Stone fruit trees of genus Prunus, like other temperate woody species, need to accumulate a cultivar-specific amount of chilling during endodormancy, and of heat during ecodormancy to flower properly in spring. Knowing the requirements of a cultivar can be critical in determining if it can be adapted to a particular area. Growers can use this information to anticipate the future performance of their orchards and the adaptation of new cultivars to their region. In this work, the available information on chilling- and heat-requirements of almond, apricot, plum, peach, and sweet cherry cultivars is reviewed. We pay special attention to the method used for the determination of breaking dormancy, the method used to quantify chilling and heat temperatures, and the place where experiments were conducted. The results reveal different gaps in the information available, both in the lack of information of cultivars with unknown requirements and in the methodologies used. The main emerging challenges are the standardization of the conditions of each methodology and the search for biological markers for dormancy. These will help to deal with the growing number of new cultivars and the reduction of winter cold in many areas due to global warming.

Self-incompatibility (SI) is one of the most efficient mechanisms to promote out-crossing in plants. However, SI could be a problem for fruit production. An example is apricot (Prunus armeniaca), in which, as in other species of the Rosaceae, SI is determined by an S-RNase-based-Gametophytic Self-Incompatibility (GSI) system. Incompatibility relationships between cultivars can be established by an S-allele genotyping PCR strategy. Until recently, most of the traditional European apricot cultivars were self-compatible but several breeding programs have introduced an increasing number of new cultivars whose pollination requirements are unknown. To fill this gap, we have identified the S-allele of 44 apricot genotypes, of which 43 are reported here for the first time. The identification of S_c in 15 genotypes suggests that those cultivars are self-compatible. In five genotypes, self-(in)compatibility was established by the observation of pollen tube growth in self-pollinated flowers, since PCR analysis could not allowed distinguishing between the S_c and S₈ alleles. Self-incompatible genotypes were assigned to their corresponding self-incompatibility groups. The knowledge of incompatibility relationships between apricot cultivars can be a highly valuable tool for the development of future breeding programs by selecting the appropriate parents and for efficient orchard design by planting self-compatible and inter-compatible cultivars.

In the recent years, an important renewal of apricot cultivars is taking place worldwide with the introduction of a large number of new releases, which are replacing traditional and local cultivars in many situations. To study the current genetic diversity, a group of 202 apricot accessions, including landraces and releases from breeding programs in several countries, has been characterized using 13 microsatellite markers. The diversity parameters showed higher diversity in modern releases than in landraces, but also suggested a loss of diversity associated with recent breeding. Two main clusters according to the pedigree origin of the accessions were clearly differentiated in the phylogenetic analysis based on Nei’s genetic distance. The first group comprised mostly European and North American traditional cultivars, and the second group included the majority of recent and commercial releases from breeding programs. Further population analyses showed the same clustering trend on the distribution of individuals and clusters, confirming the results obtained in the molecular phylogenetic analysis. These results provide a sight of the erosion and the decrease of the genetic diversity in the currently grown apricot and highlight the importance of preserve traditional cultivars and local germplasm to assure genetic resources for further breeding.

Japanese plum (Prunus salicina Lindl.) is widely distributed in temperate zones across the world. Since its introduction to USA in the late 19th century, this species has been hybridized with up to 15 different diploid Prunus species. This high level of introgression has resulted in a wide range of traits and agronomic behaviors among currently grown cultivars. In this work, 161 Japanese plum-type accessions were genotyped using a set of eight Simple Sequence Repeats (SSR) markers to assess the current genetic diversity and population structure. A total of 104 alleles were detected, with an average of 13 alleles per locus. The overall Polymorphic Informative Content (PIC) value of SSR markers was 0.75, which indicates that these SSR markers are highly polymorphic. The Unweighted Pair Group Method with Arithmetic (UPGMA) dendrogram and the seven groups inferred by Discriminant Analysis of Principal Components (DAPC) revealed a strong correlation of the population structure to the parentage background of the accessions, supported by a moderate but highly significant genetic differentiation. The results reported herein provide useful information for breeders and for the preservation of germplasm resources.

In recent years, an important renewal of apricot cultivars is taking place worldwide, with the introduction of many new releases. Self-incompatible genotypes tolerant to the sharka disease caused by the plum pox virus (PPV), which can severely reduce fruit production and quality, are being used as parents in most breeding programs. As a result, the self-incompatibility trait present in most of those accessions can be transmitted to the offspring, leading to the release of new self-incompatible cultivars. This situation can considerably affect apricot management, since pollination requirements were traditionally not considered in this crop and information is lacking for many cultivars. Thus, the objective of this work was to determine the pollination requirements of a group of new apricot cultivars by molecular identification of the S-alleles through PCR amplification of RNase and SFB regions with different primer combinations. The S-genotype of 66 apricot cultivars is reported, 41 for the first time. Forty-nine cultivars were considered self-compatible and 12 self-incompatible, which were allocated in their corresponding incompatibility groups. Additionally, the available information was reviewed and added to the new results obtained, resulting in a compilation of the pollination requirements of 235 apricot cultivars. This information will allow an efficient selection of parents in apricot breeding programs, the proper design of new orchards, and the identification and solution of production problems associated with a lack of fruit set in established orchards. The diversity at the S-locus observed in the cultivars developed in breeding programs indicates a possible genetic bottleneck due to the use of a reduced number of parents.

Self-incompatibility in Rosaceae is determined by a Gametophytic Self-Incompatibility System (GSI) that is mainly controlled by the multiallelic locus S. In apricot, the determination of self- and inter-(in)compatibility relationships is increasingly important, since the release of an important number of new cultivars has resulted in the increase of cultivars with unknown pollination requirements. Here, we describe a methodology that combines the determination of self-(in)compatibility by hand-pollinations and microscopy with the identification of the S-genotype by PCR analysis. For self-(in)compatibility determination, flowers at balloon stage from each cultivar were collected in the field, hand-pollinated in the laboratory, fixed, and stained with aniline blue for the observation of pollen tube behavior under the fluorescence microscopy. For the establishment of incompatibility relationships between cultivars, DNA from each cultivar was extracted from young leaves and S-alleles were identified by PCR. This approach allows establishing incompatibility groups and elucidate incompatibility relationships between cultivars, which provides a valuable information to choose suitable pollinizers in the design of new orchards and to select appropriate parents in breeding programs.