2017 ASHS Annual Conference

Chilling requirement (CR) is the minimum cold exposure necessary for dormant bud break in woody plants and therefore is a major limiting factor for growth of many temperate-zone plants in tropical and sub-tropical regions. Together with heat requirement (HR) CR affects the bloom date (BD) and susceptibility to late spring frosts. CR, HR and BD are among the most important factors to consider when selecting cultivars for local climatic conditions to ensure sustainable and profitable production. CR, HR and BD are quantitative traits and highly variable in peach. Climate variations in recent years are causing insufficient chilling resulting in poor bud break and inconsistent leafing and bloom. Incomplete CR fulfillment also may lead to the higher flower-bud drop and abnormalities in fruit morphology. Dormancy maintenance and release in peach is governed by the family of MADS-box genes. QTL mapping in peach and related Prunus species showed overlap between CR and BD associated regions, and candidate gene studies suggested conservation of the flowering control gene networks across Prunoideae. Climate resilient peach would have low CR and high HR. To achieve that combination, we need to clarify the CR, HR and BD phenotypes. While BD is relatively easy to phenotype, by being a product of CR and HR it potentially incorporates independent variation in those two traits, complicating efforts to identify genetic loci for breeding. Generally, CR dominates large scale BD variation between years and between cultivars. However, the role of HR variation as a cause of BD variation has been less appreciated. Attempts to quantify HR from field data is complicated by the ability of chilling temperatures to reduce the HR for bloom in a range of chilling above a minimum critical value up to a saturating chilling value. These minimum and saturating chilling accumulations are not known for most cultivars. Large scale phenotyping of CR has been done in a few cases for mapping families, but not at the level of minimum and saturating chill values. Potential HR variation in base and optimal temperatures for heat accumulation in cultivars needs to be determined to understand if there is a different quantitative HR (similar to variation in CR). Analysis of the US peach breeding germplasm for genetic variation in CR associated regions in peach and potential limitations in breeding for CR, HR and BD will be discussed.