Low temperature damage of peach floral buds as assessed by vital staining

Spring freezes in southeast US can cause damage to peach flowers after plant de-acclimation. Winter, on the other hand, is usually not cold enough to produce major damage. However, understanding the magnitude of internal damage produced by freeze events in peach floral buds is of importance. The objective of this project was to analyze the susceptibility of floral bud internal structures to freeze events using a vital staining technique. Floral buds were collected in winter of 2016-2017, and treated with low temperatures ranging from 4 to -24°C in a temperature-controlled programmable freezing chamber. Fluorescein diacetate (FDA) staining was used to identify the vital structure of floral buds that were damaged after being subjected to low temperature treatments. Viable tissues stained with FDA presented a green fluorescence under 450-490 nm blue light. Cold damage was observed as a dark non-fluorescent region. Critical bud freezing temperatures that caused damage in 50% of samples (LT₅₀) based on the fluorescence were calculated for each internal floral structure. For floral buds collected during the dormant bud stage (tight bud) on November 21, 2016, the flower pistils were short, ovaries were not swollen, and stamens were immature. We found that during this time, young pistils tended to show cold damage first, with a LT₅₀ = -15.4°C, followed by anthers (LT₅₀ = -16.8°C), and then petals and sepals (LT₅₀ = -17.2°C). As the floral buds further developed in the field in spring of 2017, internal structures in the floral buds developed with cold susceptibility of these structures being variable. Stamens tended to be most cold resistant (LT₅₀ = -15.2°C), and pistils and corollas were more cold susceptible (LT₅₀ = -12.0 and -11.0°C, respectively). This study offers important information on cold damage associated with specific floral bud internal structures.