The path to dormancy induction, maintenance, and release is a continuum and has been the topic of thousands of research articles to date. It would be an impossible task and indeed presumptuous of us to imagine that we could review all the research conducted on dormancy in the past century in this article. The multiple and complex nature of the dormancy phenomenon would require not one review but a series of in-depth reviews to cover the research on individual subdisciplines that come under the umbrella of dormancy. Its complexity and multiplicity of various subdisciplines stem from the fact that dormancy affects diverse plant structures (buds, seeds, bulbs, etc.) distinctly and that these dormant structures maintain distinct anatomical and physiological relations with neighboring parts. We, therefore, have chosen to discuss here only one, nevertheless highly significant, aspect of dormancy, i.e., bud dormancy in woody plants.As one reflects over nearly a century of work, it is apparent that, as with other disciplines, dormancy research has evolved as different aspects of bud dormancy (e.g., site of dormancy; photoperiod and environmental induction of dormancy; physiology of dormancy control, particularly phytohormones; chilling requirement-effective temperatures, bud differences, modification of chilling requirement by environment and/or cultural practices, models for calculating chilling requirement; dormancy-breaking chemicals and/or stress treatments) catching the fancy of horticulturists at different periods on the temporal curve of dormancy research. This research was extensively reviewed during the ), followed by more recent reviews and workshop proceedings in the among others). To appreciate the continuity of significant research developments in this field, we recommend them as a must-read for students of plant dormancy.Although many significant mileposts have been reached in our understanding of the induction and release of bud dormancy in the past 50 to 60 years (reviewed in the above citations), research published up until the 1980s includes little information on experimental systems and approaches for studying the genetics of bud dormancy and the cellular and molecular events-gene expression and regulation, signaling mechanism(s), or mechanistic aspects-associated with regulation of bud dormancy. Many of us must wonder how H. Muller-Thurgau had already confirmed in 1885 that a shortened growth period of the shoots caused by water stress promotes early inception of bud dormancy and shortens its duration, i.e., reduces the chilling requirement. This observation was further supported by Chandler and Tufts in 1934 based on their observation that an extended growth period of shoots delays budbreak the following spring if there is insufficient chilling. Despite these early observations, today we still do not clearly understand the cellular biology of how environmental stress regulates bud dormancy. dormancy induction and release in the past century has been, in part, due to the preoccupation with the linear ...

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The role of temperature during dormancy development is being reconsidered as more research emerges demonstrating that temperature can significantly influence growth cessation and dormancy development in woody plants. However, there are seemingly contradictory responses to warm and low temperature in the literature. This research/review paper aims to address this contradiction. The impact of temperature was examined in four poplar clones and two dogwood ecotypes with contrasting dormancy induction patterns. Under short day (SD) conditions, warm night temperature (WT) strongly accelerated timing of growth cessation leading to greater dormancy development and cold hardiness in poplar hybrids. In contrast, under long day (LD) conditions, low night temperature (LT) can completely bypass the short photoperiod requirement in northern but not southern dogwood ecotypes. These findings are in fact consistent with the literature in which both coniferous and deciduous woody plant species' growth cessation, bud set or dormancy induction are accelerated by temperature. The contradictions are addressed when photoperiod and ecotypes are taken into account in which the combination of either SD/WT (northern and southern ecotypes) or LD/LT (northern ecotypes only) are separated. Photoperiod insensitive types are driven to growth cessation by LT. Also consistent is the importance of night temperature in regulating these warm and cool temperature responses. However, the physiological basis for these temperature effects remain unclear. Changes in water content, binding and mobility are factors known to be associated with dormancy induction in woody plants. These were measured using non-destructive magnetic resonance micro-imaging (MRMI) in specific regions within lateral buds of poplar under SD/WT dormancing inducing conditions. Under SD/WT, dormancy was associated with restrictions in inter- or intracellular water movement between plant cells that reduces water mobility during dormancy development. Northern ecotypes of dogwood may be more tolerant to photoinhibition under the dormancy inducing LD/LT conditions compared to southern ecotypes. In this paper, we propose the existence of two separate, but temporally connected processes that contribute to dormancy development in some deciduous woody plant: one driven by photoperiod and influenced by moderate temperatures; the other driven by abiotic stresses, such as low temperature in combination with long photoperiods. The molecular changes corresponding to these two related but distinct responses to temperature during dormancy development in woody plants remains an investigative challenge.

There is increasing evidence that temperature, in addition to photoperiod, may be an important factor regulating bud dormancy. The impact of temperature during growth cessation, dormancy development, and subsequent cold acclimation was examined in four hybrid poplar clones with contrasting acclimation patterns: 'Okanese'-EARLY, 'Walker'-INT1, 'Katepwa'-INT2, and 'Prairie Sky'-LATE. Four day-night temperature treatments (13.5/8.5, 18.5/13.5, 23.5/8.5, and 18.5/3.5°C) were applied during a 60-day induction period to reflect current and predicted future annual variation in autumn temperature for Saskatoon, SK. Warm night temperature (18.5/ 13.5°C) strongly accelerated growth cessation, dormancy development, and cold acclimation in all four clones. Day temperature had the opposite effect of night temperature. Day and night temperatures appeared to act antagonistically against each other during growth cessation and subsequent dormancy development and cold acclimation. Growth cessation, dormancy development, and cold acclimation in EARLY and LATE were less affected by induction temperature than INT1 and INT2 suggesting that genotypic variations exist in response to temperature. Separating specific phenological stages and the impact by temperature on each clone revealed the complexity of fall phenological changes and their interaction with temperature. Most importantly, future changes in temperature may affect time to growth cessation, subsequently altering the depth of dormancy and cold hardiness in hybrid poplar.

The cells in the crown of winter wheat cv. Fredrick critical for the survival of freezing and icing stress were identified using tetrazolium staining as a viability test. In acclimated seedlings, a freezing stress which lowered regrowth (−12 °C) also lowered tetrazolium staining in the vascular transition zone in the basal portion of the crown but generally did not affect the staining of the apical meristem. The majority of cells in the crown, including the apical meristem, were able to reduce tetrazolium after a lethal freezing stress. Thus, survival was limited by the freezing tolerance of a relatively small number of cells in the basal region of the crown. These observations were confirmed using plasmolysis and mitotic figures as alternative indicies of viability. No significant variability was observed among winter wheat cultivars. However, in seedlings not acclimated to freezing stress, there was quite a different pattern of injury. In these seedlings, the sensitivity of the apical and basal regions to freezing was similar. Thus, these two regions appeared to differentially acclimate and the cells of the apical meristem developed greater cold hardiness than that of the basal area. After a lethal icing stress, all regions within the crown were able to reduce tetrazolium, but the crown was unable to regrow. The ability to reduce tetrazolium was gradually lost during the regrowth period. Unlike freezing stress, no differential sensitivity was observed within the crown, and there was no variability among the cultivars of winter wheat examined.