Winter Dormancy of Woody Plants and Its Noninvasive Monitoring

When dormant, perennial plants dwelling in the regions with pronounced seasonality of climate can withstand prolonged periods of harsh environmental conditions. The period of plant dormancy is commonly divided into pre-dormancy, endodormancy, and ecodormancy. During pre-dormancy, genetic, physiological, biochemical, and morphological rearrangements increasing stress resilience of the plant organism are completed. In the course of endodormancy, meristem cells cannot resume division even under favorable conditions. Environmental stimuli trigger dormancy release and the onset of ecodormancy when plant cell division and growth are restrained only by unfavorable environmental conditions. Frequent nowadays, weather fluctuations can lead to abnormal progression of dormancy. It results in the increased risk of damage to plants, especially crop plants, by adverse climatic conditions. This situation calls for the development of methods for noninvasive express monitoring of plant dormancy. Studies of the relationships between the dormancy status of plants and the functioning of their photosynthetic apparatus made possible the development of methods for monitoring of woody plant condition by recording the variable fluorescence of chlorophyll contained either in needles or in the endoderm of the shoots. This review briefly summarizes current knowledge about the mechanism of the dormancy induction and release. The functioning and regulation of the photosynthetic apparatus during winter dormancy as well as characteristic patterns of chlorophyll fluorescence induction in this period are considered. The difficulties of interpretation of chlorophyll fluorescence signals in the context of monitoring of plant dormancy are discussed together with its potential applications.

Perennial plants, occurring in the regions with pronounced seasonality of climate conditions, evolved a number of adaptations to survive prolonged periods of harsh environmental conditions (cold season) [1][2][3]. The period of plant dormancy characterized by minimum plant growth intensity is among the key adaptations for surviving the cold time of a year [4,5]. The period of winter dormancy is typical both of wintergreen and deciduous plants. A heated debate [2,7] associated with the dormant period's terminology has been ongoing, even though a phenomenon of plant dormancy took center stage among researchers as long as 100 years ago [6]. One of the most universal definitions of the dormant state [8] suggests it to be understood as an "…inactive state within meristems and(or) organs capable of growing characterized by the inability to initiate growth even under favorable conditions until this state is changed by environmental stimuli" [7,9]. Plants in a dormant state are characterized by a high degree of resistance to the effects of adverse environmental factors. Therefore, rhythmics of the onset and release from dormancy aligned with seasonal climate changes ensures the maximum survival of shoot tissues and vegetative and reproductive (floral) buds during cold period [3,6].
The period of plant dormancy is commonly divided into three stages (phases), which are most evident in plants of the temperate climate zone. Plants first come into a stage of predormancy. The genetic, physiological and biochemical, and morphological rearrangements, which increase stress tolerance in plants, take place during predormancy [10]. These changes are reversible to a certain degree due to a possibility of inducing a secondary growth by means of keeping plants in a prolonged photoperiod, application of high doses of nitrogen fertilizers, and exposure to shocks, such as defoliation, excessive pruning, or abundant watering. Predormancy changes into physiological dormancy of endodormancy, hampering the growth even under favorable conditions. This involves a decrease in shoot water content and an increase in water retention [11]; maximum tolerance to the effects of adverse environmental conditions; and fall senescence and abscission of leaves in deciduous species [10,12]. During the cold season and exposure to low temperatures, plants transition from endodormancy to a state of ecodormancy, during which the growth is hampered only by harsh climate conditions. The precise dormancy sustainment mechanisms are unknown, but a number of hypotheses were put forward about their nature [1,[13][14][15]. The growth resumes with the beginning of the warm season and stress-tolerance declines to the level preceding the onset of dormancy.
