Three diploid taxons (Vaccinium darrowi Camp, V. elliottii Chapm., and interspecific V. darrowi x V. elliottii) were treated with various colchicine concentrations and treatment durations to determine the best method for inducing autopolyploidy in in vitro blueberry cultures. Shoot-tip cuttings were the best in vitro planting material for induction of shoots with increased diameter, an indicator of polyploidy. Tetraploids were produced at colchicine concentrations from 0% to 0.20%. The best treatment combinations were genotype-dependent.
Late spring frosts are a major concern to blueberry growers in the southeastern United States. Cold hardiness of flower buds (stages 4 to 6) was evaluated in three southern highbush blueberry cultivars (`Cooper', `O'Neal', and `Gulfcoast'). Differential thermal analysis (DTA) and tissue browning tests revealed that the critical temperature and ovary damage occurred at –11C in `Cooper', –12C in `O'Neal', and –13C in `Gulfcoast'.
Seven highbush blueberry (Vaccinium corymbosum L.) cultivars were evaluated for their photosynthetic heat stability. Ail showed significant reductions in CO2 assimilation rates (A) as leaf temperatures were raised from 20 to 30C, although `Blue-crop', `Jersey', `Elliot', and `Rubel' (22% to - 27%) were significantly less affected than Spartan', `Bluejay', and `Patriot' (-41% to -51%). To determine whether temperature adaptations of highbush types can be broadened through hybridization with native, heat-tolerant species, `Bluecrop' was crossed with the V. darrowi Camp. selection Florida 4B, and F2, BC1, and BC2, populations were generated. This approach showed promise as genotypes were identified in all the derivative populations that were more heat tolerant than `Bluecrop' and had a high A.
Variation was studied within and among five Vaccinium taxa for the flower parameters corolla length, corolla aperture diameter, stigma location relative to the apex of the corolla tube, position of the anthers relative to the stigma and to the apex of the corolla, and style length. The objective was to determine whether there was enough genetic variation to breed cultivars with flower shapes that might favor pollination by a wider range of bee species. The taxa studied were cultivated rabbiteye (V. ashei Reade), cultivated southern highbush (mainly V. corymbosum L. with up to 30% introgression from V. darrowi Camp), F1 V. ashei × V. constablaei A. Gray hybrids, V. darrowi, and V. elliottii Chapm. Vaccinium elliottii flowers differed from all others in having short styles that were not exserted from the corolla tube. Vaccinium elliottii was also unusual in that the end of the anther tube extended nearly to the stigmatic surface. Vaccinium ashei corollas were longer and had smaller apertures than those of southern highbush, possibly making them less suitable for honeybee (Apis) pollination. For corolla length and aperture diameter, F1 V. ashei × V. constablaei hybrids were similar to southern highbush, indicating that V. constablaei introgression could be used to breed hexaploid cultivars with shorter, more open flowers. Large clone-to-clone variation within taxa for each flower characteristic indicates much potential for changing the shape of the blueberry flower by breeding, if the shape that maximizes fruit set can be determined.
Pollen deposition on the stigmatic surface of blueberry pistils was studied with regard to maximum pollen load and stigmatic fluid production (stigma receptivity). Three hybrid southern highbush cultivars (Vaccinium corymbosum L. with V. darrowi Camp, V. ashei Reade, and/or V. angustfolium Aiton), two northern highbush cultivars (V. corymbosum), and one hybrid half-high cultivar (V. corymbosum with V. angustifolium) were selfand cross-pollinated with counted pollen tetrads until saturation of the stigmatic surface occurred. Stigmatic saturation generally required 200 to 300 tetrads and was characterized by the cessation of stigmatic fluid production and the inability to absorb further tetrads. The loss of stigmatic receptivity was irreversible. Cross-pollination resulted in cessation of stigmatic fluid production at lower levels of tetrad deposition than did self-pollination, suggesting a potential pollen-stigma recognition phenomenon. Northern highbush, half-high, and southern highbush cultivars required 7% to 10%, 12% to 17%, and 14% to 21%, respectively, more self-pollen to develop the stigmatic saturation condition. The potential relation of the pollenstigma phenomenon to self-incompatibility mechanisms is discussed.