The use of differential thermal analysis in determining the critical temperatures of sweet cherry (Prunus avium L.) flower buds at different stages of bud burst.

Many studies to date on the response of cherry flower buds to frost have focused on visual evaluations by observing tissue browning after frost event in the orchard and laboratory conditions but only little knowledge is available on the consequences of intracellular ice formation on cellular ultrastructure that underlies exothermic reactions during bud burst stages. In this study, the differential thermal analysis (DTA) method was used to investigate critical frost temperatures for the sweet cherry cultivars '0900-Ziraat', 'Erzincan Macar', 'Lambert', 'Vista', 'Stella', and 'Early Burlat' under laboratory-based freeze assays. In the course of our experimental study, frost tolerance or cell death points (CDPs) of flowers of six cherry cultivars were investigated in consecutive phenological stages from the start to the end of blooming, for 2 years. The frost tolerance of flower buds changed according to different developmental stages and cherry cultivars. Our results of frost tolerance tests performed on the cherry blooming stages are rather controversial. Our findings have shown that at the open cluster stage, the frost tolerance of the flower buds is very sensitive (mCDP = -1.18°C for 'Lambert'), while the first white stage has revealed an important increase (mCDP= -9.96°C for '0900-Ziraat') in the frost tolerance of those. Averaged over 2 years, the temperatures causing 50% frost damage for flower buds were -2.08 to -3.76°C at the side green stage, -1.49 to -3.22°C at the green tip stage, -1.18 to -1.98°C at the open cluster stage, -7.92 to -9.96°C at the first white stage, and -6.29 to -9.36°C at the full bloom stage in the range of six cultivars. Based on our test results, '0900-Ziraat' and 'Vista' were regularly classified as frost-tolerant cultivars. The flower buds of 'Lambert' and 'Early Burlat' have been regularly the most sensitive, while 'Erzincan Macar' and 'Stella' were ranked into the group of medium sensitivity. These results can help farmers to estimate possible frost damages on sweet cherry flower buds due to frost events at the investigated phenological stages.