Winter dormancy is distinguished by the quantitative "dose-dependent" nature evident through a dependence of depth and duration of dormancy on an intensity and duration of exposure to the environmental factors [10]. The minimum chilling requirement (CR)-the chilling hours of plant exposure at temperatures within a range of 0-7°C [5]-is characterized by intra-and interspecies variation and is considered one of the adaptations to the regional climate patterns; mathematical models were designed for its estimation [5]. The regulators of perennial woody plant dormancy included both the internal (circadian rhythms and hormonal signals/controls) and external (photoperiod, temperature, solar intensity, and hydrologic regime) stimuli [8,10,16]. Mechanisms and factors of dormancy onset and release in plants, however, remain largely obscure.
Close attention to the winter dormancy is determined by practical importance of this phenomenon for horticulture, in particular, in regions with harsh climate conditions, including zones of risky horticulture . On the one hand, it is critical to maintain dormancy during the temperature fluctuation period since plants released from dormancy prematurely are susceptible to cold-induced damage. On the other hand, global warming causes risk associated with a lack of cold exposure and, consequently, disturbance of rhythmics of phenological stages' progression. This problem has become particularly relevant in recent years due to climate instability and increase in frequency of hydrothermal regime fluctuations in orchards, causing an abnormal progression in the period of dormancy in plants [5]. Knowledge about the influence pattern of rapid changes in climate conditions on the state of a plant at the onset of and release from dormancy would make it possible to select measures for alleviation of the adverse effects of these phenomena. This issue is particularly pressing in the modern age of climate instability.
This calls for identification of trends of dormancy progression rhythmics and likelihood of premature bloom during the period with the increased risk of frost snaps. Selection of cultivars with a winter hardiness level adequate to the local climate condition is equally important as noninvasive assessment for winter hardiness. High-throughput noninvasive express methods are optimal for solving this task. This includes, among others, a method for recording and analysis of the amplitude and kinetic characteristics of variable chlorophyll fluorescence or PAM (pulseamplitude modulation) widely utilized at high throughput phenotyping of plants [17][18][19][20]. Preliminary studies demonstrated technical feasibility of measuring the induction curves of chlorophyll a fluorescence (ChlF) in photosystem II. Additionally, they characterized seasonal dynamics of the photosynthetic activity of chlorophyll-containing shoot tissues in wild and cultivated woody plants [21][22][23]. Implementation of these methods at the state-of-the-art-level will ensure year-round automated data collection on the functioning of the photosynthetic apparatus and their express analysis, while simultaneously recording the climate conditions. Further insight into the relationship between plant dormancy rhythmics and the state of their photosynthetic apparatus is, however, necessary for confident interpretation of the obtained results within the scope of a winter dormancy study.
In light of the foregoing, the review briefly summarizes contemporary views on mechanisms of plant winter dormancy and release from it. It considers distinctive features of functioning and regulation of the photosynthetic apparatus in dormant plants and the possibility of their noninvasive analysis based on the amplitude and kinetic characteristic of the ChlF induction. Discussion on the problems and potential application of this approach concludes the review.

ENVIRONMENTAL FACTORS AS DETERMINANTS OF DORMANCY PERIOD
It is widely agreed that the onset of dormancy occurs due to the actions of environmental stimuli. The classical [2,3,6,24] and recent [1,13,15] literature accumulated a significant body of evidence on the phenomenology of regulation of rhythmics and depth of dormancy by external factors, among which the key ones are low temperatures, short photoperiod, and their combinations. The question on the primacy among the listed factors is actively debated in the literature and the solution appears to depend on plant species ( Table 1). The majority of woody plants require exposure to the combination of factors both to attain the maximum winter hardiness and for release from dormancy [25,26]. Exposure to both factors is typi-cally required by the plants growing in the regions with warm climate [27,34].
In addition, the shortening of the photoperiod first induces the onset of dormancy in a plant; winter hardiness does not develop until later. Temperature decrease inverts these two responses in plants; specifically, winter hardiness occurs first followed by the transition of a plant into a state of dormancy later [37]. Differences in dormancy induction mechanisms between long-and short-day plants appear to be associated with this distinction based on the effect of a short day and low temperatures [37]. Overall, the effect of the day-length reduction is reversible at early stages of the growth cessation; the plant can resume growth once the long day returns [38].
