Relationship between Morphology and Weight of Seeds and Ploidy Level of Seedlings Obtained by Cross between Diploid Pummelo ‘Banpeiyu’ and Triploid Grapefruit-type Citrus ‘Oroblanco’

To promote polyploid breeding efficiently in Citrus, we investigated the relationship between the morphology and weight of seeds and ploidy level of seedlings obtained from a cross between diploid pummelo ‘Banpeiyu’ [C. maxima (Burm.) Merr.] and triploid grapefruit-type citrus ‘Oroblanco’ (C. Maxima × C. paradisi Macfad.). When pummelo ‘Banpeiyu’ was pollinated with pollen of the grapefruit ‘Oroblanco’, the number of seeds per fruit decreased significantly, and many undeveloped and empty seeds appeared compared with crossing with Natsudaidai (C. natsudaidai Hayata). As a result of FCM analysis of seedlings obtained from crossing with ‘Oroblanco’, most of the developed seeds were diploid, but triploids appeared frequently from developed seeds weighing less than 200 mg, and tetraploids occurred from large developed seeds weighing above 500 mg. In undeveloped seeds, many triploids and aneuploids appeared, and two haploids were also obtained. These results suggest that various polyploids can be obtained efficiently by selecting types and weights of seeds.