S‐RNase genotyping and incompatibility group assignment by PCR and pollination experiments in Japanese plum

Most Japanese plum‐type cultivars are self‐incompatible and cross pollination is necessary to ensure fruit set. In this study, the S‐RNase genotype and the incompatibility group of 68 Japanese plum‐type cultivars were determined by PCR amplification of the S‐RNase gene. The S‐RNase genotype of 50 cultivars is first reported here and five new Japanese plum S‐RNase alleles (So, Sp, Sq, Sr, Ss) were identified. The results obtained, together with information compiled from previous studies, allowed describing 12 new incompatibility groups (VIII–XIX). The self‐incompatibility of several cultivars and the cross‐compatibility among different incompatibility groups were verified by self‐ and cross‐pollination experiments followed by observation of pollen tube growth. Five cultivars behaved as self‐compatible, but two of them do not have the Se allele, which has been correlated with self‐compatibility. Thus, additional sources of self‐compatibility different from Se appear to be involved in Japanese plum self‐compatibility.