The major sensor of the photoperiod change is a leaf phytochrome system, triggering a synthesis of VSP (vegetative storage proteins) [41], which is one of the attributes of dormancy onset [41]. The strength of manifestation of responses to photoperiod variation depends on geographic latitude of the plant origin; specifically, the northern ecotypes are more sensitive to reduced daylight, while their critical photoperiod is longer; that is, as the length of a day is reduced, the northern ecotypes cease the growth and will transition into dormancy earlier than the southern ecotypes [31], exceptions to this rule are, however, known [42]. Moreover, some species enter a dormant state (though with a delay) even under the ongoing lighting and high temperature [24].
Perception of cold signals and responses to them has not been studied as thoroughly as for photoperi-odic signals. An interesting hypothesis relates rhythmics of woody plants' dormancy to the dysfunction of "molecular oscillator," controlling the circadian rhythmics under exposure to low temperature and day-length shortening [10]. The key studies of the corresponding mechanisms were conducted using a model herbaceous plant Arabidopsis (Arabidopsis thaliana (L.) Heynh.); studies on woody plants are rather scarce. Findings obtained using the model species, however, are critical for understanding the regulatory mechanisms present in many species, including the woody plant. Thus, a case study on A. thaliana showed that circadian oscillator modulates cold signaling, expression of CBF (C repeat-binding factor) proteins, and transfer of low-temperature Ca 2+ -signals [43]. The circadian oscillator stops while triggering an expression of a number of genes (see below), which ultimately results in the onset of dormancy [44]. Interestingly, during summertime, the cumulative effect of the high, rather than low, temperatures is more important for a release from dormancy in particular plants, such as Japanese apricot (Prunus mume (Siebold) Siebold & Zucc.) [45]. A similar effect is observed under stress conditions, such as premature defoliation, watering after a period of drought, and treatment with the chemicals interrupting the dormancy [46,47].
GENETIC CONTROL OVER DORMANCY PERIOD Metabolic processes and expression levels of particular genes considerably vary even when the mor- phology of dormant shoots and buds remains unchanged. In this setting, various organs and tissues exhibit similar variations in gene expression, which might point to the universality of molecular mechanisms of induction and regulation of dormancy. It was proposed to use the most typical changes identified (established) by genomic studies as molecular marker of a dormancy period [13].
Analysis of quantitative trait loci (QTL) associated with the progression of dormancy in a poplar hybrid Populus trichocarpa Torr. & A. Gray ex. Hook and P. deltoides W. Bartram ex Marshall showed that differences in organizing the genetic control of responses to changes in photoperiod fail to fully explain the differences in timing of bud set observed in field trials [48]. It is necessary to include the effects of other factors, such as temperature, for the full picture. CR is, however, also known to be a complex quantitative trait, governing flowering time (among other things) in Prunus L. spp., including almond (P. dulcis (Mill.) D.A. Webb), apricot (P. armeniaca L.), peach (P. persica (L.) Batsch), and sweet cherry (P. avium (L.) L.) [13]. Some QTL "for CR appear to overlap orthologous genomic regions, suggesting shared underlying molecular mechanisms" of this process [49]. Thus, genomic studies on more than 400 peach genotypes revealed a close relationship between CR and QTL localized on chromosome 1 [50]. Precisely this region contains six "tandem" repeated dam1-6 genes. The cluster of dam (dormancy associated mads-box) genes, coding the transcription factors, was earlier mapped to the evg (evergrowing) locus [13,15]. Phenotypically, overexpression of dam6 gene inhibits growth similar to the one at the onset of dormancy. Deletion of four out of six dam genes results in a loss of ability to cease the growth during winter [15,51].
Experiments performed under the conditions of variations in photoperiod and temperatures demonstrated that dam genes are association with seasonal cessation of growth in shoots and formation of reproductive (fruit) buds in the end of summer [15]. Expression patterns of dam cluster genes suggest their participation in maintaining a endodormant state [15]. Additionally, the svl (short vegetative phase-like) genes homologous to dam genes are considered to be involved in the regulation of dormancy progression. Both families are regulated by a number of the transcription factors, hormones, as well as epigenetically [53]. The teosinte branched1 transcription factors are presumed to inhibit expression of dam5 and dam6 and, therefore, lead to a cessation of endodormancy.
