Relationship of Air Temperature to Water Content and Supercooling of Overwintering Peach Flower Buds

Abstract

In overwintering flower buds of ‘Redhaven’ peach [ Prunus persica (L) Batsch], seasonal changes in the supercooling point were correlated significantly with seasonal changes in water content of the whole flower bud and its parts, the flower primordium within the flower bud and the vascular traces just below the flower primordium. Both the supercooling point and the water content of the whole flower bud and its parts were correlated significantly with the 2- and 5-day mean air temperature preceding collection. Controlled-temperature studies with ‘Siberian C’ and ‘Redhaven’ revealed that the water content of the flower primordium and vascular traces and not that of the whole flower bud was critical to the level of supercooling attained. Supercooling of the flower primordium appeared to be related directly to its water content. Water content of the vascular trace appeared to be important during the initial stages of freezing for preventing spread of the ice boundary into supercooled regions of the flower primordium. Water content of the 2 critical tissues appeared to be determined by migration from and to these tissues during freezing and thawing. During freezing, water was lost from the flower primordium and vascular traces by migration to the flower bud scales which appeared to act as an ice sink. After prolonged exposure to freezing outdoors, water was also lost to the exterior of the flower bud. After thawing, the flower primordium and vascular traces regained water content by transfer from the ice sinks within the flower bud and sources external to the flower bud. Loss of flower bud hardiness occurred during storage at 0°C for 10 days, which was not due to change in water content of the flower bud or its parts. The excised flower buds of the hardier ‘Siberian C’ supercooled to a lower temperature than excised flower buds of the less hardy ‘Redhaven’ at equivalent water content.