Developmental Sensitivity of European Pear and Sweet Cherry Floral Buds to Sub-zero Temperatures From Late Dormancy Through Petal Fall

Late winter and early spring freeze events can have devastating impacts on tree fruit production.  Our primary objective was to determine the critical temperatures resulting in 10%, 50%, and 90% reproductive bud mortality at distinct phenology stages of sweet cherry and European pear cultivars from late dormancy through anthesis.  A secondary objective was to phenotype the variability in reproductive bud development throughout this period.  Samples were collected from two sites, weekly, beginning in mid-January through petal fall on commercially important cultivars of sweet cherry [‘Sweetheart’, ‘Skeena’, ‘Regina’ and ‘Bing’ (standard)] and European pear [‘Bosc’, ‘d’Anjou’, ‘Starkrimson’, and ‘Bartlett’ (standard)].  All buds were frozen at 1 °C per hour.  Ice nucleation temperatures of sweet cherry floral buds were detected by a high-throughput differential thermal analysis (DTA) system between dormancy and first swelling (earliest visible stage of bud development).  Subsequent to bud swelling, DTA lost the capacity to accurately detect individual flower exotherms; therefore, bud hardiness of advanced phenology stages was based on the visible presence or absence of oxidative browning following freezing. Ice nucleation events within dormant and transitioning pear buds were not detectable during freezing.  We experimentally established that water migration from buds, presumably to extracellular ice, was the cause of this; hence, microscopy was used to assess pear flower bud hardiness throughout the entire developmental period.  Anthers and pistils of both cherry and pear flowers were rated separately.  The relationship between temperature and bud survival was explained by a sigmoidal function.  DTA and visual assessment data were well-aligned during late dormancy and bud swell.  In fact, data suggest that DTA may be useful for detecting growth resumption and/or increased metabolism prior to visible changes in physical properties of reproductive cherry buds.  Significant differences in hardiness among cultivars were evident between late dormancy and Stage 3 of development; general hardiness ranking in cherry was ‘Regina’ >> ‘Skeena’ > ‘Sweetheart’ ≥ ’Bing’; and, for pear ‘Bosc’ >> ’d’Anjou’ > ’Bartlett’ ≥ ‘Starkrimson’.  These genotypic effects were largely attributed to early developmental differences among cultivars, and were reduced or disappeared entirely with advancing bud development.  Variability in bud stage was more pronounced in cherry than pear.  Given the significant differences among bud stages for critical temperatures, combining a frequency distribution of bud stages with their respective critical temperatures provided a comprehensive analysis of freeze susceptibility and potential crop loss on an orchard scale.