5 1. Introduction In stone fruit trees, the cycle of shoot growth is marked by the development of two bud types: the vegetative and reproductive buds. Vegetative buds comprise bud scales, leaf primordia, and the shoot apical meristem. Reproduct- ive buds have a more complex structure made up of bud scales and flower primordia represented by pistil and sta- mens. The flower buds of temperate-zone fruit trees are initiated during the previous growing season and several critical steps during bud morphogenesis were identified, from floral induction up to complete bud differentiation (Ryugo, 1990). During the autumn-winter season, the plant elaborates mechanisms for survival under unfavorable growing con- ditions by adopting a dormancy strategy to cold tempera- ture acclimation. Meristem activity becomes insensitive to growth-promoting signals (Rohde and Bhalerao, 2007) preserving the buds in a quiescent state under potential- ly damaging environmental conditions (Ćechová et al., 2012). Bud dormancy starts with the perception by the plant of rest-signals under the influence of short and cool days; this process finishes after an accumulation of chill- ing temperatures. The conventional terminology identifies dormancy evolution as: paradormancy, when growth is inhibited by endogenous factors outside the dormant structure; en- dodormancy, when growth is regulated by physiological factors within the dormant structure; and ecodormancy, when the inability to grow is imposed by environmental factors (Lang et al., 1987). In trees, winter dormancy means endodormancy: during this phase the chilling temperatures are accumulated, and flower buds are unable to respond to warm temperatures required for dormancy breaking (Fig. 1). When the chilling requirement (CR) is fulfilled, buds are in ecodormancy and can react to warm temperatures, but remain in an apparent quiescent state due to persist- ent low temperatures (Horvath et al., 2003). The release of dormancy is genetically controlled and regulated by com- plex phenomena affected by various integrated elements whose interaction(s) determine the point in time when release of bud dormancy occurs (Faust et al., 1997). En- Apricot flower bud dormancy: main morphological, anatomical and physiological features related to winter climate influence R. Viti*, S. Bartolini**, L. Andreini* * Dipartimento di Scienze Agrarie, Alimentari e Agro-ambientali, Università di Pisa, Via del Borghet- to, 80, 56124 Pisa, Italy. ** Istituto di Scienze delle Vita, Scuola Superiore Sant’Anna, Piazza Martiri della Liberta, 33, 56127 Pisa, Italy. Key words: budbreak, climate, endodormancy process, Prunus armeniaca L. Abstract: This review examines recent advances regarding flower bud dormancy in apricot, focusing on biological, ana- tomical, and physiological processes which occur during the induction and depth of dormancy. In a scenario of global climate change, the relationship between endodormancy and winter climate influence is discussed. Dormancy regulation is a complex process necessary for plant survival and development. In fruit species, the knowledge of mechanisms con- trolling dormancy and establishing its release appears crucial for successful yields. Specific studies have suggested that, when the flower buds are apparently inactive, slow and gradual changes occur in the whorls: organogenesis, such as microsporogenesis processes and vascular connections take place during the entire dormancy period. It has been indic- ated that an asynchronism between biological (i.e. endodormancy release, microsporogenesis evolution), anatomical (i.e. xylem vessel differentiation) and biochemical (i.e. changes in metabolic compounds and enzymes) events could represent further causes determining an inconstant rate of blooming. Temperature is the main factor involved in dormancy trigger- ing and releasing. In the perspective of global warming, mild winter temperatures could greatly impact apricot ecological cropping systems. Phenological process-based models are considered to be the best tool to study the climatic changes and subsequent expected phenology variation (dormancy and flowering). A new model, calibrated and validated on apricot cultivars, is proposed to predict the dormancy release date in a future scenario. Adv. Hort. Sci., 2013 27(1-2): 5-17 The authors equally contributed to the manuscript. Received for publication 21 December 2012 Accepted for publication 2 April 2013 6 vironmental factors (temperature, light, relative humidity, etc.) are directly related to biochemical changes involved in bud dormancy release. In fruit species, understanding the mechanisms controlling dormancy and establishing its release appear crucial for successful yields. Apricot (Prunus armeniaca L.) is characterized by a restricted spread in the Mediterranean basin and its adapt- ability to specific environments is strongly influenced by the ability of certain cultivars to overcome flower bud dor- mancy (Viti et al., 2010). Climatic-environmental factors may heavily influence flower bud dormancy breaking, af- fecting the entity of appearance of floral anomalies and de- termining irregular and/or insufficient fruit bearing (Clanet and Salles, 1972; Legave et al., 1984; Viti and Monteleone, 1995). Cold winter temperatures