Perspectives on the adaptation of Japanese plum-type cultivars to reduced winter chilling in two regions of Spain.

Japanese plum, like other temperate fruit tree species, has cultivar-specific temperature requirements during dormancy for proper flowering. Knowing the temperature requirements of this species is of increasing interest due to the great genetic variability that exists among the available Japanese plum-type cultivars, since most of them are interspecific hybrids. The reduction of winter chilling caused by climate change is threatening their cultivation in many regions. In this work, the adaptation perspectives of 21 Japanese plum-type cultivars were analyzed in two of the main plum-growing regions in Spain, Badajoz and Zaragoza, to future climate conditions. Endodormancy release for subsequent estimation of chilling and heat requirements was determined through empirical experiments conducted during dormancy at least over two years. Chill requirements were calculated using three models [chilling hours (CH), chilling units (CU) and chilling portions (CP)] and heat requirements using growing degree hours (GDH). Chilling requirements ranged 277-851 CH, 412-1,030 CU and 26-51 CP, and heat requirements ranged from 4,343 to 9,525 GDH. The potential adaption of the cultivars to future warmer conditions in both regions was assessed using climate projections under two Representative Concentration Pathways (RCP), RCP4.5 (effective reduction of greenhouse gas emissions) and RCP8.5 (continuous increase in greenhouse gas emissions), in two time horizons, from the middle to the end of 21st century, with temperature projections from 15 Global Climate Models. The probability of satisfying the estimated cultivar-specific chilling requirements in Badajoz was lower than in Zaragoza, because of the lower chill availability predicted. In this region, the cultivars analyzed herein may have limited cultivation because the predicted reduction in winter chill may result in the chilling requirements not being successfully fulfilled.

Simple Sequence Repeat (SSR)-Based Genetic Diversity in Interspecific Plumcot-Type (Prunus salicina × Prunus armeniaca) Hybrids.

The main objective of many fruit-breeding programs around the world is the release of new cultivars from interspecific hybridizations between species of the Prunus genus. Plum × apricot (Prunus salicina Lindl. × Prunus armeniaca L.) are the most widespread interspecific hybrids, which include plumcots, pluots, and apriums. In this work, 115 accessions of interspecific hybrids from different origins and 27 reference genotypes of apricot and other diploid plum species were analyzed using eight simple sequence repeat (SSR) markers to assess the population structure and current genetic diversity. A total of 149 alleles were obtained, with an average of 19 alleles per locus. The overall polymorphic information content (PIC) mean value of SSR markers was 0.81, indicating a high degree of polymorphism of the SSR. The genetic analysis revealed 141 unique genotypes and two synonyms. The unweighted pair group method with arithmetic averages (UPGMA) dendrogram and the population structure with five groups inferred through the discriminant analysis of principal components (DAPC) revealed a clear genetic differentiation between apricot genotypes and the rest of the accessions since the interspecific hybrids clustered with the Japanese plum genotypes. Repeated backcrosses between interspecific hybrids with plum genotypes could be the cause of the higher genetic proximity of the hybrids with respect to plum than with apricot genotypes. This corresponds to the fruit morphology and agronomic behavior observed in most interspecific hybrids, which also resemble plums more than apricots.

Establishing Pollination Requirements in Japanese Plum by Phenological Monitoring, Hand Pollinations, Fluorescence Microscopy and Molecular Genotyping.

The Japanese plum cultivars commonly grown are interspecific hybrids derived from crosses between the original Prunus salicina with other Prunus species. Most hybrids exhibit gametophytic self-incompatibility, which is controlled by a single and highly polymorphic S-locus that contains multiple alleles. Most cultivated hybrids are self-incompatible and need pollen from a compatible donor to fertilize their flowers. Establishing pollination requirements in Japanese plum is becoming increasingly important due to the high number of new cultivars with unknown pollination requirements. In this work, a methodology for the determination of pollination requirements in Japanese plum-type hybrids is described. Self-(in)compatibility is determined by hand-pollinations in both the field and in the laboratory, followed by monitoring pollen tube elongation with fluorescence microscopy, and also monitoring fruit maturation in the field. Selection of pollinizer cultivars is assessed by combining the identification of S-genotypes by PCR analysis with the monitoring of flowering time in the field. Knowing the pollination requirements of cultivars facilitates the selection of cultivars for the design of new orchards and allows the early detection of productivity problems related with pollination deficiency in established orchards.