Chilling affects hormone metabolism (see below) and activates genes of the families dam, flowering locus C-like (flc), flowering locus T (ft), and terminal flower 1. Transcriptome is enriched with the gene products controlling the postembryonic development as well as early flowering 7, raf10, zep4, and F-box, which can be involved in regulation of release from dormancy.
Exposure to cold regulates pathways associated with phytohormones and postembryonic development at bud break. This involves reciprocal regulation of two loci, ft and flc, responsible for flowering. Expression of flc is suppressed and repression of ft is relieved after a long period of cold; the plant can bloom. Abundance and regulation of these genes is the most extensively studied in model herbaceous plants, such as Arabidopsis, but homologs (homotypes) of these genes were also discovered in genomes of woody plants that have been investigated by now. Thus, a description exists of epigenetic regulation of expression of these genes with participation of changes in methylation of DNA and histones, as well as chromatin-packaging and pathways of cell cycle regulation. The corresponding mechanisms are related to an interaction between histones and promoter, the second exon and second intron of dam6 genes, and the large intronic dam5 and dam6 insertions in peach and apple [13,52].
The genes involved in regulation of dormancy are mentioned to include genes whose expression is regulated by a level of hormones and abiotic stressors, specifically, the genes coding cyclin D and stabilizing the structure of the dehydrin macromolecules. Expression of dehydrins is associated with dormancy regulation [12] and hardening, proteins CRF2 (cytokinin response factor 2), CAP160 (catabolite activator protein), and proteins from the LEA (late embryogenesis abundant) family associated with responses to coldinduced stress [54,55]. Reduced daylight and falling temperature commonly upregulate these genes; it is gradually repressed after that during the cold season. Low temperatures synergistically amplify the effect from change in photoperiod [52,56]. Evidence was found for participation of an orthologous gene first described in the Arabidopsis of floral repressor svp (Short Vegetative Phase) in a process of transduction of photoperiodic signal and signal of abscisic acid (ABA) in a hybrid of aspen and trembling poplar (P. tremula L. × P. tremuloides Michx.) [57]. Downregulation of svp interferes with the state of dormancy, whereas overexpression of this gene alleviates the disturbance at mutations, resulting in the loss of responsiveness to to abscisic acid.
Proteins CBF bind to genomic DNA prior to dam genes, while inducing a transcription of the latter [58]. Overexpression of genes coding CBF-proteins in peach delays bud break and upregulates expression of lea family genes and other genes, e.g., coding the COR (Cold-Responsive) proteins, whose expression is activated by low temperature and causes acclimation to cold exposure [59,60]. Notwithstanding, no definitive evidence exist so far with respect to involvement of the LEA-and COR-proteins in induction and sustainment of a state of dormancy in addition to the development of tolerance to low temperature exposure.
Experimental data obtained to date made it possible to propose a hypothetical mechanism of endodormancy induction, in which a decrease in temperature induces transcription of the CBF-factors, while the latter upregulate dam genes and block transduction of gibberellin signals [13]. Prolonged exposure to low temperatures leads to a gradual decrease in expression of the CBF and DAM genes, and, therefore, release from endodormancy (Fig. 1).

HORMONAL SIGNALS AND DORMANCY IN PLANTS
Growth inhibition hormones were earlier considered a primary cause of dormancy, while, in the context of dormancy progression, knowledge about mechanisms of their action largely remains fragmented and controversial, despite the large number of studies dealing with this question [8]. Contemporary studies point to a potential role of gibberellin and ABA in bud dormancy regulation [61]. Daylight reduction (Short photoperiod) induces a decrease in gibberellin content in woody plants, correlating with growth delay and cessation of cell division in subapical meristem of shoots [62], whereas treatment with gibberellins of apical (terminal) buds that received short-day signal leads to resumption of division of cells in their apical meristems.