influence the time of bud dormancy release and, consequently, the CR of a genotype (Garcia et al., 1999). Most of the known apricot cultivars are characterized by a high CR (Table 1); more than 1000 Chill Units (CU) are required to overcome bud dormancy (Guerriero et al., 2006). In Mediterranean apricot grow- ing areas, characterized by a mild-winter climate, many cultivars have an inadequate satisfaction of CR. During a short or mild rest season (Garcia et al., 1999), the unsatis- factory CR can determine an incomplete release of flower bud dormancy, a late bud break with scanty blooming and a high flower bud drop (Viti and Monteleone, 1991; Erez, 2000). Thus, CR is a key factor for breaking dormancy and knowledge about it has practical significance and econom- ic impact on the control, maintenance and production of woody plants (Fennell, 1999). This statement appears cru- cial for the crop management of apricot cultivars in most cultivation areas. The present paper examines the dormancy process of apricot flower buds, focusing on biological, anatomical, and physiological changes that occur during the endodor- mancy phase in relation to winter temperature influence within the context of global climate change. 2. Morphological and anatomical features The pattern of apricot floral organogenesis after initiation is typical of other Prunoideae. The first floral organs to ap- pear are sepals and petals followed by the stamen and pistil; the most internal whorls of the pistil derive from the gradual evolution of the receptacle (Monet and Bastard, 1968; Bar- tolini et al., 2013). This process requires about two months and is normally completed before leaf fall but not all buds reach the stage of pistil appearance (Legave, 1975). Apricot flower bud evolution was described for the first time by Baggiolini (1952). The classification has been re- vised to describe accurately the progress of flower bud phenological development by the addition of sub-phases within each phenological stage (Bartolini et al., 2004). Figure 2 depicts the evolution of flower bud phenological stages from deep dormancy (stage A) until petal fall (stage H). The stages related to the dormancy process are referred to as: 1) stage A 0 -A 2 during endodormancy; 2) stage B 1 - E 2 from ecodormancy to bud swelling; and 3) stage F 1 -H from beginning of flowering to petal fall. Dormant buds (stages A 0 , A 1 , A 2 ) are characterised by a conical shape, rounded at the base, with the brown bud scales tightly closed (A 0 ) or Table 1 - Chilling Units (CU) required to induce break of endo-dorman- cy in flower buds of several apricot cultivars. Starting date and amount (%) of flowering are reported Cultivars CU Flowering Starting date % San Castrese 870 1 Mar 57 Goldrich 950 7 Mar 18 Sarritzu I° 950 7 Mar 51 Alessandrino 1000 7 Mar 10 Baracca 1000 14 Mar 21 D’Alessandria 1000 10 Mar 5 Bebeco 1030 7 Mar 15 Canino 1030 2 Mar 34 Moniqui 1125 5 Mar 2 Aurora 1140 29 Feb 2 Amabile Vecchioni 1140 2 Mar 30 S.Nicola Grosso 1140 9 Mar 25 Bergeron 1225 17 Mar 1 Rapareddu 1250 7 Mar 23 Polonais 1300 9 Mar 5 Orange Red 1450 14 Mar 1 Stark Early Orange not defined not defined 0 Mean 1100 SD 151 Fig. 1 - Schematic representation of apricot flower bud growth dur- ing the annual cycle. 1) Paradormancy: from flower bud initia- tion to beginning of dormancy; 2) Endodormancy: from deep dormancy to bud swelling; 3) Ecodormancy: from beginning of flowering to petal fall. Solid and dotted lines represent the flower bud length (mm) and the flower bud fresh weight (mg/ FB), respectively. Evolution of the main phenological stages of flower buds are illustrated. 7 more enlarged (A 1 and A 2 ); at the beginning of bud swell- ing (stages B 1 and B 2 ) the pink sepal tips appear; at stages C 1 and C 2 sepals are clearly visible; at stages D 1 and D 2 full bud swelling takes place with the appearance of the white petal tips; at stages E 1 and E 2 petals are clearly visible; at stages F 1 and F 2 full flowering occurs; at stages G and H petal fall begins with the end of flowering. During endodormancy, the accumulation of chilling temperatures occurs and it is usually believed that flow- er bud development is arrested because no clear signs of growth are evident. However, specific studies have shown that bud growth is hardly perceptible, although constant (Alonso et al., 2005). The buds are metabolically active and continue their development during the entire winter, leading to bud break. In peach flower buds continuous anatomical development during the late autumn and win- ter dormancy period were observed, without macroscopic changes (Reinoso et al., 2002). Apricot flower buds also have a gradual and prolonged development during mor- phogenesis since their organogenesis is not generally completed until just before anthesis (Erez and Couvillon, 1987). During the first stage (A) a minimal variation in bud size and weight was detected. Indeed, during Decem- ber and January, when buds are apparently still dormant, it was possible to detect a continuous growth in weight and size. The parameters of weight increase and height/ width ratio have revealed a statistically appreciable slow and progressive buds growth from the middle of December (Guerriero et al., 1986; Scalabrelli et