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 long-range goal of obtaining cultivars with new characteristics (Lyrene, 2011). Vaccinium arboreum can develop a deep, wide-ranging root system when planted on deep, well-drained 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 mid-April 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.
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