Fifteen years of Michigan harvest data for highbush blueberry (Vacciniun corymbosum L.) were used in conjunction with daily maximum and minimum temperatures to determine appropriate heat-unit models for first-picking dates of 13 cultivars. For each cultivar, an optimal heat-unit model was chosen after evaluating the performance of a standard method with 72 combinations of three variables: a) starting date for the heat-unit accumulations (SDATE), b) low-temperature threshold (TLOW), and c) high-temperature threshold (THIGH). The optimal model sought to include the most important criteria values with respect to model performance and to minimize the average square of the prediction error (days) and the range in that error. Compared with a strict calendar-day method of estimating harvest dates, the heat-unit models reduced the standard deviation of the prediction error from 22% to 69%, depending on cultivar.
CO2 assimilation (A), leaf conductance to water vapor (gl), mesophyll conductance (gm), and water use efficiency (WUE) were compared for two cultivars of highbush (Vaccinium corymbosum L.) and a wild diploid lowbush blueberry species (Vaccinium darrowii Camp.) in response to PPF, CO2, temperature, and vapor pressure deficit (VPD) to determine if apparent tolerance of V. darrowii to high temperature and drought conditions resulted from differences in gas exchange characteristics. Cultivar differences between ‘Bluecrop’ and ‘Jersey’ in A were not significant when expressed on a leaf area, leaf dry weight, or total chlorophyll basis. Maximum CO2 assimilation rates for V. darrowii were about 35%, 50%, or 40% lower than highbush cultivars when expressed on a leaf area, leaf dry weight, or total chlorophyll basis, respectively. Differences between ‘Bluecrop’ and ‘Jersey’ were also non-significant for mesophyll conductance, transpiration, CO2 compensation points, and water use efficiency. CO2 assimilation maximized between 600-800 µmol·s–1·m–2 photosynthetic photon flux (PPF) for all three genotypes and the temperature optima ranged between 18° and 26°C for ‘Jersey’, 14° and 22° for ‘Bluecrop’, and 25° and 30° for V. darrowii. As temperature was increased from 20° to 30°, leaf conductance (gl) to water vapor was lower and water use efficiency was higher for V. darrowii, compared to ‘Bluecrop’ but not ‘Jersey’. There was a 50-65% reduction in gl as VPD was increased, but only 10–20% reduction was observed in A. Leaf conductance to water vapor was reduced for V. darrowii, which restricted intercellular CO2. Since crosses are possible between highbush and V. darrowii, it is possible that heat tolerance and/or drought resistance could be improved in Highbush blueberry through the incorporation of genes from V. darrowii.
Starch gel electrophoresis was employed to distinguish the University of California-released strawberry cultivars. Isozyme patterns of 3 enzyme systems (PGI, LAP, and PGM) were studied. Fourteen of the 22 cultivars studied were classified uniquely by using the 3 enzyme systems. The use of electrophoresis as a tool in breeding clonally propagated crops is discussed.
To determine if the net CO2 assimilation and water use efficiency (WUE) of highbush blueberry under high temperature can be improved genetically, gas exchange determinations were made for a selection of Vaccinium darrowi Camp (Florida 4B), a highbush cultivar (Bluecrop) (V. corymbosum L.), their F1 hybrid (US75), and two crosses of the F1 hybrid to another improved genotype (US239 and US245). All genotypes responded parabolically to increasing temperature at vapor pressure deficits <1 kPa. Maximum CO2 assimilation of US75 (15 µmol·s–1·m–2) was 30% to 40% higher than either parent. Carbon dioxide assimilation of US75 and Florida 4B was optimum at 30°C and that of ‘Bluecrop’ at 20°. The optimum for US239 was similar to ‘Bluecrop’, and that of US245 to Florida 4B. Florida 4B had higher WUEs than ‘Bluecrop’ at both 20° (5.64 µmol CO2/mmol H2O to 4.01) and 30° (3.73 to 2.53). US239 and US245 had significantly (P < 0.05) higher WUEs at 30° than did ‘Bluecrop’. Residual conductance to CO2 (gr) decreased in ‘Bluecrop’ when temperature was raised from 20° to 30°, but increased in all other genotypes. Due to the favorable gas exchange properties of US75 and US245 at 30°, we suggest that the high temperature tolerance of V. darrowi may be heritable and that US245 may be used to improve the heat tolerance of highbush blueberry.
Flower bud and leaf samples collected from a wide range of native North American Vaccinium populations were tested for the presence of blueberry shoestring virus (BBSSV) using the enzyme-linked immunosorbant assay. The highest disease incidence was found in Michigan (14%), although a few positive samples also were found in Virginia, New Jersey, Maine, Ontario, and Quebec. Of seven species tested, only V. corymbosum L. and V. angustifolium Ait. were infected with BBSSV.