The Effects of Ice Nucleation-active Bacteria on Temperatures of Ice Nucleation and Freeze Injury of Prunus Flower Buds at Various Stages of Development

Abstract

Abstract The effects of high (105 to 107 colony-forming units/g) and low populations of ice nucleation-active (INA) Pseudomonas syringae, strain B301D, on the ice nucleation temperatures and the amount of frost injury were determined for ‘Redhaven’ peach [Prunus persica (L.) Batsch.] and ‘Bing’ sweet cherry [Prunus avium (L.)] flower buds. The experiments were conducted from bud swell in early spring through the small, green fruit stages of development on INA bacteria-inoculated and -noninoculated flower buds that were either excised or attached to 10-cm segments of woody stem tissue. Shoots were immersed one minute in an INA bacterial suspension or sterile buffer 4 hours prior to freeze-testing. Excised-noninoculated buds supercooled to near −4°C before the first exotherm was determined, whereas inoculated buds attached to the stem supercooled only to −2.5°. Ice nucleation temperatures were about −3.0° for excised-inoculated and attached-noninoculated flower buds. The stem tissue contained a nonbacterial source of ice nucleation, active at temperatures similar to INA bacteria, that limited supercooling in the floral organs. While the ice-nucleation temperatures did not change with respect to bud development, the susceptibility of the floral organs to frost injury did change. Before the emergence of the petal tips through the calyx (frost-tolerant phase), the percentage of injury was reduced at higher ice nucleation temperatures, whether induced by stem tissue or inoculation with INA bacteria. Temperatures of −8 to −10° were required to give about 50% injury during this frost-tolerant phase compared with −3 to −4° in the later, frost-sensitive phase. Within a week of petal tip emergence from the calyx, the direct relationship between ice nucleation temperature and frost injury reversed, and the percentage of injury was inversely proportional to supercooling in flower buds. The flowers and fruit were injured by ice formation during the frost-sensitive phase regardless of whether ice was induced by INA bacteria or woody stem tissue.