Cross Incompatibility between Rhododendron sect. Tsutsusi Species and Rhododendron japonicum (A. Gray) J.V. Suringar f. flavum Nakai

Abstract

The nature of cross incompatibility between evergreen azaleas (Rhododendron section Tsutsusi) × R. japonicum (A. Gray) J. V. Suringar f. flavum Nakai, on intersubgeneric crossings, was examined using 22 plants of 13 species in section Tsutsusi. Pre-fertilization barriers were: 1) the arrest of pollen tube growth and 2) the failure of pollen tubes to penetrate into the ovules, whereas the post-fertilization ones were: 1) the failure of fertilized ovules to develop, 2) lack of seed germination, and 3) death of young seedlings. In 22 cross combinations, the number of pre-, post-, and both pre-and post-fertilization barriers that played the principal roles in cross incompatibility were fifteen, six, and one, respectively. Thus, we conclude that the variation in the propensity for the cross incompatibility existed in this intersubgeneric hybridization, and that it differed among individuals rather than among species.