al., 1991). During this apparent rest period, anatomical observa- tions revealed that processes of microsporogenesis, mac- rosporogenesis and xylogenesis take place. As regards mi- crosporogenesis, the following phases have been identified (Fig. 3) in relation to flower bud development (Nyújtó and Banai, 1975; Viti and Scalabrelli, 1988; Viti and Montele- one, 1991): a) Sporigen Cells Differentiation (sticked cells), during the paradormancy period; b) Pollen Mother Cells (diploid microsporocytes differentiated from sporogenous cells that become spherical and separated; c) Tetrads ap- pearance (four haploid microspores produced by meiosis of the pollen mother cells and surrounded by a callose wall) during the endodormancy period; d) Young Pollen Grains (isolated microspores produced when the callose wall dis- appears) at the overcoming of endodormancy; e) Mature Pollen Grains (full development of the wall made up of two layers, exine and intine) after the resumption of growth. The tetrad stage has been considered a signal marking the end of endodormancy (Bordeianu et al., 1962; Szabò et al., 2002). Nevertheless, other studies have shown that tetrads occur at a late stage of morphological bud growth and they are not closely linked to chilling accumulation, because the meiosis process took place also in buds exposed to high temperature (Martìnez-Tellez et al., 1982; Felker et al., 1983; Viti and Scalabrelli, 1988). Contradictory studies have shown that winter dormancy sets a boundary between the development of the sporogenous tissue and further microspore develop- ment (Julian et al., 2011). These authors observed that, in autumn, stamens develop until the differentiation of sporog- enous tissue, remaining in this anatomically quiescent stage during the three months of winter; microspore development took place only after dormancy. Concerning gyneceum development and the related macrosporogenesis, a later evolution than stamen and mi- crosporogenesis process was observed. Morphologically, the pistil has a lengthwise growth that can be divided into different phases: intensive growth during paradormancy; slow growth during endodormancy; and increased growth during ecodormancy, at first very slowly then followed by a significant rapid rise a few weeks before flowering (Sza- lay and Nemeth, 2010). Anatomical observations showed that the first signs of the ovary can be detected in October. Several studies have found a well-developed embryo sac at anthesis; Egea and Burgos (1995) found a frequent presence at anthesis of ovules without the sac being formed. In apricot, two to three ovules/ovary are usually noted, but at anthesis they are frequently malformed and quickly degenerate. Ovule Fig. 2 - Evolution of flower bud phenological stages in apricot: deep dormancy (stage A), pink sepal tips appearance (stage B), sepals clearly visible (stage C), achievement of bud swelling (stage D), petals clearly visible (stage E), anthesis (stage F), end of flower- ing (stage G) and petal fall (stage H). From Bartolini et al. (2004). Fig. 3 - Main microsporogenesis phases (x 400). a) Pollen Mother Cells (PMC); b), b1) Tetrads (T) and c) Young Pollen Grains (YPG). 8 number seems to be related to agronomic and climatic conditions, rather than genetics (Egea and Burgos, 1995; Burgos and Egea, 1993; Albunquerque et al., 2002). At the end of the endodormancy process (end of winter- beginning of spring), a rapid weight increase and the com- plete development of floral verticils at the transition from phenological stage B (visual beginning of bud swelling) up to stage D (appearance of the white petal tip) have been observed. In this period, elongation of pistil and stamen filaments occurs while mature pollen grains and complete gynaecium development take place only few days before blossoming (Luna et al., 1990). In concomitance with the effective warm temperatures, female and male gameto- phytes become functional. During the temporary delay of visible bud growth, the process of xylem development within the flower bud axis occurs. The bud, during early growth, is connected to the stem through a parenchymatous zone, traversed by a pro- cambial strand. In several Prunus species, it was observed that, during winter, vascular tissues are not completely dif- ferentiated and the connection between the flower primor- dium and the bud axis is formed only by the procambium (Ashworth and Rowse, 1982). This tissue, which consti- tutes the vascular strands, is made up of elongated cells and contains densely stained cytoplasm and lacks lignified secondary wall thickenings (Esau, 1965). The differentia- tion process consists of the transition from this meriste- matic tissue to xylem cells, i.e. dead cells with lignified walls producing an empty conduit through which water flows. In apricot, gradual xylem development was ob- served during winter, when no bud growth changes were visible. The acropetal progression of primary xylem dif- ferentiation along the flower bud axis was defined by five stages according to Bartolini and Giorgelli (1994): stage 1 = at the base of the axis; stage 2 = at ½ of the axis; stage 3 = at ¾ of the axis; stage 4 = at the base of the ovary; stage 5 = inside the pistil (Fig. 4). Stage 3 appears to be the most significant with