Induction and identification of tetraploids of pear plants (Pyrus bretschneideri and Pyrus betulaefolia)

Polyploid plants usually exhibit broader leaves, thicker stems, bigger flowers and fruits, dwarfing stature, as well as improved biotic and abiotic resistance. Therefore, increasing the polyploidy is one of the most important strategies used in plant breeding. Here, we reported the successful induction of tetraploids of two pear varieties, ‘duli’ pear (Pyrus betulaefolia), a wildtype pear used for rootstocks in grafting, and ‘Xinli No.7′ pear (Pyrus bretschneideri), a popular cultivated pear variety in China. This was achieved by treating their seeds, shoot tips of tissue cultured or field grown seedlings with colchicine and pendimethalin using three different methods: impregnation, mixed culture and smearing. The best tetraploids induction condition for ‘duli’ pear seeds was impregnating them with 0.4% colchicine for 24 h where the mutation rate of 2.0% was achieved with no chimera found. For shoot tips of tissue cultured seedlings the best condition was impregnating them with 0.2% colchicine where 6.67% of ‘duli’ (48 h) and 13.0% of ‘Xinli No.7′ (24 h) were mutated. In addition, we found that in the mixed culture method the best induction condition for ‘duli’ was mixing with 1.0% colchicine while for ‘Xinli No.7′ it was 0.5% colchicine. In the smear method, for the shoot tips of field grown seedlings of ‘duli’, the best induction condition was treating them with 0.4% colchicine for 24 h. Subsequently, the DNA content of these putative polyploid seedlings was estimated by the flow cytometry, confirming their ploidy nature. Also, we compared the morphological differences between the tetraploid seedlings and the diploid seedlings. Obviously, the tetraploid seedlings appeared to be dwarfed with shorter internode length and reduced internode number of their stems