Hypothetical mechanisms were proposed to support dormancy, such as preventing entry of growthpromoting hormones to cells of meristems. Thus, isolation of the apical meristem cells occurs during dormancy due to disruption of a conductive state of plasmodesmata. In apical buds, the latter is disrupted due to the accumulation of callose in them. The hypothesis suggested that gibberellins restore extracellular communication by opening the pores sealed by callose, which is essential for transduction of other chemical signals and dormancy release [63].
Additionally, daylight reduction is known to be accompanied by downregulation of the gene coding GA-20-oxidase, which is a key enzyme in a synthesis of gibberellins [62], whereas dormancy release is accompanied by upregulation of GA-20-oxidase genes in P. mume. Decrease in the level of expression of GA-2-oxidase, deactivating gibberellins (converting gibberellins into inactive form), increases the content of active forms of these hormones, such as GA3 after dormancy release [64]. It is entirely possible that regulation of the dormancy period by gibberellins may be At the onset of dormancy, the global level of gene expression gradually decreases and reaches maximum in the endodormancy followed by a further temporary increase during the transition to ecodormancy. Two groups of genes with the contrast expression parameters can be distinguished. Genes from the first group are assumed to be responsible for "accumulation of the cold-induced effect" and release from endodormancy, while the second group genes are responsible for maturation of reproductive buds and preparation to a release from dormancy (for more details, see the paper by Ju et al. [13] coordinated by the phytochrome system. Thus, the foregoing effect was not observed in hybrid poplar plants with overexpression of the phyA gene [65]. Therefore, inhibition of gibberellins' biosynthesis has its share in response to photoperiodic signal (growth cessation). Content of gibberellins increased as exposure to low temperatures continued in poplar [44]. In contrast to the gibberellins, transport of auxins is not interrupted when cellular communication is broken. The latter is carried out by specialized carriers, whose transcripts are also found in cells at rest. Auxin level in cambial cells remains the same at the onset and release from dormancy, whereas sensitivity of tissues to this hormone changes [10]. According to other sources, a significant increase in concentration of auxins is observed during ecodormancy [66]. The latter assumption is evidenced by a decreased expression of protein transporters, exporting the auxins, against the background of induction of transporters responsible for import of molecules of these hormones. Interestingly, transcription of the entire family of small auxinresponsive RNA, SAUR, is amplified at dormancy release. Currently, precisely transport of auxins is considered to play a role of a "switch," regulating the dormancy period in apple [67]. Actually, one of the auxin gene transporters in peach (Prupe.1G07180) proved to be associated with QTL qCR1d-2008, controlling an "accumulation" of cold signal.
Abscisic acid is the growth inhibitor and abscission promoter [63,68] involved in induction of dormancy by accumulating in buds during a short-day period. Effects of ABA also include break of cell communication through plasmodesmata [61,69]. Changes in a state of dormancy are assumed to be more closely connected with change in susceptibility to ABA than to variations in its concentration. Data on the role of ABA remains controversial. Thus, an increase in the content of this hormone is observed at daylight reduction; however, treatment of plants with exogenous ABA did not cause induction of dormancy [31]. Daylight reduction induced the onset of dormancy even in the forms of silver birch (Betula pendula Roth) deficient in ABA synthesis; these plants, however, featured decreased tolerance to low-temperature exposure compared to the wild type [31,69]. In silver poplar (Populus alba L.), reduction of the day length and drop in temperature similarly trigger an upregulation mediated by an increase in concentration of ABA of SVL gene potentially involved in the control of dormancy and bud break [70]. Overexpression of DAM6 gene of P. mume delays bud break by increasing the level of ABA and decreasing cytokinin content in Malus pumila Mill. plants [14]. Ultimately, ABA was recognized to participate in photoperiodic regulation of tolerance to low temperatures rather than induction of endodormancy [12]. Overall, the findings point toward the independent (distinct) character of dormancy induction and a set of dormant (resting) buds, while photoperiod and the level of ethylene modulate both processes [71]. Photosynthesis and physiological processes associated with propagation of winter buds in common duckmeat (Spirodela polyrhiza (L.) Schleid) are similarly assumed to be controlled by ABA [72].
