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.
Paul M. Lyrene
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.
Paul M. Lyrene
Vaccinium stamineum (deerberry) is a highly variable diploid species in section Polycodium. Deerberry is native on excessively drained sandy soils from southeastern Ontario, south through the Florida peninsula to Lake Okeechobee, west to eastern Texas and southeastern Kansas. The V. stamineum used in this study were tall plants (2–4 m) native in north Florida, with a plant architecture similar to rabbiteye blueberry (V. virgatum). Starting in 2013 with crosses between tetraploid highbush cultivars (section Cyanococcus) and colchicine-doubled V. stamineum, hundreds of F1 and thousands of later-generation seedlings were grown and evaluated in high-density field nurseries at Citra in North Florida. The populations studied included F1, F2, backcrosses to each parent species, and BC1 × BC1 seedlings. The goal of the study was to assess the feasibility of introgressing into highbush blueberry cultivars desirable traits from V. stamineum (drought tolerance, red-flesh berries, new flavor components, open flowers with short corolla cups and exserted anthers and stigmas) without introducing horticulturally problematic characteristics (bitter skin, berries that shatter when ripe, difficult vegetative propagation). Vigor averaged very low in F1 seedlings, higher in F2 seedlings and in seedlings from backcrosses to V. stamineum, and highest in seedlings from backcrosses to highbush. Most crosses yielded numerous plump seeds, but crosses to produce F1 hybrids yielded fewer than 10% as many seeds as highbush × highbush crosses. Most vegetative, flower, and fruit traits that differentiate highbush from V. stamineum were intermediate in F1 seedlings. Backcross seedlings more closely resembled the recurrent parent. Variability in morphological characters was high in every generation, giving much opportunity for selection. Some seedlings from backcrosses to highbush (≈5%) appeared to have the vigor, berry quality, and yield potential required in commercial cultivars. Producing highbush cultivars that strongly express a particular V. stamineum trait might best be accomplished by growing large, segregating F2 populations from which parents for backcrosses can be selected.
Rogério Ritzinger and Paul M. Lyrene
Several morphological features of Vaccinium ashei Reade, V. constablaei A. Gray, their F1 hybrids, V. simulatum Small, and southern highbush blueberry (V. corymbosum L. hybrids) flowers were compared in Gainesville, Fla. Desirable characteristics that could increase the extent of honeybee pollination, such as a large corolla aperture and a short anther-to-stigma distance, were common in V. constablaei but not in V. ashei. F1 (V. ashei × V. constablaei) hybrids were generally intermediate between the two parents. Thus, it appears that V. constablaei could be used to breed V. ashei cultivars with improved flower morphology. Vaccinium simulatum and V. constablaei flowers were similar in all features. The corollas of southern highbush blueberry flowers were wide and had wide apertures, but the distance between stigma and anther pore was also large.
Rogério Ritzinger and Paul M. Lyrene
Open-pollinated southern highbush (V. corymbosum L. hybrids) and F1 (southern highbush × V. simulatum Small) hybrid blueberry seedlings were compared for fertility in a high-density nursery in Gainesville, Fla. Most of the pollen sources in the field were tetraploid southern highbush seedlings. Berries were collected from 100 southern highbush seedlings and from 100 seedlings from southern highbush × V. simulatum crosses. The seeds were extracted and dried on a laboratory bench for several days before weighing. No significant differences were found in seed mass/berry between the two types of seedlings. Although the F1 interspecific hybrids averaged slightly lower in seed mass per berry, this was due to the smaller size of their well-developed seeds, not to poor seed development. The estimated number of well-developed seeds per berry was 35.4 and 39.1 for southern highbush blueberries and their F1 hybrids with V. simulatum, respectively. These results indicate that reduced fertility should not be a problem in using V. simulatum to breed southern highbush blueberries.
Dario J. Chavez and Paul M. Lyrene
Diploid Vaccinium darrowii Camp has been used in breeding tetraploid southern highbush blueberry (Vaccinium corymbosum L.) as a source of reduced chilling requirement, adaptation to hot, wet summers, and resistance to leaf diseases. V. darrowii in Florida is quite variable, but most crosses have involved only one V. darrowii clone, Fla. 4B. The use in breeding of a wider range of V. darrowii accessions would provide beneficial diversity in the blueberry cultivated gene pool. The purpose of this research was to determine the functional 2n gamete frequency of numerous V. darrowii genotypes when crossed with tetraploid V. corymbosum, and to study the pollen fertility and backcross ability of the interspecific (V. darrowii × V. corymbosum) hybrids to V. corymbosum. Crosses between diploid V. darrowii and tetraploid highbush blueberry cultivars had low fruit set compared with the V. darrowii × V. darrowii and highbush × highbush crosses. The unusually high number of hybrids per pollinated flower (HPF) in certain 4x-2x or 2x-4x crosses was attributed to high functional 2n gamete production in certain V. darrowii genotypes. Diploid Vaccinium fuscatum Aiton and diploid V. darrowii × V. fuscatum hybrids, when crossed with southern highbush blueberry cultivars, were equally productive of hybrids whether used as male or female parents. Variation in frequency of functional 2n gametes in V. darrowii, expressed as high HPF, was present within plants (megaspores vs. microspores) and among V. darrowii plants. Of the 114 interspecific (V. darrowii × V. corymbosum) hybrids studied, 106 had pollen stainability >50%. This indicated that most of these hybrids were tetraploid, because triploid blueberries, like most triploid plants, are highly sterile. Twenty-two V. darrowii × V. corymbosum hybrids were backcrossed to tetraploid highbush blueberry cultivars. Fruit set was variable, but large populations of vigorous hybrids were obtained. Lower fruit set was associated with hybrids that had lower pollen fertility. It should be possible to obtain plants of cultivar quality in a few generations of backcrosses.
