Plant propagule dormancy ensures that plant growth occurs during optimal environmental conditions. Plants use environmental cues such as shortening photoperiod, colder temperatures, and mild drought to regulate growth habit. Dormancy is generally broken by consistent changes in the environment that lead to favorable growing conditions (i.e., a response to erratic environmental changes could lead to plant death). Dormancy is defined as the suspension of active growth in meristematic tissue (Kalberer et al., 2006). There are three general types of dormancy (Lang, 1987): endodormancy, paradormancy, and ecodormancy. Endodormancy is related to physiological factors within plant organs, paradormancy is related to physiological factors outside the affected plant organs but within the plant, and ecodormancy is related to environmental factors. Helianthus tuberosus L. is thought to be regulated by endodormancy (Kays and Nottingham, 2008). The biochemical mechanisms that affect the tuber dormancy period have been well studied in H. tuberosus and these studies have implicated an increased level of polyamines in actively growing tuber cells relative to dormant tuber cells (Bagni and Serafini-Fracassini, 1985; Kays and Nottingham, 2008; Tassoni et al., 2010).
Breaking dormancy artificially is important for propagation of plants outside of field or wild conditions. The use of cold treatments to break dormancy among different ecotypes of H. tuberosus was studied during the first half of the 20th century (Boswell, 1932; Steinbauer, 1932, 1939; Traub et al., 1929). H. tuberosus tubers quantify exposure to cold, and once the cold period has reached a certain length, the tubers initiate growth (Kays and Nottingham, 2008). The time required to break dormancy for H. tuberosus is dependent on ecotype and ranges between 30 and 200 d (Steinbauer, 1939; Traub et al., 1929). Cold acclimation temperatures between –2 and 5 °C have been recommended, because tubers rot above 5 °C and freeze below –2 °C (Whiteman, 1957).
Various chemical treatments have also been studied with regard to breaking tuber dormancy in both Helianthus and Solanum species. In potato (Solanum tuberosum L.) tubers, ethanol caused apical bud dormancy to be broken more quickly than in untreated controls (Claassens et al., 2005). In H. tuberosus, an ethanol treatment caused apical bud dormancy to be masked (during treatment tuber cell growth occurred), but after the ethanol treatment stopped, cell growth stopped and dormancy was restored (Petel et al., 1993). 2,4-dichlorophenylacetic acid has been shown to break dormancy in cultured H. tuberosus tuber cells (Bennici et al., 1982). In potato tubers, ethylene and gibberellic acid have been implicated in both breaking dormancy and initiating growth after dormancy, but the role of these hormones has not been clearly defined (Coleman, 1998). Cytokinins have been implicated in the maintenance of dormancy in potato tubers (Coleman, 1998). Because these hormones have been implicated in potato tuber dormancy, they may have similar effects on the tubers of H. tuberosus.
H. tuberosus has an important history as a specialty food crop. Initial breeding efforts date back to the 17th century although the first systematic breeding program did not begin until the early 20th century (Kays and Nottingham, 2008). The major goal of these breeding programs has been to increase tuber yield and inulin content. Inulin, the primary storage carbohydrate of H. tuberosus, is proposed as a source of carbohydrate that will help fight the obesity epidemic because inulin is of low caloric value and can be helpful to digestion (Kays and Nottingham, 2008). In addition, H. tuberosus has been used as a resource for disease and insect resistance in Helianthus annuus L. breeding programs (Charlet and Brewer, 1995; Hulke and Wyse, 2008; Miller and Gulya, 1987). Cultivated H. annuus is the world’s second largest hybrid crop in acreage and is the fifth highest yielding oil seed crop (Jan and Seiler, 2007).
Recently, there has been interest in increasing landscape ecosystem services through the use of perennial crops (DeHaan et al., 2005; Hu et al., 2003; Hulke and Wyse, 2008; Sacks et al., 2003; Wang et al., 2009). One of the promising wild species being investigated for creating a perennial sunflower crop is H. tuberosus. Interspecific hybrids between H. annuus and H. tuberosus also show promise as breeding material for a perennial crop. Hybrids have been shown to have potential for both biomass and tuber production (Kays and Nottingham, 2008). In addition, hybrids have substantial variation in seed, agronomic, and tuber traits (Kays and Nottingham, 2008). Despite the large amount of research on tuber dormancy in H. tuberosus, the dormancy requirement has not been characterized for interspecific hybrids. It has been difficult to break dormancy in the interspecific Helianthus tubers, delaying the breeding process with these materials by making it difficult to grow more than one generation per year. The goal of this research was to determine a reliable method to break tuber dormancy in genotypes of wild H. tuberosus and in interspecific hybrids of H. annuus × H. tuberosus to have two full growth cycles per year.
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