regard to breaking dormancy be- cause it is in concomitance with the first morphological sign of bud growth resumption. A good relationship be- tween an advanced xylem differentiation (‘stage 3’) and endodormancy release was observed in cultivars with a low chilling requirement (i.e. ‘San Castrese’). The avail- ability of nutritional elements throughout xylem supply in correspondence with bud growth reactivation could be a factor determining flowering regularity. A correlation be- tween an increase of certain elements (i.e. potassium and boron) by the xylem acropetal transport and bud swelling has been observed (Hanson and Breen, 1985; Essiamah and Eschrich, 1986; Bartolini and Giorgelli, 1995). On the other hand, in cultivars with a high chilling requirement (i.e. ‘Orange Red’), xylem ‘stage 3’ occurs when flow- er buds are still in endodormancy. This early anatomical trait of flower buds does not coincide with the reactiva- tion of bud growth; it could be hypothesized that the newly formed vessels might preserve the ability to function in water transport. This feature was confirmed by a recent research where a good syncronism between overcoming dormancy, xylogenesis and microsporogenesis, was found in cultivars with a low CR, while in cultivars with a medi- um or high CR an asynchrony between such processes was observed (Bartolini et al., 2006 a), leading to inconstant blooming and fruit yield. 3. Physiological features Control of plant dormancy is the result of multifactorial regulatory networks in which nutrients, phytohormones, genes, proteins and climatic factors (namely temperature and photoperiod) are involved at some point in time (Chao and Anderson, 2010). Light and temperature climate pa- rameters are important determinants in several aspects of dormancy. In northern temperate regions, the stimulus for induction of growth cessation and dormancy has been considered to be primarily controlled by short photoperiod (Allona et al., 2008). Increasingly, several authors have shown that temperature may replace or strongly mediate this short photoperiod dormancy response in woody spe- cies (Kalcsits et al., 2009; Tanino et al., 2010). The onset and release of dormancy state might begin with the perception of a signal by the plant upon exposure to chilling temperatures (Or et al., 2002). Then, it would be followed by transduction of this signal via a cascade of biochemical events to the stage where it imposes or re- leases repression of bud meristematic activity (Faust et al., 1997). Many cellular activities leading to morphological, physiological and biochemical changes take place inside the bud during the transition period from endodormancy to active bud growth, including respiratory rate, reserve carbohydrate mobilisation, water content increase, energy transport, and gene expression. Fig. 4 - Representation of xylem vessel differentiation along the flower bud axis, from Bartolini et al., 2006: stage 1 (at the base of the axis); stage 2 (at ½ of the axis); stage 3 (at ¾ of the axis); stage 4 (at the base of the ovary); stage 5 (inside the pistil). 9 ‘Sink strength’ and carbohydrates Considering that the bud of a woody plant is morpho- logically complex and constituted by organs differing in structure and physiology, the bud anabolic potential is par- tially regulated by the “sink strength” of dormant and non- dormant tissues influencing the subsequent capacity to ac- cumulate metabolites (Crabbé and Barnola, 1996). During early bud growth, the parenchymatous zone, which repre- sents the connection between bud and stem, is fed through the symplasm of this region (Pétel and Gendraud, 1996). Thus, plant dormancy and dormancy breaking appear to depend on peculiar short distance relationships between the bud and its underlying tissues (Champagnat, 1973; Gendraud and Pétel, 1990). In peach, during the dormant period, the parenchyma shows a strong ATPase activity driving a powerful proton extrusion, with a consequent pH cytoplasmatic alkalisation linked to enzyme activity (Pétel et al., 1992; Petél and Gendraud, 1996). A relation- ship between changes in intracellular pH (pHi) and bud dormancy release has been observed, first, in Jerusalem artichoke and peach vegetative buds (Marquat et al., 1996; Aue et al., 2000), and subsequently in apricot buds (Zanol and Bartolini, 2003), suggesting the involvement of pHi changes as signalled by growing evidence (Zimmermann et al., 1999). In particular, when apricot flower buds were still in deep dormancy, an increase of pHi in the flower primordia tissues was found, just before the first sign of growth reactivation, usually denoted by a significant bud weight increase (Fig. 5). The changes in pHi values might be useful to detect in advance bud growth capacity, show- ing the potential competitive sink for nutrients between the different bud tissues from dormancy to growth resumption (Zanol and Bartolini, 2003). The pHi measurement is a good parameter to estimate ‘sink strength’ determining the nutrient fluxes (Gendraud and Pétel, 1990; Bonhomme et al., 1999; Robert et al., 1999). The energy source for budbreak comes mainly from the mobilization of products stored in the perennial parts of the tree. Carbohydrates are the main source of energy for the metabolic changes