Understanding Flower Frost Tolerance in Almond (Prunus dulcis): The Role of Phenology, Cultivar and Sugars Content

Freezing temperatures during the flower and fruitlet stages are considered one of the most limiting factors for almond cultivation. Understanding the minimum temperature that reproductive organs can resist without damage is crucial for adapting the crop to different environmental conditions and for breeding cultivars with enhanced tolerance to frost damage. Accordingly, this study examined frost tolerance progression across various phenological stages as well as assessed frost tolerance in 20 almond cultivars during full bloom. Almond cultivars exhibited a noticeable decline in frost tolerance as they advanced through the studied phenological stages, with the highest vulnerability occurring after the fruit set. Phenotyping for frost tolerance at the flowering stage revealed significant differences within cultivars, with most experiencing 50% flower damage at temperatures around −4.0°C. Among the studied cultivars, ‘Vairo’, ‘Tarraco’, ‘Lauranne’, ‘Marinada’, ‘Tuono’ and ‘Penta’ exhibited the highest tolerance to flower freezing, in contrast to ‘Marta’, ‘Marcona’ and ‘Francolí’ which showed the least. To further explore the relationship between physical and chemical traits and lethal temperatures, ovary and pistil weights were measured, along with the determination of the sucrose, fructose and glucose content in the pistils. Correlation analyses revealed that higher pistil sucrose content was associated with increased flower tolerance to freezing temperatures, suggesting that sucrose content enhances, to a certain extent, tolerance to frost damage at the flowering stage. This study provides valuable insights into assessing freezing tolerance within the almond germplasm, offering growers and breeders crucial information for selecting the most well-adapted cultivars in each environment.