Japanese plum (Prunus salicina Lindl.) is native to China1. In the 19th century, this crop was introduced from Japan to the United States, where it was intercrossed with other North American diploid plums2. In the 20th century, some of these hybrids were spread to temperate regions around the world. Nowadays, the term “Japanese plum” refers to a wide range of interspecific hybrids derived from crosses between the original P. salicina with up to 15 other diploid Prunus spp.3,4,5.
Japanese plum, like other species of the Rosaceae family, exhibits Gametophytic Self-Incompatibility (GSI), which is controlled by a single and highly polymorphic S-locus containing multiple alleles6. The S-locus contains two genes that encode a ribonuclease (S-RNase) expressed in the pistil, and an F-box protein (SFB) expressed in the pollen grain7. In the self-Incompatibility reaction, when the S-allele expressed in the pollen grain (haploid) is the same as one of the two expressed in the pistil (diploid), the growth of the pollen tube across the style is arrested due to the degradation of the pollen tube RNA by the action of the S-RNase8. Since this process prevents fertilization of the female gametophyte in the ovule, GSI promotes the outcrossing between cultivars.
Although some Japanese plum cultivars are self-compatible, most cultivars currently grown are self-incompatible, and need pollen from inter-compatible donors to fertilize their flowers3. In stone fruit species of genus Prunus such as almond9, apricot10,11,12 and sweet cherry13, pollination requirements of cultivars can be established by different approaches. Self-(in)compatibility can be determined by self-pollination of flowers in the field and subsequent monitoring of fruit set, or by semi-in vivo self-pollinations at controlled conditions in a laboratory and the observation of pollen tubes under the microscope14,15,16,17,18. Incompatibility relationships among cultivars can be determined by cross pollinations in the field or the laboratory using pollen of the potential pollinizer cultivar, and by the identification of S-alleles of each cultivar by PCR analysis14,15,16,19,20,21,22. In species such as sweet cherry or almond, self-(in)compatibility can be also assessed by the identification of particular S alleles associated to self-compatibility, as S4’ in sweet cherry13 or Sf in almond23.
Several plum breeding programs from the main producing countries are releasing a number of new cultivars2,14, many of them 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 self-pollinations in both the field and the laboratory, followed by observations of pollen tubes under the fluorescence microscopy. Selection of pollinizer cultivars combines the identification of S-genotypes by PCR analysis with the monitoring of flowering time in the field.