that occur during the dormant period and for spring sprouting and blooming (Flore and Layne, 1996; Sherson et al., 2003). Changes in the con- tent of carbohydrates in different tissues from vegetative and reproductive structures were found during dormancy, associated with chilling temperatures (Wang and Faust, 1987; Valentini et al., 2006). There was a significant decrease in starch concentration in the bark tissue of Prunus sp. due to exposure to chilling temperatures (González-Rossia et al., 2008). The effect of low temperatures on starch and sugar concentration dur- ing the rest period can be explained: amylase activity is induced by cold temperature, increasing starch hydrolysis and, consequently, sugar concentration (Elle and Sauter, 2000; Bonhomme et al., 2005). Soluble sugars, important signalling molecules in- volved in many processes in the life-cycle of plants, are also related to the dormancy period and involved in in- creased frost resistance (Tabuenca, 1975; Sheen et al., 1999; Smeekens, 2000). In particular, starch levels were negatively correlated with hardiness but most soluble sug- ars were positively correlated (Jones et al., 1999). Glucose, fructose, and sorbitol were the main sugars in the bark tissues of peach, nectarine, plum and apricot; sorbitol concentrations varied significantly with chilling accumulation (Bonhome et al., 2005; González-Rossia et al., 2008). In other sorbitol-synthesizing plants, such as sweet cherry (Prunus avium L.), sucrose is the most pre- dominant soluble carbohydrate during dormancy (Keller and Loescher, 1989). In peach primordia, especially the floral ones, very high concentrations of transport forms of carbohydrates (sucrose and sorbitol), imported during growth capacity recovery, were found (Bonhomme et al., 2005). From autumn to mid-winter, a significant relation- ship between total sugars and starch concentrations, with a marked increase in amounts of sorbitol, fructose, glu- cose and sucrose, was found in the bark tissue of stone and pome fruits, coinciding with a decrease in starch content (Wang and Faust, 1987; González-Rossia et al., 2008). Free radicals and antioxidant mechanisms Studies have shown that free radicals, activated oxygen species (AOS), implicated in a number of biological phe- nomena, are produced in dormant buds of some fruit spe- cies, where their removal seems to be associated with bud break as a result of changes in antioxidant systems (Wang et al., 1991). The generation of AOS, particularly H 2 O 2 , during stress has been proposed as part of the signalling cascade leading to plant response (Anderson et al., 1998). The plant’s antioxidant defence system, via enzymatic and non-enzymatic mechanisms (i.e. amino acids, glutathione, acid ascorbic, carotenoids, α-tocopherol), provides protec- tion against high levels of free radicals responsible for the Fig. 5 - Intracellular pH at different phenological stages (from St A 0 to St C). Changes in different tissues: Cushion, Vegetative Bud (VB), Flower Bud (FB) axis, Flower Bud (FB) primordial. 10 peroxidation of membrane lipids and the destruction of proteins (De Kok and Stulen, 1993). Antioxidative enzymes, individually or cooperatively, have been viewed as a defensive team that protect cells from active oxygen damage performing a detoxifying function (Kranner and Grill, 1996). Superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guai- acol peroxidase (GPX) and glutathione reductase (GR) are the main antioxidative enzymes involved in the reduction of H 2 O 2 to H 2 O (Schmidt and Kunert, 1986; Noctor and Foyer, 1998; Rojas-Beltran et al., 2000). Maximum H 2 O 2 accumulation could act as a signaling molecule to trigger the sequence of reactions to break endodormancy (Kuroda et al., 2002). During the dormancy process, biochemical analysis conducted on apricot flower buds revealed CAT and GPX as the most involved enzymes, with significant increases of activity at the release of endodormancy, par- ticularly in cultivars with a low or medium CR (Viti et al., 2013). Other metabolites such as glutathione and antioxi- dant proteins were associated with increased capacity for free radical scavenging (Siller-Cepeda et al., 1991). Glu- tathione (γ-glutamyl-cysteinyl-glycine) is generally con- sidered to be a ubiquitous sulfhydryl-containing tripeptide in living cells (Fahey et al., 1975), being the main reserve and long-distance transport form of reduced sulfhydryls which are indispensable for protein synthesis (Rennem- berg, 1982). It is an important metabolite in stabilizing the cell redox state during the cold hardening process (De Kok and Stulen, 1993). In particular, in stressed plants, reduced glutathione (GSH) protects protein thiol groups from au- to-oxidation (Kranner and Grill, 1996). GSH is oxidized to glutathione disulfide (GSSG) and, under normal con- ditions, is reduced efficiently back to GSH by the action of glutathione reductase (Foyer et al., 2001). The major portion of the glutathione in the cell is maintained in the reduced state, and a high reduced/oxidized ratio (GSH/ GSSG) is necessary for numerous physiological functions. Glutathione was suggested as one of the strong factors in controlling bud dormancy, first, in grape (Tohbe et al., 1998); later, in apricot