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J.A. Beaver and A.F. Iezzoni

Inheritance for seven enzyme loci was determined in seeds produced from crosses and self-pollinations involving four sour cherry parents and one open-pollinated ground cherry (P. fruticosa Pall.) parent. Segregation data were used to identify allozymes and determine whether sour cherry is a naturally occurring allo- or autotetraploid. Three allozymes were identified at the 6-Pgd-1 locus, and two were identified at each of the following loci: Pgi-2, Lap-1, Adh-1, Idh-2, Pgm-2, and 6-Pgd-2. Segregating allozyme patterns for the diagnostic loci Idh-2, Pgm-2, 6-Pgd-1, and 6-Pgd-2 tit disomic inheritance models and thus confirmed the allotetraploid hypothesis for sour cherry. Chi-square tests of independence between loci indicated that Pgi-2, Adh-1, Idh-2, 6-Pgd-1, and 6-Pgd-2 were not linked.

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Amy F. Iezzoni and Colleen A. Mulinix

Yield components were measured from 115 sour cherry (Prunus cerasus L.) hybrid seedlings from 13 full-sib families to investigate the potential of breeding for increased yield. Those families with the highest number of fruit and reproductive buds had the highest yields. In general, increased fruit size was not able to compensate for low fruit count. Fruit set and flower count per bud were inversely related, suggesting compensation between these two components. Yield components from six selections chosen for differing fruiting habits were measured for an additional 2 years. In year 1, those selections with a majority of their fruit on l-year-old wood had higher yield efficiencies (yield per branch cross-sectional area) than those with fruit on spurs; however, but year 3, the higher-yielding selections were those that fruited primarily on spurs. The data are discussed relative to selecting for yield in a sour cherry breeding program.

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Amy F. Iezzoni and Colleen A. Mulinix

Bloom times were evaluated for seedlings from four full-sib and 14 open-pollinated families of sour cherry (Prunus cerasus L.). Time of anthesis for individual seedlings ranged over 17and 16-day periods in 1989 and 1990, respectively. In both years, most seedlings bloomed later than `Montmorency', the only commercially important sour cherry cultivar in the United States. `Pitic de Iasi', the parent of the latest-blooming family, is a natural interspecific hybrid between sour cherry and the cold-hardy Russian ground cherry (P. fruticosa Pall.). Hybridization between sour and ground cherry and intense selection pressure in the colder areas of the sour cherry habitat may have favored selection of the late-blooming character.

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Christopher L. Owens, J.F. Hancock and A.F. Iezzoni

Sour cherry and strawberry are examples of two Rosaceous species that often suffer crop reductions due to spring freezes. Breeding for improved floral freezing tolerance has the potential to mitigate the susceptibility of these plants to spring frosts. In model plant systems, researchers have been able to identify genes that play a role in freezing tolerance by initially searching for mRNAs regulated in response to cold temperatures. To search for cold-responsive freezing-tolerance genes in strawberry and sour cherry, it is necessary to first define their cold acclimation response. To test the hypothesis that sour cherry and strawberry flowers have the ability to cold acclimate, blooming plants were exposed to 4 °C and 16 h light for 14 days. Sour cherry styles and strawberry receptacles from open, fully developed flowers were excised, and electrolyte leakage curves were generated over a range of subzero temperatures. The temperature at which 50% electrolyte leakage (EL50) occurred was used to compare treatments. The flowers of two strawberry cultivars were tested for the ability to cold acclimate. Non-acclimated `Chandler' receptacles had an EL50 of -2.9 °C, while non-acclimated `Honeoye' had an EL50 of -3.4 °C. Conversely, acclimated `Chandler' receptacles had an EL50 of -7.7 and acclimated `Honeoye' receptacles had an EL50 of -8.7 °C, both are significantly different from non-acclimated values (P ≤ 0.01). Additionally, sour cherry styles were collected from the field at full bloom from a mapping population of 86 individuals from the cross `Rheinische Schattenmorelle' × `Erdi Botermo' and acclimated as previously described. The EL50 of the 86 progeny ranged from approximately -2.0 to -6.0 °C.

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Christopher M. Long, Colleen A. Mulinix and Amy F. Iezzoni

Microspore-derived callus cultures were obtained by anther culture of `Emperor Francis' sweet cherry (Prunus avium L.). Branches were removed from the field in January and March and forced in the laboratory. When the microspores reached the uninucleate stage, anthers were placed on modified Quoirin and Lepoivre liquid culture medium containing 4.4 μm BA and 4.5 μm 2,4-D. After ≈60 days, callus that emerged from the anthers was placed on woody plant medium supplemented with 1 μm 2,4-D and 3 μm 2iP and routinely transferred. The resulting 270 callus cultures were screened for two allozymes heterozygous in `Emperor Francis', Pgi-2 and 6-Pgd-1. Of the 270 callus cultures, 154 expressed only one allele each for Pgi-2 and 6-Pgd-1; thus, they were considered microspore-derived. The microspore-derived callus cultures can be used as a linkage mapping population. Chemical names used: 6-benzyladenine (BA); 2,4-dichlorophenoxyacetic acid (2,4-D); N6-(2-isopentenyl)-adenine (2iP).

