Patterns and Strength of Pollen Tube Arrest in Self-incompatible Citrus Accessions (Rutaceae)

Pollen tube growth was examined to demonstrate the degree of self-incompatibility reaction in the pistils of 121 Citrus accessions including 77 pummelo accessions. One hundred of the 121 accessions were determined to be self-incompatible with the aid of an epifluorescent microscope. Based on the difference of self pollen tube growth between the self-incompatible and self-compatible accessions, the degree of self- incompatibility reaction in four portions of the pistils was roughly divided into three categories, i.e., high (H), moderate (M) and low (L). The degrees of the reaction in the stigmas, upper styles, middle styles and basal styles of these accessions were high-high-high-high (H-H-H-H), moderate-high-high-high (M-H- H-H), moderate-moderate-high-high (M-M-H-H), low-high-high-high (L-H-H-H), low-moderate-high- high (L-M-H-H) or low-low-high-high (L-L-H-H) in order from the stigma to lower style, and were designated as H, MH, MMH, LH, LMH and LLH types respectively. All of 77 pummelo accessions examined were self-incompatible and all of the seven types of self-incompatibility reaction were observed in the accessions of which about 80% showed either H or MH type of reaction. The degree of self-incompatibility reaction was more variable in the pummelo accessions originating in Southeast Asia than those in Japan. In three mandarin relatives and four yuzu relatives, the H and MH types of self-incompatibility reaction were exclusively found. These results suggested that the degree of self-incompatibility reaction in each part of style was different in different Citrus accessions and showed geographical diversity. The reason for this is discussed.