Abstract
In temperate trees, growth resumption in spring time results from chilling and heat requirements, and is an adaptive trait under global warming. Here, the genetic determinism of budbreak and flowering time was deciphered using five related full-sib apple families. Both traits were observed over 3 years and two sites and expressed in calendar and degree-days. Best linear unbiased predictors of genotypic effect or interaction with climatic year were extracted from mixed linear models and used for quantitative trait locus (QTL) mapping, performed with an integrated genetic map containing 6849 single nucleotide polymorphisms (SNPs), grouped into haplotypes, and with a Bayesian pedigree-based analysis. Four major regions, on linkage group (LG) 7, LG10, LG12, and LG9, the latter being the most stable across families, sites, and years, explained 5.6-21.3% of trait variance. Co-localizations for traits in calendar days or growing degree hours (GDH) suggested common genetic determinism for chilling and heating requirements. Homologs of two major flowering genes, AGL24 and FT, were predicted close to LG9 and LG12 QTLs, respectively, whereas Dormancy Associated MADs-box (DAM) genes were near additional QTLs on LG8 and LG15. This suggests that chilling perception mechanisms could be common among perennial and annual plants. Progenitors with favorable alleles depending on trait and LG were identified and could benefit new breeding strategies for apple adaptation to temperature increase.
Highlights
Global warming has an impact on tree phenology, and rising temperatures in late autumn and winter can lead to problematic conditions regarding bud dormancy release
Dormancy has been described as being composed of three states: (i) paradormancy, which is regulated by physiological factors outside the bud, such as apical dominance; (ii) endodormancy, which is regulated by physiological factors inside the bud and during which the perception of chilling temperatures is active; and (iii) ecodormancy, which is regulated by environmental factors, especially temperature (Lang et al, 1987)
For Angers, the dormancy release date predicted for ‘Golden Delicious’ seemed to be stable, with a maximum of a 3 d difference between years, whereas a larger variance was observed in the number of days to fulfill the heat requirement (HR), which varied between 77 d and 95 d
Summary
Global warming has an impact on tree phenology, and rising temperatures in late autumn and winter can lead to problematic conditions regarding bud dormancy release. Dormancy release is mainly driven by exposure to chilling temperatures (Heide and Prestrud, 2005; Schoot et al, 2013), whereas growth rate is driven by warm temperatures (Wigge, 2013). The chilling requirement (CR) corresponds to the amount of exposure time to cold temperatures required for dormancy release and is usually expressed in chilling hours (Anderson and Richardson, 1986). The heat requirement (HR) represents the amount of exposure time to warm temperatures required to reach a particular physiological stage, generally budbreak or flowering time.
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