Cranberry (Vaccinium macrocarpon Ait.) is a low-growing, woody, perennial vine native to eastern North America that grows in moist acidic soils with variable organic matter content. In New Jersey, large-fruited cranberry is a common native species in wet, sandy, peaty bogs, and along the edges of streams in the Pine Barrens (Beckwith, 1922). Cranberry distribution ranges from Newfoundland to Minnesota in the west and Delaware in the south, with populations also present at higher elevations in the Appalachian Mountains of North Carolina and Tennessee (Vander Kloet, 1988). Cranberry domestication from selected wild plants and cultivation began in the early 1800s in Massachusetts and New Jersey (Eck, 1990). Cranberry is now commercially cultivated in the United States, Canada, and Chile, which produced 53% (359,110 t), 26% (172,440 t), and 21% (141,338 t) of the global cranberry production, respectively, in 2019 (FAOSTAT, 2019). In 2019, Wisconsin was the highest cranberry-producing state in the United States (202,791 t) followed by Massachusetts (90,839 t), Oregon (23,854 t), New Jersey (20,279 t), and Washington (6433 t) (Cranberry Marketing Committee, 2019).
Cranberry propagates asexually from stolons that spread over the ground and rapidly form a dense mat, and sexually by seed (Eck, 1990). Seeds are produced through outcrossing by native pollinators or European honeybee (Apis mellifera L.), or through self-pollination (Marucci and Filmer, 1964; Sarracino and Vorsa, 1991). Historically, growers cultivated cranberries from wild selections that were clonally maintained and propagated (Eck, 1990). Genetic improvement since the 1950s has been conducted either by crossing wild selections to produce first-generation hybrids or by crossing first-generation hybrids with elite wild selections (Eck, 1990; Fajardo et al., 2013). Hybrid cultivars currently grown by growers are considered to have higher levels of heterozygosity than cultivars selected from wild populations (Bruederle et al., 1996). Maintaining heterozygosity may be advantageous for improving cranberry tolerance to abiotic stresses, therefore contributing to a higher fruit yielding capacity (Davenport and Vorsa, 1999; Ortiz and Vorsa, 1998). Commercial cranberry beds are generally maintained for long periods of time, often surpassing 20 years. This longevity provides opportunities for seeds contained in rotten or unharvested fruit to germinate and become established, thereby increasing the genetic heterogeneity of cranberry plantings over time. Using random amplified polymorphic DNA (RAPD) markers, Novy et al. (1996) observed high genetic heterogeneity and identified 15 genetic profiles in 12 Washington ‘McFarlin’ beds, including four associated with low-producing cranberry beds and not corresponding to the true ‘McFarlin’ profile. Reduced pollen viability, mean seed/berry, and fruit set indicated that these divergent genotypes were less fertile. It was hypothesized that genotypes with reduced fertility would be more vegetatively competitive and preferentially selected when stolons were collected for establishing new cranberry beds, thus contributing to yield reduction in beds contaminated with these genotypes (Novy et al., 1996; Polashock and Vorsa, 2002; Vorsa and Johnson-Cicalese, 2012).
The importance of off-types as an endogenous source of genetic diversity has been emphasized by studies investigating the impact of cranberry fairy ring (Thanatophytum sp.) disease on crop losses and cultivar homogeneity (Oudemans et al., 2008; Polashock and Oudemans, 2006). Higher fruit morphological diversity was noted in areas recovering from fairy ring disease than in healthy areas with ‘Ben Lear’ cranberry beds. This was correlated with greater genotypic diversity within areas affected by fairy ring, with an average of seven haplotypes compared to an average of two haplotypes in healthy areas (Oudemans et al., 2008). Cranberry seedlings recolonizing open areas where vines were killed by fairy ring disease may originate from a soil seed bank. The proportion of off-types with more vigorous vegetative development and reduced yield potential will increase and contribute to long-term decline of the productivity of cranberry beds (Oudemans et al., 2008).
Because cranberry cultivars are traditionally propagated vegatatively to ensure the preservation of genetic characteristics and rapid fruit set (Vorsa and Johnson-Cicalese, 2012), few studies have investigated the effects of biotic and abiotic factors on seed germination. Devlin and Karczmarczyk (1977) assessed the influence of light intensity, abscisic acid (ABA), and gibberellic acid (GA) on cranberry seed dormancy; they demonstrated that cranberry seeds are photoblastic and will germinate if exposed to sufficient light, whereas GA will promote seed germination under dark conditions. Paglietta (1977) indicated that premoistened seeds germinate 2 to 3 d quicker than dry seeds, whereas Demoranville (1974) reported that a temperature of at least 22 °C was required for >90% cranberry seed germination.
We hypothesize that openings in the cranberry canopy may induce changes in the local environmental conditions that may be favorable to the germination of cranberry seeds from the seedbank. A better understanding of cranberry germination in relation to environmental conditions may help provide cranberry growers with practical information about minimizing the onset of off-type varieties in renovated cranberry beds or in open areas that result from damage to the cranberry canopy. Therefore, the objectives of this research were to explore the effects of light, temperature regime, solution pH, water stress, and seeding depth under controlled conditions on cranberry seed germination.
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