ASHS 2025 Conference

Freezing temperatures limit blackberry (Rubus spp.) growers from meeting their full yield potential in the Pacific Northwest (PNW). Damage from freezing winter temperatures spans a gradient and ranges from complete mortality to partial damage of floral buds and supporting vascular and structural tissues. Partial damage can permit resumption of plant growth but is poorly characterized. Moreover, understanding the mechanisms contributing to this gradient of damage could lead to novel practices focused on mitigation and recovery. The objective of this study was to investigate the relationship between freeze damage and floral bud re-growth. Once chill unit accumulation was sufficient, floricane samples of ‘Victoria’ blackberry were collected for five consecutive weeks during late winter from a commercial farm in Skagit County, Washington. Sampled canes were cut into smaller cuttings, each containing one single floral bud, and exposed to freezing temperatures (-4 C°, -6 C°, -8 C°, -10 C°, and -12 C°) in a glycol bath. The freezing temperature treatments were based on historical freezing temperatures in the area over the past decade during the sample collection period. Treated samples were then placed in suitable growing conditions (22 C°, 16 h light), and shoot growth, chlorophyll fluorescence (Fv/Fm), and gas exchange were measured after three weeks. Shoot growth was delayed from buds exposed to -8 C°, -10 C°, and -12 C° compared to the untreated control. The delay in re-growth is likely explained by limitations in water transportation to the leaves rather than carbohydrate depletion, considering leaf transpiration and stomatal conductance were significantly lower in treated canes compared to the untreated control, while Fv/Fm and carbon assimilation remained unaffected. Findings from this study demonstrate partial re-growth from freeze-damaged blackberry canes and floral buds, and damage to the xylem is likely responsible for reductions in re-growth.