Cultivar and developmental differences in S-protein concentration and self-incompatibility in the Japanese pear

Abstract

Cultivars of the Japanese pear [Pyrus pyrifolia (Burm.) Nakai] have variable degrees of self-incompatibility (SI) and can be classified into at least three groups: strong, intermediate, or weak SI; as shown by the extent of self-pollen tube growth in the style, and the percentage of fruit set following self-pollination. Following self-pollination, the elongation of pollen tubes in the detached styles of 'Kosui' and 'Kikusui' became increasingly suppressed from 4 days before anthesis (-4 DAA) to 2 days after anthesis (2 DAA). Tube growth of 'Kosui' was more suppressed than that of 'Kikusui' during this period. In 'Osa-Nijisseiki', however, the rate of tube growth did not vary with stage of stylar development, from -8 to 2 DAA. Pollen tubes elongated much better after cross-pollination than after self-pollination at all stages tested, and the extent of the elongation increased as the styles matured. The concentration of total S-protein (sum of two S-proteins per buffer-soluble protein) increased with stylar development, but the rate of increase varied with the cultivar. The rate was significantly greater in the strongly self-incompatible 'Kosui' than in the moderately self-incompatible 'Kikusui', and was slowest in the weakly self-incompatible 'Osa-Nijisseiki' at all developmental stages. During stylar maturation, the concentration of S4-protein, which is common in all cultivars, was highest in 'Kosui', followed by 'Kikusui' and 'Osa-Nijisseiki'. Thus, the cultivar differences in SI expression in the Japanese pear are determined about -4 DAA and appear to be regulated, in part, by the concentration of S-proteins produced in the style.