Ice Nucleation and Supercooling in Freezesensitive Peach and Sweet Cherry Tissues

Abstract

Abstract Ice nucleation temperatures were measured in peach [Prunus persica (L.) Batsch ‘Redhaven’] and sweet cherry (Prunus avium L. ‘Bing’) flowers, fruits, and stems by differential thermal analysis and by controlled freezing in test tubes and of droplets. The mean nucleation temperatures of peach and sweet cherry flowers were –4° to – 6°C, while those of 0.5-cm stem segments were –6° to –7°. The highest ice nucleation activity in peach flowers appeared to be associated with the pedicels. A logarithmic function described the relationship between stem weight and ice nucleation temperature, with 1.5-g (16-cm) stem segments freezing as high as –3°. Apparently, stem tissue contained a lower concentration of ice nucleators active above –5° than did the floral tissue. Homogenization of the tissues reduced their ice nucleation activity. Structural integrity may be necessary for the manifestation of optimal activity. Ice nucleation-active (INA) Pseudomonas syringae bacteria were isolated from the surfaces of peach and sweet cherry floral tissues only occasionally and then at low populations. The absence of epiphytic INA bacteria from these tissues suggested that ice nucleation was promoted by intrinsic ice nucleators within the plant. These intrinsic ice nucleators appeared to be active at temperatures similar to INA bacteria. Although these nucleators may be less numerous than bacterial ice nuclei on heavily colonized plant surfaces, their presence nonetheless suggests that control of INA bacteria or inhibition of the ice nucleation activity of INA bacteria alone may be insufficient to prevent ice nucleation and freezing injury of freeze-sensitive flowers and fruits during spring frosts in the orchard.