采用传统的自然变异繁殖方法育种周期长，效果又不明显。
Natural variation is one of the traditional reproduce methods, not only its breeding cycle is too long, but also produces little effect.
苹果的遗传转化技术通过分子手段改良苹果，有助于缩短其育种周期。
Techniques of apple genetic transformation can improve apple trees and shorten the breeding cycle by molecular methods.
然而，这些育种技术均存在育种周期长、优良性状遗传不稳定等不足之处。
However, these breeding technologies have differently some serious weaknesses such as breeding cycle period being too long and heredity of good characters being unstable.
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