To induce haploid plants in Citrus maxima (Burm.) Merr. 'Banpeiyu', we evaluated the effect of pollination with soft X-ray-irradiated pollen on fruit set and seed development, and carried out ovule culture. When 'Banpeiyu' pummelo pistils were pollinated with X-ray-irradiated pollen of 'Fukuhara' sweet orange [C. sinensis (L.) Osbeck], the exposure doses affected the fruit set. The number of seeds per fruit was also affected by the exposure dose, and tended to decrease as the dose increased; however, all developed seeds obtained from these crosses were diploid. In the ovule culture of 'Banpeiyu' pummelo, six embryoids shown haploidy were obtained in all treatments. One haploid plantlet with 9 chromosomes was regenerated from an embryoid in a culture of ovules established 40 days after pollination with 400 Gray (Gy)-irradiated pollen of 'Tosa-buntan' pummelo (C. maxima). This haploid was suggested to be derived from 'Banpeiyu' pummelo by random amplified polymorphic DNA (RAPD) and cleaved amplified polymorphic sequence (CAPS) analysis.

Somatic hybrids were produced hetween embryogenic callus protoplasts of 'Shogun' mandarin (Citrus reticulata Blanco, 2n = 2x = 1 8) and leaf protoplasts of grapefruit (C. paradisi Macf., 2n = 2x =18) by electrofusion. Hybridity of the two regenerated plants was confirmed by leaf characteristics and nuclear genorne analysis using RAPD and PCR RFLP. Flow cytometry and root tip cell count of the two hybrids revealed a chromosome number of 27. These triploid somatic hybrids on transfer to field showed norma] morphology and vigorous growth. Field performances as well as disease resistance are under investigation.