1. Introduction
Self-incompatibility is a genetically controlled mechanism in
flowering plants which prevents inbreeding and promotes out
crossing. Since Japanese pear (Pyrus pyrifolia Nakai) exhibits
S-RNase-based self-incompatibility, artificial pollination is conducted
in Japan because fertilization and subsequent seed
formation are inevitable for fruit setting and fruit growth. After
pollination, furthermore, fruit thinning should be done at fruit:leaf
ratio = 1:25 to promote fruit growth. Breeding of self-compatible
cultivars using self-compatible ‘Osa-Nijisseiki’ has been proceeded
and several new cultivars released recently in Japan (Washio et al.,
2006), but they need much labor for fruit thinning. Therefore,
development of cultivars with moderate fruit set and/or suitable
chemical thinners would be desirable; only 5% of flowers are necessary
for fruit production (Hiratsuka et al., 2002)