It is interesting that plants can eventually acclimate fully in response to low temperatures in the absence of inductive photoperiods (67). This suggests that there is more than one route to resistance or more than one ignition key to start the machinery. In either case, the ability of plants to acclimate in response to more than one environmental stimulus provides adaptive flexibility which enhances survival potential.
The freezing tolerance or cold acclimation of plants is enhanced over a period of time by temperatures below 10°C and by a short photoperiod in certain species of trees and grasses. During this process, freezing tolerance increases 2-8°C in spring annuals, 10-30°C in winter annuals, and 20-200°C in tree species. Gene upregulation and downregulation have been demonstrated to be involved in response to environmental cues such as low temperature. Evidence suggests ABA can substitute for the low temperature stimulus, provided there is also an adequate supply of sugars. Evidence also suggests there may be ABA-dependent and ABA-independent pathways involved in the acclimation process. This review summarizes the role of ABA in cold acclimation from both a historical and recent perspective. It is concluded that it is highly unlikely that ABA regulates all the genes associated with cold acclimation; however, it definitely regulates many of the genes associated with an increase in freezing tolerance.
The role of photoperiod and temnperature in the cold acclimation of living Haralson apple (Pyrus malus L.) bark was studied in the autumn under field conditions in Minnesota. Whole trees, or different parts of the same tree, were exposed to either natural conditions, artifically lengthened days, or artificially warmed iiights, or they were subjected to manual leaf removal. The results indicate that acclimation occurs in two stages which are induced by short days and frost (or low Cold acclimation is necessary for the survival of perennial plants over large areas of the earth's surface. In these studies the roles of photoperiod and temperature in controlling the cold acclimation of the living bark of apple trees were examined under field conditions in Minnesota.Low temperatures (1,2,5,7,9,10,(16)(17)(18) and, in fewer instances, short days (2, 3,5,7,8,17) (to -18 C) was ascribed to short days, and the second stage (to below -196 C) to frost.Photoperiodic induction was found to be a necessary prerequisite to the frost-induced stage. In studies of the same species, Hurst et al. (3) observed that even limited amounts of foliage (one pair of leaves) effectively promoted the cold acclimation of bark. In English ivy vines, Steponkus and Lanphear (14) found that the light-induced phase of acclimation involved translocatable hardiness promoters.These results imply that substances are produced in the leaves which inhibit (6) or promote the acclimation of overwintering tissues (3), or do both. This study was designed to clarify the relationship of the photoperiod-and temperature-induced hardiness responses in apple, and to determine whether either involves translocatable hardiness-promoting substances. MATERIALS AND GENERAL METHODSNine studies were conducted on Haralson apple trees (Pyrus malus L.) over a 2-year period. Two of the studies characterized the natural sequence of cold acclimation in mature orchard trees in the autumn of 1967 and 1968. The other seven studies were conducted on 3-year old trees which were potted in 3-gallon containers in April of 1967 and pruned to two leaders. In the autumn of 1967 and again in 1968, three experiments were conducted on these plants in the field where they were subjected to treatments involving natural or modified photoperiod, natural abscission or artificial leaf removal, and natural or modified temperatures. In 1968 a greenhouse experiment and a growth chamber experiment were conducted on similar plants to supplement the field photoperiod and temperature studies, respectively.The following features were common to all experiments. Bark hardiness was determined periodically by subjecting excised 1-year-old branches to a controlled freezing stress as previously described (2). At each sampling date branches from three replicates were cooled (10 C/hr) in vacuum flasks in a freezer. The sample flasks were removed at 5 C intervals and slowly warmed. After 5 days of incubation the sample branches were dissected and rated visually for injury. The cambium, phloem, and corte...
Red—osier dogwood (Cornus stolonifera Michx.) clones were collected from 21 locations in the natural range of the species and grown in a uniform plot at St. Paul, Minnesota, to determine the extent and nature of geographic variation. Significant variations in plant form and growth rate were noted. Genetic influence among the climatic races was apparent in autumnal phenological events such as onset of rest, change of bark color, and leaf abscission. These events were related to provenance features such as winter minimum temperatures, length of growth season, and to a lesser extent to latitude. The time at which cold hardening began in the autumn differed among clones, but no differences were noted in the ultimate degree of hardiness in midwinter. Spring bud break and flowering occurred at about the same time in all clones, while the loss of red bark color varied. Controlled environment and field tests indicated that photoperiod was the prime factor triggering autumn phenomena, while temperature was most important in the spring.
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