Timothy F. Wenslaff and Paul M. Lyrene
Two clones of anthocyanin-deficient (AD) Vaccinium elliottii (2×, homozygous recessive, yy) were used as seed parents in experiments combining normal and AD pollen. AD gives a seedling marker to distinguish the pollen parent. In the first experiment, flowers were pollinated daily for one, two, three or four days. AD and normal V. elliottii pollen were applied on separate days. The last day of each treatment received the opposite genotype from the previous day(s). The first pollination gave as many, or more, seedlings as later applications but the number of seeds per fruit was higher with multiple pollen applications. The second experiment used pollen from normal V. corymbosum (4×) alone or mixed with AD pollen from the 2× clones. Results depended on the seed-parent genotype. There was no difference between treatments in the number of hybrids produced by W81-1, which tends to set fruit even with only one seed per berry. Only mixed pollen yielded hybrids from clone FL83-139, which was never observed to produce berries with only one seed; apparently the mentor AD pollen helps to set the fruit, thereby allowing the rare hybrid seeds to develop.
Timothy F. Wenslaff and Paul M. Lyrene
A study was done to test whether inheritance is tetrasomic or disomic in tetraploid Fl hybrids between 2x Vaccinium elliottii Chapm. and 4x V. corymbosum L. Seventeen Fl hybrids derived from V. elliottii homozygous for recessive anthocyanin deficiency (AD) were confirmed by isozyme analysis and, where V. elliottii was the seed parent, by the presence of anthocyanin. Fertile hybrids with high pollen stainability were assumed to be 4x and duplex for the AD allele, having arisen from 2n gametes in V. elliottii. In nine Fl × Fl crosses, all progeny populations segregated for AD phenotype at or above the expected tetrasomic ratio of 1 AD:35 normal; no AD would be expected with disomic inheritance. Tetraploid AD progeny were used in testcrosses on sixteen Fl hybrids in 1991. Progeny segregated tetrasomically, 1 AD:5 normal. Isozyme loci PGD-2 and PGI-2 also segregated tetrasomically.
Sylvia J. Brooks and Paul M. Lyrene
Fertility of F1 hybrids and their open-pollinated progeny was studied for the intersectional cross Vaccinium darrowi Camp × V. arboreum Marsh as part of a project to determine the feasibility of using V. arboreum to breed vigorous, drought-tolerant southern highbush blueberry cultivars. The 16 F1 hybrids that were studied were vigorous but very low in fertility. Second generation hybrids [MIKs (mother is known) obtained by open-pollination of the F1s] and MIK derivatives were extremely variable in vigor and fertility, but averaged far higher in fertility than the F1s as evidenced by pollen stainability and amount of pollen produced. F1s produced an average of 0.4 seedlings per 100 pollinated flowers when hand-pollinated in a greenhouse with pollen from V. darrowi, 0.2 when pollinated by V. arboreum and 3.4 when pollinated by cultivated highbush. Some MIKs that were crossed with other MIKs and with cultivated southern highbush were very high in male and female fertility. Female fertility was estimated in greenhouse crosses from fruit set, berry weight, number and weight of seeds, number of plump seeds per berry, and number of seedlings obtained. Male fertility was estimated by pollen stainability with acetocarmine and amount of pollen shed. Chromosome counts showed that three F1s were diploid and that four fertile MIKs were tetraploid. One MIK appeared to be aneuploid. Aneuploidy may explain much of the low fertility found in MIK populations. These results indicate that good progress is being made in returning the hybrid plants to cultivar quality in only a few generations of backcrossing.
Dario J. Chavez and Paul M. Lyrene
In Vaccinium L., most tetraploid hybrids between tetraploid cultivated highbush blueberry (V. corymbosum L.) and diploid darrow's evergreen blueberry (V. darrowii Camp) have been produced by exploiting V. darrowii's tendency to produce 2n gametes and a strong triploid block, which greatly reduces the number of triploid hybrids produced. Colchicine-derived tetraploids offer an alternative method of producing V. darrowii plants that will easily cross with tetraploid Vaccinium species. V. darrowii (2n = 2x = 24) seeds were imbibed in 0.2% aqueous colchicine solution for 24 h. The seeds were germinated and seedlings whose morphology suggested colchicine effects were selected at various stages of development. No macromorphological changes were consistently associated with chromosome doubling. However, stomatal guard cells and pollen size increased substantially as a result of chromosome doubling. Several types of plants were identified after colchicine treatment: 1) plants with a doubled LI (epidermal tissues) and LII (internal tissues) plant layers; 2) periclinal chimeras with a doubled LI layer and a normal LII; and 3) periclinal chimeras with a doubled LII layer and a normal LI. Of ≈4000 seedlings that emerged from colchicine-treated seeds, 200 were selected for further examination based on leaf and stem morphology. Of the 200, five appeared to be tetraploid in LI, LII, or both layers based on stomatal guard cell size and pollen size. Crosses between colchicine-derived V. darrowii (4x) plants and southern highbush blueberry (V. corymbosum) cultivars (4x) were successful compared with 4x–2x and 2x–4x crosses using diploid V. darrowii and tetraploid southern highbush blueberry. Stomatal guard cells and pollen screening of the colchicine-treated plants were used as indicators of doubled V. darrowii plants and periclinal chimeras. The results from the crosses between colchicine-treated V. darrowii plants and tetraploid highbush blueberry cultivars confirmed the information obtained by stomata and pollen screening.