Freezing stress damage and growth viability in Vaccinium macrocarpon Ait. bud structures

After a freezing event, it can be challenging to extrapolate levels of freezing damage to plant growth viability based on the presence or absence of symptoms in specific bud tissues. This study investigated the relationship between freezing damage in terminal buds during ecodormancy and their viability during the subsequent growing season. We identified the bud structure that best explained this relationship, and developed a model to explain the changes in bud cold hardiness. Vertical shoots (uprights) of Vaccinium macrocarpon Ait. were sampled in central Wisconsin during Spring of 2018 and 2019. Sets of uprights with terminal buds were subjected to controlled freezing tests, followed by either visual freeze damage evaluation or assessment of shoot viability by growth assays. We determined the Browning Lethal-Temperature₅₀ (BLT₅₀ ), as temperature for 50% damage (tissue browning) at each bud structure, and Growth Lethal-Temperature₅₀ (GLT₅₀ ) temperature where 50% reduction in growth viability occurred. Two models were constructed to explain: (1) bud structure damage and growth viability, and (2) GLT₅₀ 's seasonal changes, representing the cold hardiness variations, and environmental factors. The correlation between the BLT₅₀ and GLT₅₀ values was closest for the bud scales and bud axis, indicating the better correspondence between levels of freezing damage with the impact on the growth potential. In addition, the latter was also the most suitable candidate for modeling due to easier damage evaluation. The freezing stress damage of the bud axis explained comparatively best the resulting growth viability. Seasonal changes in GLT₅₀ were best explained by temperature indices based on daily minimum and on maximum temperatures over 10-day periods. However, among the model components, daily maximum temperatures had the greatest influence on V. macrocarpon cold hardiness changes during ecodormancy.