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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 3 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. Cultivated highbush blueberry, which is tetraploid (2n = 4x = 48), was developed by crosses and selection using plants in Vaccinium section Cyanococcus, including V. corymbosum and its close relatives. Vaccinium arboreum (section Batodendron) is a diploid wild blueberry, native and locally abundant in the southeastern United States. On favorable sites, where fire is infrequent, V. arboreum takes the form of a small tree, up to 10 m tall, with diameter at breast height up to 35 cm (Vander Kloet, 1988). In the southeastern United States, V. arboreum is used in native-plant landscaping and is valued for its ornamental, brushy, winter form, which is attractive to songbirds. Abundant white flowers are fragrant in springtime, and shiny black berries persist on the tree through fall and early winter. Highbush blueberry cultivars have been crossed with V. arboreum with the longrange goal of obtaining cultivars with new characteristics (Lyrene, 2011). Vaccinium arboreum can develop a deep, wide-ranging root system when planted on deep, welldrained soil. A larger root system would allow blueberry growers to irrigate less frequently and would reduce the amount of irrigation water used annually in areas where heavy rains alternate with dry periods. Vaccinium arboreum can tolerate higher soil pH than cultivated highbush blueberries. Vaccinium arboreum has long peduncles and pedicels, giving a very open flowering raceme. An open raceme has become more important as blueberry cultivars with larger berries have been developed. Large berries in a tight cluster make hand harvesting difficult and mechanical harvesting almost impossible. Crowded berries in a tight cluster can result in misshaped berries. Ripe berries of highbush blueberry and related species in section Cyanococcus have green or white flesh inside the blue or black skins. Native Vaccinium arboreum populations in the southeastern United States are polymorphic for internal flesh berry color. Berry flesh of some plants remains green or white when the berries are fully mature. On other plants, berries begin to develop internal anthocyanins at the start of the ripening season, and by the time all the berries on the plant are ripe, the flesh inside the skin is dark purple to black. The dark purple flesh of wild European bilberries (Vaccinium myrtillus) has market appeal, because consumers associate the purple color with health benefits. The ability of V. arboreum to produce a large plant on a single stem could be used to develop cultivars better suited to mechanical harvest. The late flowering of V. arboreum (flowers typically open in midApril and berries ripen in October in north Florida) avoids most spring freezes. Local wild races of tetraploid V. corymbosum flower in late February and early March in north Florida and ripen in May. Yields are often reduced by hard freezes in February and early March. Late-ripening blueberries are not of commercial interest in Florida but might be grown profitably for October harvest in states farther north. Home-garden blueberries that ripen in September and October in the southeastern United States, and are larger and more palatable than the berries of V. arboreum, would be valuable for gardeners and for wildlife. Vaccinium arboreum produces numerous berries, but these are small and scarcely edible. They contain large seeds and sclerids and have a dry and gritty texture. Berries from some plants are sweet, but sparkleberries are often astringent or have a ‘‘grassy’’ taste. Two barriers make it hard to cross cultivated highbush blueberries with V. arboreum. First, the cultivars are tetraploid and V. arboreum diploid. The triploid block is strong in Vaccinium, and if the diploid parent makes few or no unreduced gametes, as seems to be the case with V. arboreum, few hybrids are obtained from crosses between diploids and tetraploids. The second barrier is the evolutionary divergence that separates section Cyanococcus from section Batodendron. This barrier is not highly restrictive; crosses between diploid V. darrowii of section Cyanococcus and V. arboreum give hybrids rather easily (Brooks and Lyrene, 1998; Chavez and Lyrene, 2010). Some of these diploid hybrids are vigorous and flower heavily but are sterile, except that they make a few unreduced gametes, which allow them to be crossed with tetraploid cultivars. Some BC2 and BC3 (backcrosses were to highbush cultivars) selections that were grown for 10 years in 15-plant field plots were vigorous, early-ripening, and produced berries of commercial quality. The success of the crosses that used V. darrowii as a bridge to bring V. arboreum genes into highbush cultivars made it seem desirable to attempt direct crosses between the cultivars and V. arboreum. We attempted such crosses with several thousand highbush flowers pollinated in several different years and obtained no seedlings. Subsequently, over a period of eight years, we treated thousands of V. arboreum seeds with colchicine as a pre-germination seed soak and selected plants that looked like they might be tetraploid. Initial selection was by seedling morphology, and the retained seedlings were re-selected when they flowered based on the size of pollen tetrads. Information on these crosses has been published (Lyrene, 2011). I report the results of intercrossing 34 BC1 seedlings and present information on the phenotypes of F1 and BC1 plants. Materials and Methods Vaccinium arboreum parents FL 06-730 and FL 06-753 were selected from plants Received for publication 15 Nov. 2012. Accepted for publication 18 Dec. 2012. This work was supported in part by USDA CSREES grant 2009-51181-06201. To whom reprint requests should be addressed; e-mail lyrene@ufl.edu. 146 HORTSCIENCE VOL. 48(2) FEBRUARY 2013 grown from colchicine-treated seed because they produced unusually large pollen tetrads. In 2007, crosses were made by pollinating flowers of 16 southern highbush cultivars with pollen from either FL 06-730 or FL 06-753 (Lyrene, 2011). Additional F1 hybrids were made in 2008 using different highbush parents and the same two V. arboreum plants. Seedlings were grown in a field nursery, and 450 seedlings from the first year’s crosses were verified as F1 hybrids in late winter, 2009, based on morphology after one growing season in the field. Hybrids were easily distinguished from non-hybrids based on leaf shape and color, on the appearance of bark on one-year-old wood, and on the presence or absence of obvious flower buds in midwinter. Highbush seedlings that flowered in the spring had large, conspicuous flower buds by midwinter, whereas F1 hybrids that flowered in the spring showed no indication of flower buds until 1 or 2 weeks before the corollas became visible. When they flowered, hybrids were confirmed by raceme morphology and by the presence of anther awns (Camp, 1945). Anther awns are absent in section Cyanococcus, present in section Batodendron, and were present (although reduced in size) on the anthers of all F1 hybrids that were examined. In Feb. and Mar. of 2009 and 2010, 57 F1 plants were selected for use in backcrosses to highbush cultivars. The plants were selected because they were vigorous, had the most flower buds after one growing season in the field, shed the most pollen, and had pollen that appeared, under the microscope, to include at least some potentially viable microspores. The selected F1 plants included one or more seedlings from each of the crosses ‘Lenoir’ 3 V. arboreum ‘FL06-753’ and ‘Carteret’ 3 V. arboreum ‘FL06-753’ plus one or more seedlings from each of 11 crosses of highbush 3 V. arboreum ‘FL06-730’, where the highbush parents were ‘Abundance’, ‘Bluecrisp’, ‘Brigitta’, ‘Jewel’, ‘Scintilla’, ‘Springhigh’, ‘FL95-50’, ‘FL95-138’, ‘FL01-151’, ‘FL01-297’, and ‘FL06-526’. The selected F1 plants were dug, potted, and moved to a bee-proof greenhouse. Each F1 plant was crossed with one highbush clone that was not one of its parents. Seeds from these backcrosses were sown in a greenhouse in Nov. 2009, and the seedlings were transplanted to a field nursery in Apr. 2010. The number of seedlings transplanted to the field nursery varied from cross to cross depending on the number of seeds that germinated and the vigor of the seedlings in the greenhouse grow-on trays. For each of nine crosses, 200 to 300 BC1 seedlings were transplanted to the field. For each of 10 additional crosses, 50 to 100 seedlings were transplanted. For the nine crosses having the fewest seedlings, five to 15 seedlings per cross were placed in the field. In Feb. and Mar. 2011, 37 BC1 plants were selected from the field based on high vigor and the presence of visible flower buds. These were potted and moved to a greenhouse. When the BC1 plants fl

hortscience

Fertility and Other Characteristics of F1 and Backcross1 Progeny from an Intersectional Blueberry Cross [(highbush cultivar × Vaccinium arboreum) × highbush cultivar]