Gaseous "hormone of senescence," ethylene, also participates in regulation of dormancy progression together with other hormones. The effect of ethylene on rhythmics of dormancy progression was studied based on transgenic birches insusceptible to the action of this hormone due to the expression of aberrant ethylene receptor gene ETR1 from the Arabidopsis [73]. In these plants, the growth was delayed, but no failure of apical bud set was observed under daylight reduction. Similar data was obtained in the case of hybrid poplar (P. tremula × P. alba) overexpression of homolog of ABI3 [71,73].

STRUCTURAL AND FUNCTIONAL TRANSFORMATIONS OF PHOTOSYNTHETIC APPARATUS DURING DORMANCY
Adaptation of plants to low temperature is largely directed to maintain balance between absorbed light energy and metabolic needs of cells for photosynthates [74,75]. Accordingly, plants increase the capacity of the metabolic sink (pathways downstream) and/or decrease photochemical efficiency of the photosynthetic apparatus of cells [76][77][78]. Simultaneously, the onset of dormancy in a cell may trigger change in the proportions of photosynthetic pigments, increase in content of unsaturated fatty acids in membranes, accumulation of mono-and oligosaccharides, synthesis of cryoprotective proteins, cytoplasmic viscosity, and changes in the structure of the photosynthetic apparatus [78][79][80]. At the same time, to reach the balance between the absorbed and utilized light energy, evergreen plants use the maintenance of highly functional photosystem I (PSI), which dissipates excess of absorbed light energy at low temperatures [75,78]. This factor is considered to be critical in governing the dominance of evergreens in high latitudes [81].
Thus, release from winter dormancy occurs more rapidly in Picea obovata Ledeb. and Abies sibirica Ledeb. than Pinus sibirica Du Tour and P. sylvestris L., but, as a consequence, the photosynthetic apparatus of A. sibirica is more vulnerable to spring frost. This distinction is related to the fact that the above-mentioned species utilize different mechanisms of photosynthesis reactivation in spring. In pine, the capacity for CO 2 assimilation declines during winter and an alternative (dependent on Proton Gradient Regulation proteins, PGR5 and PGRL1) electron sink is activated through PSI in spring until assimilation of CO 2 is recovered. Content of PGR5 in pine needles, as well as rates of electron transport through PSI, decreases by the end of spring. Transport through PSII and rate of CO 2 assimilation increase. Together with the controlled and flexibly regulated PGR5 content, the photosynthetic apparatus utilizes another pathway to increase sink from PSI, specifically, flavodiiron proteins [81]. As opposed to pine, spruce does not have the ability to switch between different electron sinks throughout the year and, as a result, suffers photooxidative damage in spring [82]. Consequently, the value of the variable chlorophyll а fluorescence (F v /F m ) more strongly decreases under a frost event in spruce than in pine. This distinction between pine and spruce strategies appears to be associated with their different ecological strategies, that is, a pine is a pioneer species of disturbed habitats, while spruce is a shade-enduring plant of late successional stages [81]. In the course of dormancy induction and developing cold stress tolerance (hardiness), frost onset triggers a partial loss of the PSII antenna component in pine's photosynthetic apparatus, specifically, protein CP43, as well as accumulation of the PsbS protein, participating in the defense through thermal dissipation of excess absorbed light energy [83]. Importantly, both CP43 loss and PsbS accumulation take place at an early stage of hardening, when the photoperiod starts shortening and temperature decreases. Structural and functional restructuring of the photosynthetic apparatus in conifers at the onset of winter dormancy are summarized in detail in a review by Chang et al. [84] (Fig. 2).
