Evaluation location: New Jersey, United States
Plant material.
In total, 25 rabbiteye blueberry cultivars were used for this study. Notes on the origins and unusual germplasm compositions are given in Table 1. Shoots came from pairs of mature plants in experimental plantings at the P.E. Marucci Center for Blueberry and Cranberry Research and Extension (Rutgers University) in Chatsworth, N.J. Detached shoots were collected on 8 Jan. 2004 and on 10 Jan. 2005 to determine floral-bud cold hardiness. Previous studies of highbush and rabbiteye have shown this time period to be one at which plants have achieved maximum cold hardiness (Muthalif and Rowland, 1994). All plants used for shoot collection were at least 4 to 5 years old, and were selected for as much uniformity as possible. Most samples were from terminal shoots taken from upper portions of the bush.
Determination of flower bud cold hardiness.
For the freeze-thaw protocol, three 5- to 6-cm-long shoots, each with three to eight flower buds, were used for each treatment as described previously by Arora et al. (2004). Treatment temperatures chosen for the fully cold-acclimated buds covered a range from -10 to -28 C (the lowest temperature that the glycol freezing bath would consistently reach) in 2 C increments, to cover a potential range of 0 to 100 percent injury to blueberry buds for most genotypes (Arora et al., 1997). Shoots were removed from the freezing bath at their respective selected treatment temperatures, thawed overnight at 4 C, then incubated at 20 C for 24 hours. The three most apical buds were then dissected and observed for injury (visual browning of the ovaries in individual flowers) (Arora et al., 2000; Flinn and Ashworth, 1994). Each bud was rated for percentage of injured ovaries and bud cold hardiness was defined as the temperature causing 50 percent injury overall (LT50).
Statistical analysis of cold hardiness data.
Bootstrap estimates (Manly, 1997) of LT50 values and their 95 percent fiducial confidence intervals were calculated using Proc Probit (SAS Institute, 1999) for each cultivar x year combination. The nine observed data points (three proximal buds on each of the three shoots) for each temperature were resampled (n = 9 with replacement) 10,000 times. A sigmoidal (i.e., logistic) regression model was fit to percentage of injury (browning) vs. treatment temperature for each of the 10,000 sets of resampled data, and the 10,000 resulting values of LT50 and their lower and upper confidence limits were averaged to obtain a bootstrap estimate for each cultivar x year. A one-way ANOVA was conducted on these 50 LT50 estimates to compare the 25 cultivars using the duplicate year data. Cultivar x year (i.e., error) variability was partitioned into 5 sizes of variance groups so that the appropriate size of error variation was associated with each cultivar in the means comparison. The cultivar means comparison used the Sidak adjustment to ensure alpha = 0.05.
Bud size evaluations.
On 11 Jan. 2005, the cultivars in were evaluated for uniformity of flower bud size. Ratings were done on a scale of 1 to 5 with 1 = uniformly sized buds and 5 = a substantial difference between the smallest and largest buds on the cultivar.