Many woody perennial species require a period of low temperatures during dormancy, referred to as “chilling.” A chilling requirement allows plants, especially those in temperate zones, to avoid damage during cold winter months. From late summer into fall, shorter photoperiods and decreases in temperature start the process for the plant to enter into endodormancy. Whereas several woody plants respond to both low temperature and photoperiod to start dormancy, it has been demonstrated that grape are able to start dormancy in response to short days only (Fennell and Hoover, 1991a; Salzman et al., 1996; Wake and Fennell, 2000). Buds in endodormancy are inhibited from growing and cannot resume growth even in a warm environment (Londo and Johnson, 2014). However, once buds receive a certain amount of chilling, they will burst if environmental conditions are conducive. For grape, the optimum chilling fulfillment has been reported to occur between 0 and 7 °C (Dokoozlian, 1999). Lack of chilling can result in delayed budbreak, uneven budbreak, reduced shoots and cluster development, change in fruit ripening, and an overall decrease in vine productivity (Dokoozlian, 1999; Londo and Johnson, 2014). Low-chill cultivars planted in cold winter areas may be subjected to frequent frost damage because chilling requirements are satisfied too early. Chilling requirements vary by species, cultivars, and growing conditions (Arora et al., 2003; Dokoozlian, 1999; Dokoozlian et al., 1995).
Studies suggest that budbreak can be affected by factors other than temperature, such as photoperiod, after bud set (Myking and Heide, 1995). It has been shown that tree species may be responsive to both photoperiod and temperature regarding dormancy induction and budbreak (Li et al., 2002; Welling et al., 2002). One theory is that photoperiod and temperature both stimulate hormones to act in concert (Arora et al., 2003; Heggie and Halliday, 2005) and dictate the timing of dormancy release. Hellmers and Hesketh (1974) showed that time to budbreak in loblolly pine (Pinus taeda) seedlings was influenced by both photoperiod and temperature. Short photoperiods may protect dormant buds from possible frost damage (Häkkinen et al., 1998; Heide, 1993a), whereas long photoperiods may mitigate inadequate chilling accumulations by reducing bud sensitivity to warmer temperatures (Häkkinen et al., 1998; Heide, 1993b; Sanz-Pérez et al., 2009). Wareing (1953) concluded that long day is the main factor for induction of budbreak in european beech (Fagus sylvatica) and the amount of chilling has no considerable effect on photoperiod requirement. Caffarra and Donnelly (2011) also showed that photoperiod had an effect on budbreak when the chilling requirement was not fully met in multiple tree species. In many cases, when chilling requirements are fulfilled, plants become sensitive to longer photoperiods as a signal for budbreak (Körner and Basler, 2010). The release from bud dormancy is likely a complicated process involving many factors including temperature and light (Lavee and May, 1997). Because of these complicated factors, field studies may not satisfactorily estimate chilling requirements (Hanninen et al., 2007; Hunter and Lechowicz, 1992); therefore, in this study, we chose to use controlled environments.
In regions with warm winters, inadequate chilling may lead to abnormal budbreak; therefore, artificial application of high temperature (e.g., flaming) or chemicals such as hydrogen cyanamide have been shown to be useful for budbreak in grape (Londo and Johnson, 2014). Although using these techniques can reduce budbreak problems, they are expensive and have potential phytotoxicity consequences (Or, 2009). Ultimately, using adapted, low-chill cultivars is the most sustainable practice. In addition, understanding how chilling hour accumulation affects budbreak induction can help grape breeders combat rising temperatures brought on by climate change. It has been estimated that a significant percentage of worldwide grape production is affected by changing climate conditions as it relates to chill accumulation (Hannah et al., 2013).
Regions like the gulf coastal area of the United States with high relative humidity and strong temperature swings during fall, winter, and spring do not have ideal growing conditions to produce the best quality grapes. Old cultivar options are available and new selections are being tested that may improve grape production in the southern United States.
The aim of the study was to assess the effect of photoperiod on budbreak initiation on cuttings that received different chilling hours in three hybrid grape cultivars adapted to the Gulf Coast region of Mississippi.
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