Dynamics of the photosynthetic apparatus at the onset and release from dormancy in deciduous trees is studied much less extensively than in conifers. Reasons for a decrease in photosynthetic rates at the onset of dormancy were investigated using the case of peach (P. persica) [37]. Data of differential proteomics demonstrated that photosynthesis was limited by the key proteins of the carbon cycle, specifically, Rubisco and phosphoenolpyruvate carboxylase, whose content declined at the onset of dormancy. The leaves exhibited enzymatic degradation of these enzymes, which progressed deeper and more rapidly during short days than during long days.
An interesting example would be vegetative propagules (turions) of aquatic plants, such as common duckweed (Spirodela polyrhiza), which contain a welldeveloped photosynthetic apparatus but overwinter in dark [72]. During dormancy, drop in an F v /F m value in S. polyrhiza occurs on account of a decrease in maximum fluorescence, F m , and increase in the minimum fluorescence, F o . In these conditions, the level of nonregulated quenching, Y(NO), rises, whereas regulated quenching, Y(NPQ), declines. This response can be considered rather unusual since the deepoxidation level of xanthophylls of violaxanthin cycle significantly rises simultaneously, but absolute content of these pigments does not increase [72].

POSSIBILITIES FOR NONINVASIVE MONITORING OF DORMANCY
Based on the foregoing literature analysis, physiological and biological transformations at dormancy onset and release are accompanied by significant changes in the functioning of the photosynthetic apparatus. These changes further affect the amplitude and kinetic parameters of induction of ChlF, measuring which is widely employed for analysis and monitoring of the physiological state in plants [85,86]. Thus, changes in ChlF variable, F v /F m , and photosynthesis efficiency were established to be of a biphasic character at the onset of dormancy: the first phase is induced by a daytime shortening, while the second phase is induced by a temperature decrease. The development of the chlorophyll PAM fluorometry methods allowed an efficient use of variable fluorescence for recording winter dormancy in conifers [87]. Currently, testing the physiological state of seedlings in the woody plant lots is a common practice in various regions of the United States, Canada, Sweden, and the United Kingdom. It is intended to characterize quantitatively the endogenous parameters, such as general resistance to stress, winter hardiness, status of state of dormancy, as well as detect latent injuries when no visible indications are evident. To the best knowledge of the authors of this review, however, no successful testing has been completed to analyze the dormancy effects separately from these effects of low temperature and/or intensity of solar radiation.
The technique of PChl induction is used along with the traditional methods [37]. Measurement of chlorophyll fluorescence not only in leaves but also in shoots has become a major step forward in the development of its application for monitoring winter dormancy, which determined the applicability of this technique to deciduous trees during the cold period of a year [42,88]. Thus, bark phelloderm contains chloroplasts and is photosynthetically active in woody plants. This allows year-round monitoring of the state of the photosynthetic apparatus [21]. Therefore, ChlF monitoring basically allows us to track dynamics of winter dormancy onset, the degree of freezing damage to plants in winter, and release from dormancy in spring. The use of this approach is, however, associated with some difficulties with respect to interpretation of dynamics of PChl signals. Thus, decrease in the maximum ChlF level, F m , is linked both to improved cold tolerance and acclimation to daylight. Thus, leaves and stems in 20-year old plants were established to exhibit similar patterns of F v /F m changes in the spring-summer period, although F v /F m is commonly lower in stems (0.78 vs. 0.81) [88]. Additionally, parameters of PChl induction (F v /F m ) in stems were shown to be the markers of cold hardiness and the extent of cambial injury in clones of willow (Salix viminalis L. and S. dasyclados Wimm.) [89] the essential differences between the mechanisms of acclimation to low temperature [90]; decrease in F v /F m in P. sylvestris can be used to predict the cold tolerance [91]. Plants with various levels of frost hardiness were shown to be characterized by different temperature dependencies of F v /F m . Thus, linear decrease in F v /F m with temperature was found in late-flowering almond cultivar (P. dulcis) with pronounced susceptibility to frosts, while quadratic curve with an inflection point at -1°C was observed in early-flowering cultivars of the same species [92]. Operating quantum yield of PSII is, however, more sensitive than F v /F m since the former can change more rapidly [93], whereas decrease in F v /F m can only be detected at deeper stages of hardening or winter stress [94].
