Comparison of flowering and fruiting of seedlings from reciprocal crosses between diploid and triploid apple cultivars

To clarify a strong possibility of performing cross breeding between diploid (2x) and triploid (3x) apple cultivars, flowering and fruit bearing of 2x x 3x seedlings were compared with those of 3x x 2x and 2x x 2x. Rates of flowering and fruiting in seedlings, and number of fruits per fruiting tree, were highest for 2x x 2x and lowest for 3x x 2x. Ten years after sowing, the fruiting rate of seedlings was 49.5 to 65.8% for 2x x 3x, but only 10.9% for 3x x 2x. The trunk circumference of 2x x 3x seedlings was estimated as the circumference relative to the average circumference of 2x x 2x seedlings at 6 years after sowing. These values were used as a circumference index. The seedling fruiting rate and number of fruits per fruiting tree, 10 years after sowing, were highest in seedlings with an index of more than 75, and lowest in those with an index of less than 50. These results show that seedlings from 2x x 3x can be used in apple breeding, and that the trunk circumference index can probably be used as an indicator for judging and selecting at an early stage whether or not the seedlings will flower and bear many fruits. Seedlings from 3x x 2x are considered unsuitable for breeding new apple cultivars, based on a much lower fruiting rate and number of fruits per fruiting tree 9 to 10 years after sowing.