Parthenocarpy in Citrus accessions with special focus on relatives of Kunenbo (C. nobilis Lour. var. kunep Tanaka)

Parthenocarpy is one of the most essential factors for production of seedless citrus fruits that is a desirable trait for consumers and producers. However, little research has been done on the degree and type of parthenocarpy in citrus, except a few Citrus cultivars. To increase the knowledge about parthenocarpy for cultivation and breeding of seedless citrus, 113 Citrus accessions including pummelo, mandarin and their relatives were investigated. In self-incompatible accessions flower buds just before anthesis were bagged, while in self-compatible accessions they were non-pollinated to examine autonomous parthenocarpy or style-removed three days after pollination to examine stimulative parthenocarpy. Fruit set rate in each treatment in the 113 accessions was recorded one month after anthesis and the mature fruits were harvested to examine their size and seedlessness. There was no clear difference between stimulative and autonomous parthenocarpy in Citrus accessions examined, indicating that fruit set and development in the accessions is due to autonomous parthenocarpy. The parthenocarpic fruit set occurred in 22 of 50 accessions for pummelo, 26 of 32 accessions for pummelo relatives, 3 of 9 accessions for yuzu and relatives, 12 of 21 accessions for mandarin and relatives, and 63 of 113 accessions in total. Of the 63 accessions, matured seedless fruits were harvested form 19 accessions, of which five accessions 'Shishiyuzu', 'Kunenbo', 'Okitsu-wase' and 'Aoshima' satsuma mandarins and 'Kiyomi' tangor showed very high degree of autonomous parthenocarpy, followed by 'Kunenbo' hybrid accessions such as 'Iyokan', 'Hassaku', 'Kabosu' that showed high to moderate degree of autonomous parthenocarpy. Since 'Kishu' mandarin (a seed parent of satsuma) and yuzu (a seed parent of 'Kabosu') showed very little or no parthenocarpic fruit growth, it was suggested that 'Kunenbo' carried parthenocarpic genes to satsuma mandarin and the other 'Kunenbo' relatives showing medium to high autonomous parthenocarpy. It is suggested that these and the other accessions determined to have high to moderate degree of parthenocarpy are useful for production of seedless fruits and breeding of seedless Citrus cultivars.