SummaryThe effects of the ice nucleation-active (INA) bacterium Pseudomonas syringae were studied on the physiological response and ultrastructure of the floral organs of ‘Longwangmao’ apricot (Prunus armeniaca Linn.). Inoculation with INA bacteria increased the super-cooling point (SCP) of petals, stamens, and pistils by 2.1ºC, 1.6ºC, and 1.4ºC respectively. INA bacteria also enhanced the relative electrolyte leakage (REL) of membranes in the floral organs, indicating that INA bacteria increased the permeability of membranes. INA bacteria enabled apricot floral organs to maintain relatively lower superoxide dismutase (SOD) and peroxidase (POD) activities, and led to an accumulation of malonyldialdehyde (MDA) under cold stress. The ultrastructures of pollen walls, organelles in the pollen cell, ovules and egg cells in control flowers which were subjected to –3ºC changed slightly, compared with the structures at 18ºC. However, under cold stress at –3ºC, inoculation with INA bacteria resulted in disintegrated floral organs, such as a disrupted and extruded pollen exine, and even destroyed organelles in pollen cells. Plasmolysis and the formation of small vesicles and cavities in nucellar cells occurred in INA bacteria-inoculated ovules. INA bacteria also induced swelling and the eventual disappearance of the nuclear envelope of the egg cell in the ovule. These results, the decreased activities of protective enzymes, the increased SCP, REL, and MDA contents, and the disintegrated ultrastructures of floral organs inoculated with INA bacteria under cold stress, suggest that INA bacteria decreased the tolerance of floral organs to frost and aggravated the level of damage.