flower buds a significant relation- ship between the end of endodormancy and the increase of GSH/GSSG ratio was found (Bartolini et al., 2004). In cultivars with low-medium CR the reduced and oxidized glutathione ratio (GSH/GSSG) increased in accordance with the end of endodormancy, while in cultivars with high CR the GSH/GSSG ratio was kept low during the rest period. This could suggest that a minimum threshold in antioxidant activities could be crucial for scavenging free radicals during the rest season. Hormonal involvement The endogenous system of hormonal regulation medi- ates the annual transition from growth to dormancy due to the shortened length of autumn days and reduction of daily mean temperatures (Thimann, 1985). Knowledge of hor- monal regulation of dormancy processes has become in- creasingly more complex, particularly with recent findings of auxin-and ethylene-triggered abscisic acid induction re- vealing more responses mediated by abscisic acid (ABA) than originally considered (Tanino, 2004). ABA has long been studied as a potential mediator of short-induced ces- sation of growth and initiation of bud dormancy in trees (Guak and Fuchigami, 2001). An increase in its activity or its accumulation in the fall is an indispensable condition for the onset of apical growth inhibition, and an essential prerequisite for emergence of the dormant state and the ability to adapt to winter frosts in apricot tress (Kuzina and Kalinina, 1993). Kawamata et al. (2002) found that free ABA levels in buds increased suddenly at onset of dormancy and decreased afterward. Chen et al. (2002) suggested that changes in dormancy status are more close- ly related to changes in ABA receptivity than to changes in ABA levels. ABA affects dormancy progression through its action on dehydrins or membrane permeability (Jacob- sen and Shaw, 1989). Moreover, the regulatory effect of ABA in growth inhibition, dormancy, and thermoregula- tion in woody plants is realized only with the action of other phytohormones: auxins, gibberellins, and cytokinins (Back and Richmond, 1971). Gibberellin GA 3 was tested in peach buds to promote bud burst under conditions of prolonged dormancy (Erez et al., 1971). The comparat- ively high levels of GA 3 found in mid-winter could be one of the factors that control the process of anther and gyn- oecium development (Basconsuelo et al., 1995). Molecular features Population and quantitative genetics studies indicate that phenological traits such as time to bud set, chilling requirement and time to bud flush show significant ge- netic variation, and that such traits are often controlled by multiple genes exhibiting small effects (Howe et al., 2003; Rohde et al., 2011). Dormancy signals, impact- ing numerous physiological processes, involve changes in the expression patterns of numerous regulatory genes that could play a key role in dormancy transition (Horvath et al., 2008). In contrast to paradormancy, the molecu- lar aspects of endodormancy are poorly understood. The molecular biology of endodormancy has been analysed in several recent studies by global approaches where an initial set of candidate genes involved in cold- or light-in- duced dormancy in tree species were described (Bielen- berg et al., 2008). Gyllenstrand et al., (2007) found a significant and close correlation between growth rhythm (both bud set and bud burst) and the expression pattern of an FT (flowering locus T) homologue, suggesting that FT is a key integrator of photoperiodic and thermal signals in the control of growth rhythms in gymnosperms. Addition- ally, the substitution of a single amino acid can transform an FT protein from an activator into a suppressor of flow- ering. Thus, the limited but tantalizing linkage between the floral regulatory machinery and seasonal growth ces- sation and bud set, through regulation of FT and FT-like genes, suggests a general model for endodormancy regu- lation (Horvath, 2009). Flowering locus C (FLC)-like genes have been shown to be regulated differentially during the satisfaction of 11 CR in vegetative buds of poplar (Chen and Coleman, 2006). Furthermore, Bielenberg et al. (2008) revealed a cluster of six MADS-box transcription factors (named dormancy-associated MADS-BOX or DAM genes) as candidate genes for the regulation of terminal bud for- mation in evergrowing peach. The expression of two of these genes, DAM5 and DAM6, is suppressed by chill- ing temperatures and inversely correlated with bud break rate in peach (Jimenez et al., 2010), whereas DAM4 and DAM6 expression is promoted by short photoperiods (Li et al., 2009). Similar genes were expressed differential- ly during dormancy induction, maintenance and release also in apricot. In Prunus mume, Yamane et al. (2008) have generated two SSH/MOS (subtractive hybridization supplemented with mirror orientation selection) libraries containing gene pools that are expressed preferentially in endodormant buds in comparison with paradormant or ecodormant buds to search for the genes that are up- regulated by endodormancy induction or down-regulated by endodormancy release. Differential screening and sequencing indicated that genes involved in gibberellin metabolism, stress resistance, cell wall modification, and signal transduction, such as transcription factors, are up- regulated in endodormant buds. At transition from dor- mancy to active bud growth, genes related to carbohydrate and energy metabolism have been specifically identified in Prunus armeniaca L. Céchová et al., (2012) have ob- served strong expression of xyloglucan endotransglyco- sylase/hydrolase (XTH) and EXGTA1 (endoxyloglucan transferase) in the week before, and during, the exit of apricot flower buds from endogenous dormancy. Research is still in progress to study changes in the ex- pression in regulatory genes involved in numerous physi- ological signals related to the dormancy process. 