In can be concluded that study of the temperature dependence of the PChl parameters can generate an algorithm for identification of frost-resistant plants, but parameters of the PChl dynamics specific to the onset of and release from endodormancy have not been identified to date. The hypotheses, however, exist on mechanisms of the PChl modulation during winter dormancy. One of them states that the impairment of intersystem electron transport due to inactivation or damage (injury) to the plastoquinone pool is an attribute of dormancy [95]. More recent assumptions link physiological plasticity of conifers, including during dormancy, to the induction of a high, slowly relaxing level of NPQ (nonphotochemical quenching). The mentioned mechanisms include phosphorylation of thylakoid proteins and more intensive induction of the violaxanthin cycle (see [96,97] and references in these Fig. 2. Generalized sequence of events during cold-induced acclimation of the photosynthetic apparatus and dormancy induction in conifers. Short photoperiod and low temperatures trigger growth cessation, photosynthesis delay, and development of cold hardiness. Decline in photochemical utilization of absorbed energy launches photoprotection mechanisms, including accumulation and deepoxidation level of pigments of violaxanthin hin cycle (VAZ) and transition from the energy-dependent to constitutive nonphotochemical quenching (NPQ) (after Chang et al. [84]

CONCLUSIONS
The present review summarizes contemporary ideas about mechanisms of induction and regulation of various phases of winter dormancy in woody plants and their phenotypic manifestations at a level of photosynthetic apparatus. It is evident even from this brief communication that winter dormancy appears to be a complex phenomenon, whose dynamics is governed by superposition of the environmental stimuli (photoperiod and temperature regime), pathways of their perception and transfer, and responses of plants to these signals.
Recent developments in the recording techniques and theory of analysis of the PChl signals open up new perspectives for monitoring the rhythmics and depth of winter dormancy. Mounting the multiple PChl stand sensors connected to the "cloud" through wireless telecommunication channels allows monitoring the state of dormant plants with unprecedented time resolution both in the natural and anthropogenic ecosystems.
Currently, the main problems of the method's application lie in a lack of knowledge about the fundamental mechanisms of winter dormancy and independent express techniques for assessment of dormancy status in plants. Clearly defined criterion for detection of plant transition from endodormancy to ecodormancy still remains unavailable; it is not even clear whether this is a smooth or discrete transition. Traditional methods, such as growing from shoot cuttings, anatomical and histochemical techniques, etc., continue to be applied largely for this purpose. Epigenetic and "omix" approaches have been discussed recently [10,13]; however, they cannot yet be considered extensively universal and selective. Thus, more detailed profiles of expression are necessary to prove the involvement of candidate genes identified to date for matching to phenotypic data, characterizing the depth and stage of dormancy. High level of time resolution of phenotyping is critically important in this setting. The use of a state of photosynthetic apparatus of shoots as an "internal probe," indicating metabolic activity of tissues, apparently related to the depth of dormancy in plants can be considered a viable approach. The recently developed solar-induced fluo-rescence (SIF) method to probe plants with the involvement of PChl shows particular promise [99].
There is an obvious need for additional research to identify relationships between various manifestations of dormancy at the morphological, genetic, and physiological and biochemical levels, including regulation of dissipation of an absorbed light energy in the photosynthetic apparatus. Specifically, the criteria are required that would allow us to distinguish between a pattern of comparatively long-term induction of photoprotective mechanisms, such as NPQ at the onset of dormancy and "real-time" acclimation to stressors, such as high intensity light and/or low temperatures. The problem of selecting the most informative PChl parameters, reflecting a dormancy status in plants, is essential. In this regard, it can be also assumed that the detailed recording of the state of the photosynthetic apparatus for prolonged time periods comparable in duration with dormancy phase will allow for application of mathematical methods to eliminate "high frequency" interference caused by variations in daily temperature and light intensity.

ACKNOWLEDGMENTS
The results were obtained using the resources of the Research Equipment Sharing Center of Derzhavin State University, Tambov.