Differential scanning calorimetry: a novel approach to study spring frost tolerance in pome tree floral tissues

Spring frosts occur in many pome fruit-producing regions globally. They are highly detrimental to floral tissues and yield, making frost tolerance of the reproductive organs one of the major breeding challenges. Currently, frost tolerance of flowers and floral buds is determined by meticulous observations of frost damage symptoms, or by methods that relate the damage with the accumulation of physiologically-relevant biochemicals. These methods are often inaccurate and are only feasible in instances of severe frost. We propose differential scanning calorimetry (DSC) to assess the frost tolerance of pome fruit floral tissues by measuring the heat flow of tissue samples when passing the freezing transition. DSC was applied to floral organs isolated from recently open king flowers of the apple 'Jonagold' (<italic>Malus domestica</italic> Borkh.) and the European pear 'Conference' (<italic>Pyrus communis</italic> L.) to simulate frost and determine freezing temperature as a quantitative indicator of frost tolerance. Freezing, crystallization, and melting points were measured by cooling and heating isolated ovules, stamens, and stigmas of mature king flowers from 20 to −40 °C and back to 20 °C. In pear, tissue-specific effects were observed, with ovules showing the highest (−9.6 °C) and stamens showing the lowest average freezing temperature (−13.1 °C), indicating that stamens are less susceptible to frost. In contrast, no significant differences in frost tolerance were detected among apple organs (freezing temperature of −11.1 °C). This study shows that DSC is an efficient method for monitoring and evaluating frost tolerance in pome fruit floral tissues and could be utilized for high-throughput phenotyping in breeding programs.