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D. Struss, M. Boritzki, R. Karle and A.F. Iezzoni

Two rootstocks from the Giessen (GiSelA) series of dwarfing cherry (Prunus sp.) rootstocks, GiSelA (GI) 5 (syn. 148/2) and GI 6 (syn. 148/1), are becoming commercially important and five other Giessen cherry rootstocks are being evaluated for horticultural traits. Since GI 5 and GI 6 are morphologically similar, a DNA fingerprinting project was undertaken to identify molecular markers that could be used by the nursery industry to differentiate these two rootstocks. The project was extended to include six additional Giessen rootstocks of varying pedigrees. Fourteen DNA primer pairs were tested for their ability to differentiate among the eight rootstocks. None of the primer pairs could differentiate all eight rootstock selections; however, three primer pairs could differentiate all but two selections. Two primer pairs, PMS 15 and PceGA59, were identified as the most suitable for high throughput screening of GI 5 and GI 6 due to the simplicity and the size of the base pair differences among the polymorphic fragments. These results demonstrate the utility of molecular markers to differentiate the Giessen cherry rootstocks.

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L.S. Chang, A.F. Iezzoni, G.C. Adams and F.W. Ewers

Eight open-pollinated peach families [Prunus persica (L.) Batsch] were selected from a germplasm collection that was screened for tolerance to Leucostoma persoonii (Nits.) Höhn. [imperfect state, .Leucocytospora leucostoma (Pers.) Höhn] following field inoculation. The eight peach families were either susceptible or tolerant to L. persoonii infection based on canker length measurements. Three open-pollinated seedlings per family were chosen for evaluation. Following artificial inoculation, measurements of hydraulic conductance per pressure gradient (Kh) were made on 2-year-old branch segments from the 24 seedlings, and safranin dye was used to mark the conductive xylem pathways. For the peach families tolerant to L. persoonii, the specific Kh of the canker branch segments was greater than that for the most susceptible peach families. The inoculated branch segments from the tolerant peach families maintained ≈15% to 30% of the water transport of control segments. Safranin dye movement indicated that the sapwood in inoculated branch segments of seedlings from the susceptible peach families was almost completely blocked. Isolation experiments indicated deeper penetration of the fungus into the xylem of seedlings of susceptible than tolerant families. Xylem dysfunction appears to be correlated with a reduction in Kh, and the seedlings in the tolerant peach families are better able to maintain water transport through the stem segment invaded by the fungus.

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A. Kahn, A.F. Iezzoni, S. Kalisz, CA. Mulinix and V. Delasalle

The number of flowers produced by sour cherry greatly exceeds the number of fruits developed. Two hypotheses to explain this disparity were investigated: (1) pollen may be limiting, and (2) a large flower display is important for pollinator attraction. Self-incompatibility, which is common in sour cherry, was considered. Fruit set, floral morphology, and flower density were measured on 18 sour cherry selections, both self-compatible and self-incompatible (SI), in the MSU sour cherry germplasm collection following open- and bulk-pollination. Although supplemental hand pollination resulted in a significant increase in fruit set, the final fruit set was still low (18% vs 14%) indicating that lack of pollination alone was insufficient to account for the low fruit set. The SI selections had significantly larger flower cups, pistil/petal size ratio, and more flowers/branch cross-sectional area suggesting that flower display may have an increased role in pollinator attraction in the SI selections. These results will be discussed in relation to 2 additional hypotheses: insufficient maternal resources and genetic factors resulting in pre- or post-zygotic selection.

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K. Ikeda, A. Watari, K. Ushijima, H. Yamane, N.R. Hauck, A.F. Iezzoni and R. Tao

S4′ is a pollen-part mutant in sweet cherry (Prunus avium L.) that is extensively used to develop self-compatible cultivars. The S4′-haplotype is known to have a functional stylar component and a nonfunctional pollen component. The pollen component in sweet cherry necessary for the specificity of the pollen reaction is believed to be an S-haplotype specific F-box protein gene, called SFB. This study describes two molecular markers that distinguish between SFB4 and SFB4′ by taking advantage of a four base pair deletion in the mutant allele. The resulting polymerase chain reaction (PCR) products can either be separated directly on a polyacrylamide gel or they can be subjected to restriction enzyme digestion and the different sized products can be visualized on an agarose gel. The latter technique utilizes restriction sites created in the PCR products from the SFB4′ allele, but not the SFB4 allele. Because the primer sets created differential restriction sites, these primer sets were termed dCAPS (derived cleaved amplified polymorphism sequence) markers. These molecular assays can be used to verify self-compatibility conferred by the S4′-haplotype.