Characterization of a giant-fruit mutant exhibiting fruit-limited polyploidization in pear (Pyrus communis L.)

Fruit size is one of the most important traits in fruit trees. We found a bud sport mutant bearing giant fruits of pear in an orchard in Yamagata prefecture, Japan. The fruit weight of this mutant was more than twice that of an original cultivar ‘La France’. The size of the receptacle was already larger in the mutant than in the original cultivar at full bloom, and size differences between the mutant and the original cultivar were observed throughout fruit development. Microscopic observation of fruit cortex cells revealed that the size of cells was larger in the mutant than in the original cultivar, and that the number of cells was comparable between the mutant and the original cultivar. Flow cytometric analysis revealed an increase in the number of cells having doubled amounts of DNA (4C cells) in the fruit cortex, but such an increase was not found in leaves, suggesting chimeric polyploidization or endoreduplication in fruits of the mutant. Mutant fruits exhibited significantly higher titratable acidity and lower firmness than those of the original cultivar. These differences were observed from 110 days after full bloom to 165 days (the commercial harvesting time). However, the concentration of soluble solids was not different between the mutant and the original cultivar. There were no significant differences in polyuronide contents between the mutant and the original cultivar fruits, but hemicellulose content in fruits of the mutant was significantly lower than in those of the original cultivar.