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  • Author or Editor: Paul M. Lyrene x
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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.

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The effects of environmental factors, including chilling duration during dormancy and temperature during flower bud expansion, were studied on the following blueberry 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. Flowers on plants that were chilled over 1400 hours differed little from those that received only 310 chill units. Flowers that developed under warmer temperatures had significantly wider corolla apertures. In one experiment but not the other, corolla length and style length increased under warmer temperatures. For nearly every parameter in each of three experiments, there were significant environment × clone interactions. Overall, however, it appeared that neither lack of chill units during dormancy nor warm temperatures during flower development changed flower morphology enough to affect fruit set.

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Fertility and morphological traits were studied in the F1 and BC1 generations of intersectional crosses between tetraploid highbush blueberry cultivars (Vaccinium section Cyanococcus) and colchicine-induced tetraploid V. arboreum (Vaccinium section Batodendron). The goal of the introgression project was to combine desirable plant characteristics from V. arboreum with the large fruit and high fruit quality of highbush cultivars. Highbush × V. arboreum crosses were hard to make, but large numbers of BC1 seedlings were easily obtained using the most fertile F1 plants as parents in backcrosses to highbush. Anther awns, a character from V. arboreum, were present in all F1 seedlings, but fruit sclerids, another V. arboreum trait, were absent in most seedlings. Berry size in the BC1 generation was twice as large as in the F1 generation and was twice as large in the F1 as in V. arboreum. The BC1 generation was extremely variable in vigor and berry quality. Although berries of most BC1 plants were smaller, darker, and less desirable in texture and flavor than highbush berries, the high fertility of BC1 plants and the high variability among plants indicate that useful clones could be selected or developed by further breeding.

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Over several years, we obtained no hybrids after pollinating thousands of flowers of cultivated tetraploid highbush blueberry (Vaccinium corymbosum L. hybrids, section Cyanococcus) with pollen from diploid V. arboreum Marshall (section Batodendron, sparkleberry), a drought-tolerant blueberry relative native in the southeastern United States. In an effort to produce tetraploid V. arboreum that could be crossed with highbush blueberry, more than 30,000 seeds were soaked in aqueous colchicine (0.1% to 0.2%) for 24 h or more. The seeds were germinated, and putative tetraploid plants (selected based on morphological characteristics) were grown long enough to obtain pollen for microscopic examination. Twelve selected seedlings that produced unusually large pollen tetrads were used as pollen parents in crosses with more than 40 different tetraploid highbush cultivars and advanced selections. Eighty-six crosses, in which a total of 17,968 flowers were pollinated, gave 1,569 plants that were verified as hybrids after one growing season in the field. Hybrids varied from very weak to quite vigorous, some equaling highbush cultivars in vigor. A few vigorous hybrids were male-sterile, but most had at least some pollen fertility. Of the most vigorous F1 hybrids, 12 of the most fertile, based on the amount of pollen shed and on the microscopic appearance of the pollen, were backcrossed to highbush cultivars, and 3919 backcross seedlings were obtained. These varied widely in vigor but averaged higher in vigor than their F1 interspecific hybrid parents.

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Vigorous, upright shoots on mature V. ashei Reade cv. Aliceblue plants growing in a commercial field planting were used to study the effects of premature defoliation on flower bud formation. Three treatments (total shoot defoliation, alternate-node defoliation, and no defoliation) were applied on each of three dates (20 Aug., 17 Sept., and 15 Oct. 1987). For the August defoliation, the number of flower buds present per shoot on 6 Jan. of the following year averaged 1.3 for shoots that were totally defoliated, 3.7 for shoots on which alternate nodes had been defoliated, and 4.2 for control (nondefoliated) shoots. Shoots treated on 17 Sept. averaged 2.6 buds per shoot for total defoliation, 4.1 for alternate-node defoliation, and 4.8 for controls. Defoliation on 15 Oct. did not reduce flower bud formation. Reduction in flower bud formation due to defoliation was localized at the defoliated nodes. For shoots on which alternate nodes were defoliated on 20 Aug., 59.8% of the apical five nodes that were not defoliated produced flower buds compared with 1.4% of the defoliated nodes.

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Genetic variation was surveyed within and among 5 Vaccinium species and several hybrid taxa for 6 aspects of flower size and shape. Sufficient variation was found to allow radical changes in flower architecture through breeding. The goal is a flower that pours pollen directly from the anthers onto the stigma without the assistance of pollinating insects. The flowers of V. elliottii had very short styles (mean in mm 5.3 compared to 10.2 for rabbiteye cultivars and 8.5 for highbush cultivars), and certain short-style recombinants from highbush cultivar × V. elliottii crosses came close to the desired positioning of stigmas relative to anthers. The distance (in mm) from the anther pore to the stigma averaged: V. ashei 2.7; V. corymbosum 2.4; V. darrowi 2.3; and V. elliottii 1.0. Compared to highbush cultivars, rabbiteye cultivars tended to have long corollas and narrow corolla apertures, two features believed to be related to poor honeybee pollination. These features were much more favorable in V. ashei × V. constablaei hybrids, with values averaging close to those for highbush cultivars.

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Breeding to adapt temperate-zone fruit to subtropical production areas has been a formidable objective because so many different characteristics have to be changed, most of which are controlled by many genes. Recurrent selection is the only breeding method that can accomplish the required wholesale reorganization of the physiology of the plant. The principles of recurrent selection, developed and tested using short-generation organisms like fruit flies, rats, and maize, have been applied to the development of low-chill highbush blueberry (V. corymbosum L.) and peach [Prunus persica (L.) Batsch.] cultivars for northern and central Florida. These principles include using many parents per generation of crosses, minimizing the time between cycles of selection, and selecting simultaneously for all heritable traits that are important in the final product, with traits of highest economic importance and highest heritability being given the highest weight in selecting parents. Many characteristics changed during the breeding of low-latitude peach and highbush blueberry cultivars, including chill requirement, photoperiod response, resistance to various disease and insect pests, fruit chemistry, and growth patterns during a long growing season.

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‘Emerald’ is a low-chill tetraploid southern highbush blueberry hybrid that is well adapted to northeast and central Florida and to other areas receiving similar winter chilling (100 to 400 h below 7 °C). Emerald produces a vigorous bush with stout, semierect canes. It has medium to good survival in the field in north Florida. In northeast Florida, ‘Emerald’ flowers from mid-January to mid-February and ripens from mid-April to mid-May. ‘Emerald’ is capable of producing high yields of berries that are large, firm, and medium-dark in color with a small, dry picking scar and good flavor.

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