4. Influence of mild temperatures during the dorman- cy process The complex process of dormancy is affected by a close interaction between genotype and environment, where photoperiod and temperature are the main factors involved in triggering and releasing. Their individual and combined effects change during the transition from a dormant to a non-dormant state (Caffarra et al., 2011). Apricot culture is greatly restricted by climatic condi- tions, with a decisive influence on development and pro- ductivity (Quamme et al., 1982; Guerriero and Bartolini, 1991). It is well known that some cultivars are closely linked to their geographical origin and, consequently, have a low adaptability to other climatic conditions (Bassi et al., 2006). Autumn-winter temperature trends seem to be the main cause of this low plasticity related to the need for adequate and specific satisfaction of CR for dormancy breaking (Viti et al., 2010). A warm climate may prevent or delay this process. In fact, during winter mild tempera- tures have a negative impact on endodormancy release due to an unfulfilled CR. In this situation, apricot is frequently affected by the appearance of floral anomalies, e.g. pistil abortion and/or browning and/or necrosis of flower buds, which are usually attributed to unfulfilled CR. However, a correlation between flower anomalies and CR satisfaction was not always achieved (Guerriero and Bartolini, 1991; Viti and Monteleone, 1993; Legave, 2002). At present, knowledge about the mechanisms and nature of anomalies is still scant but, on several genotypes, a genetic determ- inism has been found through analysis of different apricot progenies (Legave et al., 2006). From a physiological point of view, climatic conditions affect inductive signals regarding metabolism and phyto- hormones involved in the control of several events such as primary vascular differentiation micro- and macrosporo- genesis (Aloni, 1980; Creber and Chaloner 1984; Fukuda, 1996). In apricot flower buds, xylem differentiation was found to be slow when the winter minimum mean tem- peratures were predominantly below zero (Bartolini et al., 2006 a). Moreover, during the autumn-winter season, af- ter a minimum threshold of chilling amount, the supply of constant warm temperatures stimulated the development of vascular elements, at least in cultivars with a low-medi- um CR (Bartolini and Giorgelli, 1995). As regards the mi- crosporogenesis process, the post-meiotic phase is crucial because temperature may influence the transition from tet- rads to pollen grains (Viti and Scalabrelli, 1988). A lack of synchronism between dormancy release, xylem differen- tiation and microsporogenesis evolution has recently been demonstrated under warm winter conditions (Bartolini et al., 2006 b; Andreini et al., 2012). The negative role of certain temperatures is confirmed also during the ecodormancy stage. In particular, tempera- ture fluctuations affect different stages of reproductive development and this stress might lead to developmental asynchrony in pollen–pistil–ovule functioning, leading to reduced fertilization levels (Hedhly, 2011). Considering such events, determining the effective- ness of temperature regimes on dormancy release has, for a long time, been a focus of many studies. Several models have been proposed to predict the response of buds to chill- ing, establishing the CR of each specific genotype. The most simple one was introduced by Weinberger (1950) who simply defined the ‘chilling hours’ as the number of hours at or below 7°C. The method of Bidabé (1965) calculates the effect of given temperatures either for chill- ing or heat requirements by exponential models. The Utah model weights the efficiency of different temperatures for CR fulfillment (Richardson et al., 1974 and 1975). This model is the best tailored for cool and temperate regions (Seeley, 1996), while under warm conditions (i.e. the sub- tropical regions) Erez et al. (1990) proposed the ‘dynamic model’ as a better indicator for peach CR estimation. The model assigned negative values to high temperatures (ne- gation of rest) during endodormancy (Allan et al., 1993). Bonhomme et al. (2010), indicate a simplified smoothed Utah model as the best for French conditions, introduc- ing a more broad range of effective temperatures to break dormancy. 