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Temperate tree species have the ability to cease meristem activity in the fall and establish a dormant state (endo-dormancy or true dormancy) in which the meristem is rendered insensitive to growth-promoting signals before it is released (Rohde & Bhalerao, 2007). The chilling requirement (CR) refers to the duration of low temperatures required for the release of temperate trees from endo-dormancy. CR prevents trees from initiating growth in response to transient warm temperatures, thus avoiding damage by subsequent frost(s) in the late winter or early spring. CR is the result of long-term climatic adaption of genotypes of tree species developed in different regions. Conversely, it limits the climatic distributions of the genotypes of temperate fruit trees (Sherman & Beckman, 2003). CR is the major factor determining the bloom date (BD, also referred to as the flowering time) (Egea et al., 2003; Ruiz et al., 2007; Alburquerque et al., 2008), which is an important agronomic trait affecting the seed and fruit development of temperate fruit tree species. Genotypes with low CR bloom early in cold regions/years and are susceptible to late frost damage (Scorza & Okie, 1990). Genotypes with high CR could suffer inadequate chilling in warm regions/years, resulting in irregular floral and leaf bud break and thus poor fruit set, which is potentially problematic with the current global warming trend (Topp et al., 2008). On the other hand, in temperate fruit tree species, early ripening cultivars are often preferred because of better early market prices for their fruits (Ruiz et al., 2007; Topp et al., 2008). Breeding for earlier BD (often associated with low CR) is one approach to obtaining earlier ripening fruit with adequate size.
Heat requirement (HR) is another factor determining the BD of cultivars of temperate tree species (Richardson et al., 1974; Citadin et al., 2001). It is unclear whether heat accumulation for floral or vegetative bud break starts before or after the release of endo-dormancy. It has also been reported that extended chill (more than CR) results in a reduction in HR of tree buds (Scalabrelli & Couvillon, 1986; Citadin et al., 2001; Harrington et al., 2009). These two issues complicate the quantification of the variation of HR among different genotypes. Currently, the growing degree hour (GDH) model developed by Richardson et al. (1975) is the most widely used (Citadin et al., 2001; Egea et al., 2003; Ruiz et al., 2007; Alburquerque et al., 2008), but it only counts the heat accumulation from endo-dormancy release to full bloom.
Among the three interrelated traits, BD is considered to be quantitatively inherited in most fruit tree species (Anderson & Seeley, 1993), CR is considered to be semi-qualitatively inherited in apple (Malus × domestica Borkh.) (Hauagge & Cummins, 1991), and no study has yet been reported on the genetic nature of HR.
Quantitative trait locus (QTL) mapping results for BD in various genomic regions in Prunus has been reported. Using the terminology of the almond (Prunus amygdalus L.) (cv. Texas) × peach [Prunus persica (L.) Batsch] (cv. Earlygold) map (T × E Prunus reference map) on linkage groups (G), four QTLs on G1, G4, G6 and G7 were detected by Joobeur (1998) in an almond × peach F2 population, two QTLs on G2 and G7 by Dirlewanger et al. (1999) in a peach F2 population, one major gene (Late blooming) on G4 by Ballester et al. (2001) in an almond F1 population, and one QTL on G4 by Verde et al. (2002) in a peach backcross (BC1) population. A candidate gene approach associated only two of ten candidate genes homologous to LEAFY and MADS-box genes in Arabdopsis with two QTLs in almond (Silva et al., 2005), suggesting that direct application of the knowledge of the genetic control of the flowering time of annual plants to perennial tree species may be more complicated than expected.
There have been no reported results on the successful mapping of QTLs associated with CR for floral bud break in temperate tree species. However, two genetic studies have indicated that CR is in control of at least one major gene with dominant low CR allele(s) (Hauagge & Cummins, 1991;Tzonev & Erez, 2003). With regard to HR, almost no genetic studies have been reported. It is even unclear whether HR is an intrinsic characteristic in several fruit tree species (Couvillon & Erez, 1985; Ruiz et al., 2007).
In temperate and subtropical regions, peach is widely grown and economically important. As a proposed tree model species (Abbott et al., 2002), its self-compatibility and short generation cycle (2–3 yr) enable the relatively easy development of true F2 populations and early characterization of floral and seed-related traits. Its diploidy and the availability of a large number of mapped simple sequence repeat (SSR) markers transferable within Prunus greatly facilitate linkage map construction. The small genome size (c. 220 Mbp; B. Sosinski, North Carolina State University, Raleigh, pers. comm.) and extensive genomics/genetics resources available at the Genome Database for Rosaceae website (http://www.bioinfo.wsu.edu/gdr/) enable the map-based cloning and annotation of genes controlling important agronomic traits for tree arboriculture, and the development of markers inside or tightly linked with these genes for marker-assisted breeding applications. However, to achieve these goals, it is critical to have a detailed resolution of the location of the genomic regions (QTLs) harboring these genes.
The major objective of this research was to identify QTLs associated with CR and CR-related traits using a peach F2 population derived from two genotypes with contrasting CR values: the high CR cv. ‘Contender’ and the low CR selection ‘Fla.92-2C’. The F2 progenies segregate in a continuous fashion for a variety of traits, including CR, HR and BD. Utilizing this mapping population, we obtained the first data on the genomic regions (QTLs) determining floral bud CR and HR, and provided the first picture of potential genetic interrelationships among CR, HR and BD in temperate tree species.