One of the first attempts to study protein changes associated specifically with the changes in cold hardiness or dormancy was through the use of sibling genotypes of peach (Prunus persica), segregating for deciduous and evergreen habits. The deciduous genotype typically enters dormancy during fall and exhibits CA. The evergreen genotype, on the other hand, exhibits CA, but the apical meristem of these trees remains nondormant throughout the seasonal cycle. Researchers have characterized seasonal patterns of cold hardiness and protein profiles in bark tissues of these genotypes. Comparative analyses of the seasonality and the degree of CA with that of protein changes in the two genotypes (one lacking dormancy, whereas the other not) have enabled these researchers to specifically associate
certain protein changes with CA and others with dormancy (Figure T1.2).
Differential Induction of Dormancy and Cold Acclimation
Researchers have also used systems in which the developmental program of dormancy can be induced separately from CA. For example, Vitis labruscana, a grape species, exhibits a rather unique developmental programming, in that it is able to fully enter dormancy in response to short photoperiods without cold acclimating. By employing controlled-environment treatments, researchers have exploited this system to characterize differential accumulation of proteins in grape buds during superimposed dormancy and CA programs (use of short photoperiods and cold treatment), and in the buds that had entered only the dormancy program (use of only short photoperiods). By analyzing the profiles of bud proteins from these treatments, they have identified gene products (proteins) that are specific to cold acclimation and those specific to dormancy development.
Differential Regulation of Chill Unit Accumulation and Cold Hardiness
Chilling requirement (CR), a genetically determined trait, is defined as the need for exposure to low temperatures for a genetically determined period of time in order for buds to overcome dormancy and resume normal growth the following spring. The CR of a species is described as the number of hours (chill units, or CUs) of low-temperature exposure needed, and the progress toward meeting the requirement, as the chill unit accumulation (CUA). Temperatures of 0 to 7°C, which also induce cold acclimation, are typically considered to contribute toward CUA. However, temperatures above and below that range do not contribute to CUA (Rowland and Arora, 1997). Exposure to relatively warmer temperatures (10 to 15°C) may also cause cold-acclimated buds to deacclimate in certain species without negating CUA (dormancy neutral treatment). This premise has been used by researchers as the basis to differentially modify CUA and cold hardiness transitions in certain fruit crops and, thereby, to identify physiological changes specifically associated with these events.
338 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT Dehydrins
Biochemical and molecular studies of cold acclimation in plants have led to the discovery that numerous environmental cues (dehydration, low temperature, increased concentration of cell sap) and treatment with ABA induce accumulation of a similar class of proteins called "dehydrins" (Close, 1997). A functional role for de-hydrins in freezing tolerance of plants is suggested, in part, by their hydrophilic properties (thereby protecting cell membranes and other organelles from desiccation), and follows the logic that, since plant cells undergo dehydration during freezing stress, the cellular responses invoking desiccation tolerance should also be involved in freezing tolerance mechanisms. Biochemical and physiological studies employing the aforementioned three strategies show the accumulation of specific dehydrins in cold-acclimated tissues of certain fruit crops (Wisniewski and Arora, 2000, and references therein). Moreover, data also indicate that these dehydrins typically accumulate at much higher levels in hardier siblings, cultivars, and tissues compared to less hardy ones (Rowland and Arora, 1997). However, studies to date have only established correlative relationships between dehydrin accumulation and increased cold hardiness, and no "cause and effect" relationship has yet been established in tree fruit crops. Whereas there is little doubt that dehydrins are key biochemical factors in the cold acclimation process, their specific role in increasing a plant's freezing tolerance remains to be unraveled and will likely be a subject of future investigations.
Responses of fruit trees to low temperatures are both varied and complex. Different tissues within the same plant respond differently to subzero temperatures. This is further complicated by the fact that fruit trees undergo seasonal transitions in cold hardiness that are superimposed by changes in dormancy status of the plant. The economic importance of fruit production and limits placed on it due to low temperature stress, however, will ensure that research continues to develop better understanding of the adaptation and response of fruit trees to cold temperatures.
Related Topics: DORMANCY AND ACCLIMATION; FLOWER BUD FORMATION, POLLINATION, AND FRUIT SET; GEOGRAPHIC CONSIDERATIONS; SPRING FROST CONTROL
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