During the past few years, unusual winter temperature patterns have caused serious damage to Colorado blue spruce (Picea pungens) and Fraser fir [Abies fraseri (Pursh) Poir] in mid-Michigan nursery and Christmas tree plantations. Damages expressed after bud flushing were cold desiccation and burn and were the most serious on trees located on the south-facing areas of the field. The phenomenon was hypothesized to have been caused by several days of warm winter temperatures. It is believed that as temperatures increased over 50 °F for several days in January and February, plants responded by rehydrating plant tissues, losing their winter-hardiness and progressively triggering growth. Such a phenomenon has been observed and studied in several landscape and horticultural plants. Jones and Cregg (2006) evaluated budbreak and winter injury in exotic firs and concluded that trees that broke bud early were more prone to late spring frost damage. Similar phenomena have been reported in blueberry (Ehlenfeldt et al., 2006), pecan (Smith, 2002), apples (Forney et al., 2000), apricot (Gunes, 2006), and several grass species (Busey, 2003; Iriarte et al., 2005; Webster and Ebdon, 2005). The warmer the temperatures and the greater the duration of the warm spell, the greater the loss in hardiness observed. Consequently, sudden subfreezing temperatures after those warm days are likely to cause severe injuries to buds and needles, resulting in serious loss in growth (Kang et al., 1998) as a result of reduced plant winter-hardiness.
Plant winter-hardiness is well studied in plant physiology. It is reported to be genetically controlled and driven by temperature, photoperiod, and water stress, which induce the metabolic hardening mechanism and produce an increase in cold-hardiness in the fall and a decrease in the spring (Beuker et al., 1998; Jones and Cregg, 2006; Wisniewski et al., 2003). The process is initiated by the continuous exposure to temperatures below 5 °C (chilling requirement) necessary to prepare apical meristems of temperate perennial plants to resume growth when temperatures become favorable in the spring. In boreal conifers, chilling is reported to be the main factor promoting rest completion and growth when warm temperatures become prevalent (Hannerz and Westin, 2000). Knowledge of chilling requirements is critical to the success of conifer plantations, and the suitability of species to a given site is largely dictated by the number of days of low temperatures at that location (Hällgren and Öquist, 1990). In regions with mild winters, warm temperatures in late fall and winter regularly cause chilling deficit and delay bud burst when conditions become favorable in the spring. In regions with cold winters, however, chilling requirements are far exceeded by winter temperatures, and climatic warming would force bud burst at an earlier date and increase the risk of frost injuries (Hamann et al., 2001). This appears to be the case in Michigan where the chilling requirement of most plant species is largely met by December, and warm winters cause dehardening and growth initiation resulting in serious frost injury when temperatures returned to normal winter subfreezing levels. This practical issue creates a challenge for researchers to develop treatments or approaches that will improve tree resistance to frost damage in case of warm winter temperatures. One potential solution is the use of plant growth regulators (PGRs) to influence the natural hormonal processes that triggered dormancy and growth.
Several studies have investigated the influence of growth regulators on growth and stress resistance of ornamental and fruit plant species. For instance, Zhou and Leul (1998) used uniconazole to attenuate the hormonal balance and induce freezing injury in winter rape. A number of research projects have investigated the use of paclobutrazol on drought resistance in Douglas fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), white spruce (Picea glauca), and jack pine (Pinus banksiana) (Marshall et al., 1991). Paclobutrazol has also been reported to affect shoot and root growth in Douglas fir (Wheeler, 1987) and loblolly pine (Pinus taeda) (Barnes and Kelley, 1992). Another study conducted by Albrecht et al. (2004) investigated the use of prohexadione–calcium, vitamin E, and glycerin to reduce frost injury in apple (Malus domestica) and reported a significant reduction in frost injury resulting from the application of prohexadione-Ca. Similar studies conducted by Rossi and Buelow (1997) and Burnell (2005) explored the use of PGRs to reduce winter injury on blue grass (Poa annua L.). Results indicated enhanced freezing stress tolerance with low rates of trinexpat-ethyl, flurprimidol, and paclobutrazol. No published study reports on the influence of PGR treatments on the susceptibility of ornamental and Christmas tree species to cold injuries after warm winter spells.
The objective of this study was to evaluate the influence of three PGRs on the susceptibility of Fraser fir and Colorado blue spruce species to cold damage after simulated warm winter temperature conditions.
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