12 …and what about dormancy processes with regard to cli- matic changes? Global warming of the climate system is unequivocal from observations of increases in average air temperatures in many parts of the world (Legave et al., 2009). Since the end of the 1980s, this temperature change has influenced plant phenology, and in the future further climate changes will probably have an impact on crop yields (Chuine and Cour, 1999; Chmielewski et al., 2004). Mean temperatures will probably rise between 1.8 and 4.0°C by the end of the 21st century, according to IPCC reports. Over the period 1910-2003, climate warming was al- ready in motion with a minimum temperature increase of 0.25°C per decade, as reported for the Californian San Joaquin Valley (Baldocchi and Wong, 2007). The most striking feature of climate change in eastern Asian countries during the past century may be the remarkable winter-season warming (Kwon et al., 2008). As a con- sequence, significant impact can be expected on winter dormancy and spring bud-burst for crops and natural vegetation in this region, as has been observed in Eu- rope (Chmielewski et al., 2004). In the Mediterranean area, the winter climate shows a tendency to become progressively milder (Guerriero et al., 2010; Menzel et al., 2011). In this context, certain years (i.e. 2006-2007) were characterized by constantly very mild autumn and winter temperatures: the minimum values occasionally went below 0°C, while the maximum values often ex- ceeded 15°C (Luterbacher et al., 2007; Viti et al., 2010). Thus, over the past twenty years a progressive reduction of chilling amount was recorded (Fig. 6). In the global warming scenario, phenological pro- cess-based models are considered the best tool to study the climatic changes and subsequent expected phenology alteration (Chuine et al., 2003). BRIN is a recent phen- ological model, calibrated for grapevine, able to predict, simultaneously, timing of budburst and flowering (García de Cortázar-Atauri et al., 2009). This model computes the dormancy period using Bidabe’s Cold Action model (Bid- abé, 1965), and the post-dormancy period by the sum of hourly temperatures (growing degree hours-GDH, method of Richardson et al., 1974). Recently, the BRIN model has been calibrated and validated also for apricot cultivars un- der the climatic conditions of southern France allowing an effective prediction of dormancy release date (Andreini et al., 2013). As a consequence, BRIN model could be ap- plied in a “A1B” future scenario taking into account two aspects valuable for all cultivars: a delayed dormancy break and an early flowering. Advances in modeling will be made using the experimental data of dormancy release to calibrate forecast models that can reproduce the physio- logical behaviour of the three. According to Bonhomme et al. (2010) further study will consist in testing a wide panel of data and also assessing the introduction of the optimized endodormancy release model as sub-model into bud break/ bloom phenological model. This will likely improve them and could be very interesting for phenological predictions in the global change context. Concluding remarks and perspectives Survival and competitive success of woody plants depend on a precise timing of growth, winter dormancy in synchrony with seasonal changes in temperature. In particular, apricot is a species sensitive to satisfying the CR and thus achieving the release of dormancy and bud break. Several cultivars of different geographical origins are heavily affected by the problem of dormancy release, which is one of the main causes of inconstant yields. The effect of temperature on the induction and depth of dor- mancy may explain some of the observed annual variation in dormancy and CR for its release. The picture emerging from this work points to a com- plex relationship between winter temperatures and several biological processes in response to overcoming dormancy in flower buds. In particular, it has been demonstrated that climatic conditions characterized by mild autumn-winter seasons can affect the regular development of flower bud organs, leading to the appearance of anomalies. Moreover, several studies have indicated that an asynchronism among biological (i.e. endodormancy release, microsporogenesis evolution), anatomical (i.e. xylem vessel differentiation) and biochemical (i.e. changes in metabolic compounds and enzymes) events could represent further causes for an inconstant rate of blooming. At this time, knowledge of the mechanisms involved in dormancy is still fragmentary although recent studies of global gene expression in dif- Fig. 6 - Chilling Units accumulation from 1991 to 2012 : a) during win- ter season, b) on 31 January. Data recorded under the climatic conditions of the Tuscany coastal area. 13 ferent species have carried us several steps forward, pro- viding an excellent basis for elucidating function of genes and components thought to be involved in temperature and other environmental signaling. In a context of global warming, mild winter temperat- ures could greatly impact apricot cropping systems. Forth- coming research on this particular topic will be crucial and of great economic importance. A new model, calibrated and validated on apricot cultivars, is proposed to predict the dormancy release date in a future scenario. Specific breeding programs, focusing on rustic cultivars more ap- propriate for particular environmental conditions, should provide additional improvements in apricot culture in the Mediterranean basin. 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