Last updates on cell death point, bud death time and exothermic characteristics of flower buds for deciduous fruit species by using differential thermal analysis

The fine details of freezing behavior in flower buds during deacclimation and their exothermic characteristics are poorly understood with the use of differential thermal analysis (DTA). Understanding how spring frosts affect cold tolerance of flower buds is a fundamental question in deciduous fruit species and will be useful in developing cold-tolerant varieties and species. The occurrence of cell death point (CDP), bud death time (BDT) and exothermic characteristics in apple (Malus domestica L. 'Golden Delicious'), pear (Pyrus communis L. 'Santa Maria'), quince (Cydonia oblonga L. a native wild Type-24'), sour cherry (Prunus cerasus L. 'Kutahya'), wild apricot (Prunus armeniaca L. 'Zerdali-24'), nectarine (Prunus persica var. nectarine L. 'Armking'), peach (Prunus persica L. 'Loring'), plum (Prunus salicina L. 'Formosa') and cherry (Prunus avium L. 'native wild type') flower buds in pink-while bloom and at the full bloom stages were determined using DTA. During controlled freezing of flower buds in deciduous fruit species one distinct freezing events (single exothermic peak) was detected by DTA, and it was considered the cell death point of the flower buds. The fruit species were classified as most (sour cherry, peach, quince and cherry), moderately hardy (apple and pear) and least hardy (wild apricot, plum and nectarine) according to tolerance to low temperatures. The highest mean BDT values of flower buds were observed in pear and nectarine species compared to other species. The flower buds of nectarine and pear species showed large and wider single exothermic peaks compared to quince, sour cherry, wild apricot, plum, apple, cherry and peach species. The single exotherms in the flower buds of the fruit species support the hypothesis that supercooling in buds is lost mainly starting with bud swell and usually spreads rapid ice nucleation into the flower buds due to established xylem continuity between the woody tissues and the bud scales. This hypothesis, however, reflects the complexity of the freezing process in flower buds, and so indicates that comprehensive observational studies are essential to understanding freezing tolerance traits in deciduous fruit species, after and during budbreak.