in the extracted oil-reduced germination by 47%, and furthermore, plant growth was reduced by dipping the root tips in the oils (Ahring and Todd, 1977). However, from these studies it is not clear if the oil is at high-enough concentrations within the bur coat to impede germination. Crocker (1916) suggested that seed structure could inhibit the exchange of oxygen and carbon dioxide or prevent water absorption into the seed, effectively slowing the germination process (Crocker, 1916). The objectives of our study were to compare the effectiveness of KNO3 and water postharvest seed treatments for increased germination, to study how the soaking treatments affected hormone profiles, specifically the ABA/GA ratios in the burs, and to quantify treatment effects on water permeability of the bur. The intent was to gain a deeper physiological understanding of seed dormancy in this important amenity grass.
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