Compatibility Relationships of some Sour Cherry Cultivars and Genotypes Using PCR and Florescence Microscopy

Sour cherry (Prunus cerasuc L.) is one of the most important stone fruit of Rosaceae family which have a high nutritional value. Based on FAO statistics, Iran was ranked the 3rd to 6th among the main sour cherry producer countries in the world. Most of the sour cherry cultivars are self-compatible, but self-incompatible cultivars have been reported currently. Allotetraploidy of sour cherry complicates the self-incompatibility segregation in the progeny, so this studies have been less accomplished in Iran. The sour cherry is an allotetraploid species which possibly stand up as a result of usual hybridization between the dwarf cherry P. fruticosa and sweet cherry P. avium. Similar to other species of the Prunus, the phenomenon of gametophytic self-incompatibility (GSI) happens in sour cherry. It prevents self-fertilization and facilitate fertilization with pollen of other genotypes within same species. Self-incompatibility consists of arresting pollen-tube growth in the pistil if the pollen tube contains the same S-allele that is present in the pistil. The phenomenon of self-incompatibility in plants of Rosaceae family is controlled by two genes at the S-locus. The specificity of the pistil is encoded by a gene responsible for the synthesis of the ribonuclease protein (S-RNase), whereas the specificity of the pollen is controlled by the gene encoding an F-box protein (S-haplotype-specific F-box protein – SFB) 3a). The tightly linked genetic unit of the pistil S-allele (SRNase) and pollen S-allele is called S-haplotype.

In the present study, pollen-pistil compatibility relationships among some sour cherry cultivars and selected genotypes were investigated by controlled pollination and pollen tube penetration by fluorescence microscopy. Cultivars were include ‘Meteor’, ‘Mont Morency’, ‘Majarestani’ and ‘Mahalli’ so, genotype1 were selected. After controlled pollination fruit setting was studied and the pollen tubes growth along the style was studied using fluorescent microscope. Crosses were selected based on synchronized flowering time among cultivars and genotypes and included ‘Mont Morency’ × ‘Meteor’, ‘Meteor’ × ‘Mahalli’, ‘Meteor’ × ‘Majarestani’, ‘Meteor’ × ‘Genotype 1’, ‘Mont Morency’ × ‘Mahalli’, ‘Mont Morency’ × ‘Majarestani’, ‘Mont Morency’ × ‘Genotype 1’, ‘Mahalli’ × ‘Majarestani’, ‘Mahalli’ × ‘Genotype 1’ and ‘Majarestani’ × ‘Genotype 1’, respectively. For each cross, 2 branches were randomly selected in female cultivars in winter. On each branch, including 30 floral buds at ‘balloon stage’ were tagged and bagged to prevent the entrance of any foreign pollen. On the day of anthesis, ready-to-dehisce anthers were collected from male cultivar, maintained on a moist filter paper in petri plates at 4 ºC in refrigerator till they dehisce. Pollens from these anthers were used as a source of male gametes later. Once stigma turned receptive, cross-pollination treatments were carried out in the field as designed scheme for 12 successive days. After every 24 hrs. up to 120 hrs. post-pollination, 12 pistils per treatment were fixed in FAA (formaldehyde/glacial acetic acid/70% alcohol in the ratio of 1:1:18) for 24 hrs. Followed by washings with water to remove traces of alcohol, if any, and cleared in 5% KOH at 60 ºC for fluorescence microscopy. Staining regime, as described in Sabbaghpour et al. (2020) the number of pollen tubes in the upper and mid-parts of the style and in the ovary was employed to enable visualization of the pollen germination on the stigma. Percentage of pollen germination was determined as the proportion of pollen grains that germinated on the stigma to the total number present on a scale of 100. Pollen tube number at three levels was calculated as an average of 5 (n=10). For an easy understanding of growth pattern of pollen tubes inside the style and ovary, the stylar portion, which measures 1.8 ± 0.07 cm on an average, was divided into two parts: the middle and the base. A single, longitudinal incision was then carefully given on one portion of the style using a fine needle and made wide open. Pollen tubes inside the style were critically observed under fluorescence microscope at 100x magnification. Number of them was carefully recorded and data generated for different experimental sets.

Results showed that maximum pollen tube number penetrated to the ovary and final fruit set were observed in cross ‘Mahalli’בMajarestani’. Also, minimum pollen tube in the ovary and final fruit set were observed in ‘Meteor’בMahalli’ and ‘Meteor’בGenotype 1’ crosses.

Based on the obtained results cross-incompatibility was not observed among the studied cultivars and genotypes. Therefore, they could be planted in the sour cherry breeding programs and orchards establishment based on their blooming time.