Trees suffer from lethal and sublethal injury when severely low temperatures occur before they develop cold hardiness, to a degree that exceeds their midwinter hardiness, or in the spring after they lose cold hardiness. In most regions of apple production, tree loss to low temperature injury occurs in fall when trees are acclimating or in spring during deacclimation. Apple trees acquire cold hardiness in late summer in response to shortened daylength and exposure to cold, and then to low temperatures in fall in the range of 1.3 to −5.0 °C (Hong and Sucoff, 1982; Howell and Weiser, 1970a; Mathers and Stushnoff, 2005). When the occurrence of nonlethal freezing temperature is delayed, acclimation may also be delayed which creates conditions of plant vulnerability (Palonen and Buszard, 1997). Spring deacclimation occurs in response to high temperatures (Howell and Weiser, 1970b), and may occur transiently during brief warm spells followed by renewed cold.
Fruit tree rootstocks affect many aspects of scion performance, one of which is cold hardiness, particularly in the scion trunk (Layne, 1994; Layne et al., 1977; McArtney and Obermiller, 2011; Simons, 1970; Westwood and Bjornstad, 1981). When the trunk is damaged or girdled by low temperature injury, every part of the tree becomes stressed because of reduced functioning of vascular tissues. Tree or orchard replacement is necessary when tree mortality occurs, causing substantial losses for growers. Sublethal effects, including reduced shoot growth and small fruit size, also cause economic losses. In addition, repeated injury to xylem can favor invasion of damaged tissues by pathogens, such as wood-rotting fungi (Quamme et al., 1972; Tanino, 2012). The full economic costs may not be immediately evident, as sublethal damage may go unnoticed or may be confused with associated problems such as wood rot and cankers (Holubowicz et al., 1982). Injury to the trunk from the use of tender rootstocks also occurs in the nursery and can lead to tree shortages for growers and losses for the nursery industry.
Selection of adapted genotypes is the most effective method of preventing low temperature injury (Cummins and Aldwinkle, 1974). Following a December freeze in Washington state, trunk injury occurred in trees grafted to Malling rootstocks, but trees on ‘M.7’ showed better recovery than those on ‘M.9’ (Westwood and Bjornstad, 1981). Among the Malling cultivars, hardiness differences in stem tissue are minimal in November, but in May, ‘M.9’ had more injury than ‘M.7’, ‘M.26’, and ‘MM.106’ from natural freezing events (Wildung et al., 1973). Freeze-thaw cycles also damage tree trunks with the degree of damage governed by the rootstock genotype. After a severe freeze-thaw cycle in January in North Carolina, ‘Golden Delicious’ scions on ‘M.9’ and ‘M.26’ had greater trunk injury and subsequent reductions in tree growth than the same scion on ‘G.935’ and ‘G.16’ (McArtney and Obermiller, 2011). Naturally occurring freezes have been used to characterize rootstock cold hardiness, but such assessments are limited to genotypes in cultivation and can be confounded by other factors that influence tree performance. In controlled studies ‘M.26’ and ‘Ottawa 3’ have greater midwinter hardiness than ‘M.7’ and ‘MM.106’ (Embree, 1988), whereas ‘B.9’ has greater hardiness than ‘G.30’ and ‘G.11’ (Moran et al., 2011). In March, ‘Ottawa 3’ and ‘Ottawa 4’ have greater hardiness than Malling rootstocks (Holubowicz et al., 1982). Additional controlled studies are needed to determine the full seasonal low temperature tolerance in new cultivars. The Vineland rootstock breeding program, initiated in 1958, developed a series of several size-controlling rootstocks that were adapted to Canadian winters (Elfving et al., 1993). Geneva rootstocks were also selected for low temperature tolerance (Cummins and Aldwinkle, 1974). Cultivars and selections from these programs are likely to have greater midwinter low temperature tolerance based on genetic parents, but survival of freezing temperatures during acclimation and deacclimation is largely unknown.
Visual browning as a measure of injury has a stronger correlation with survival than electrical conductivity of leachates (Lapins, 1961). Blackheart, as a measure of xylem browning in the scion trunk, varies among rootstocks, but when less than 50% to 60% of the trunk cross-sectional area or branch is affected, there is no measurable effect on tree growth (Domoto et al., 2001; Moran et al., 2011; Steinmetz and Hilborn, 1938; Warmund et al., 1996). In apple trees, shoot mortality occurs with 80% xylem damage as measured in cross section, but not 60% (Pramsohler et al., 2012). Visual browning is useful for assessing injury in different tissues, but the level of browning that leads to mortality may differ between the xylem, cambium, and phloem.
Acclimation begins in distal parts of the tree and progresses toward the base, so the trunk is the last part of the aboveground tissues to acquire hardiness (Palonen and Buszard, 1997). For this reason, the trunk is typically less hardy than younger shoots on the same tree (Ketchie and Beeman, 1973; Quamme and Hampson, 2004), and displays more severe injury than other tree parts following injurious temperatures (Steinmetz and Hilborn, 1938). Crucially, it is because of this variation within each tree that 1-year-old shoots may not represent the hardiness of the rest of the tree (Quamme and Hampson, 2004).
In addition to the distal-basal variation in shoot hardiness, different tissues within the cross section of a shoot also differ in hardiness (Mathers, 2004; Quamme, 1976). In the fall, the phloem of apple and peach shoots is more tender than xylem and cambium, but develops greater hardiness than these tissues in midwinter (Nesmith and Dowler, 1976; Quamme et al., 1972). Cambial hardiness is not frequently measured, so injury to this tissue can go unnoticed. For these reasons, all three tissues should be considered in the measurement of shoot tissue hardiness.
The objective of this research was to measure low temperature tolerance in Geneva and Vineland apple rootstock genotypes during fall, winter and spring to identify genotypes with the least tolerance. Low temperature tolerance of xylem, cambium and phloem were evaluated through two experiments, the first conducted in the 2013–14 dormant season, and the second conducted in the 2014–15 dormant season.
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